Communicating BY Light > 자유게시판

The gear in the picture above consists of, from left to proper: meccano hand-cranked gramophone with acoustic recorder (out of body) to chop 78 rpm disc information on wax or celluloid; 'Crosley' 1926-vintage two valve regenerative receiver (above) with 1925-vintage crystal radio under it. Stromberg-Carlson portable battery-valve broadcast band receiver c.1948 used as a BFO for my major transistor receiver. Small multimeter in front of S-C radio. National Panasonic 4-band 11-transistor (all germanium!) radio set receiving 160 metres (VK3AML on that occasion) with my Emmco headphones linked to it. The curly wire above the transistor radio was our communal personal telephone line to my buddies David and Bruce Bowden, who lived across the corner in Pleasant Road. HMV portable wind-up gramophone kind C101, circa 1923, with carbon microphone to transmit 78 rpm discs down the cellphone line. Various broadcast-band and shortwave DX QSL cards and a single-sided disc on the noticeboard above (sung by a Madame Alma Gluck, from memory!). In my left hand, I held the oldest disc document then in my collection, London, circa 1903, of a brass band playing the 'Tancredi' overture. On the fitting, spare parts for our personal neighbourhood line, and an outdated electrodynamic speaker. This was the one photo of my 'radio shack' taken in the 1960s. It was the 'sleep-out' behind 6 Torring Road, East Hawthorn, five miles East of Melbourne's GPO (Australia). The home was demolished in 1997. Three home items now stand there.

Author's note, 28 April 2005: As this page offers with my own modulated gentle experiments I hope readers will tolerate these reminiscences before I reproduce my 1979 article on the topic from 'Amateur Radio' journal:

WHY MODULATE Light?

Today's young Australians can barely think about the frustration of teenagers in the so-called 'radical' 1960s the place electronic communication was involved. By modern standards, radio was locked in legislative totalitarianism, dominated by the governmental communication monopoly of the Australian PMG's Department, and run on inflexible submit-colonial British public service strains. Something as innocuous as an intercom line run alongside your back fence to adjacent mates was illegal - 'a financial menace to the monopoly of the general public telecommunications community' within the eyes of the PMG. I discovered this when i set up a neighbourhood telephone community to other native youngsters' locations early in 1967! Music transmission by way of novice radio had been banned in Australia since 1939, in order that the fabric legally conveyed on any 'wireless' system was limited, particularly for music-mad teenagers. CB radios, largely of the only channel hand-held selection, have been imported in restricted quantities and offered freely - but paradoxically they couldn't be legally licensed or used. You needed to be fifteen years outdated to take a seat for the beginner radio license, then involving a trifecta of exams in full concept, rules, and Morse at 12 phrases per minute. For the speculation, discursive essay answers had been required, https://theprimefans.com/ and there was no degree of concept examination below the one most normal. From a young particular person's perspective, you needed to drag your self over a area of damaged glass to achieve a legal entry point. Even then, you had to wait in your sixteenth birthday to function ham radio legally in Australia.

If, like me, you were 13 years old in 1967 and had electronic experimenter buddies close by, your aspirations to communicate confronted years of seemingly endless frustration. There have been good reasons to research 'different' communication applied sciences - together with modulated mild.

Together with that we had the Vietnam War; the imminent threat of the obligatory navy draft; a conservative authorities in power since our start; and British-styled school uniforms unsuited to our scorching summers. Australian teenagers of the 1960s had many valid reasons for discontent. The protest movement had its roots in lots of features of Australian society, not just the political points for which any avenue march was a handy excuse...

During the following decade Australians saw the appearance of full citizenship for Aborigines (1967); the Vietnam Draft Resister's Union, and its pirate radio station '3DR' (1971); a radical Labor government coming to power (1972); the first Sunbury rock festival, Australia's 'Woodstock' (1972); novice amateur radio licensing (circa 1973); neighborhood broadcasting (1974); ethnic, multicultural broadcasting (1975); and the native legalisation of CB radio (1977). Finally, within the 1980s, private carriers in the telecommunication marketplace were allowed to compete with the PMG and its descendant organisation, Telstra. Communication monopolies have been damaged and the authorized restraints on private digital communication eased markedly. With the advent of the Internet, how might or not it's otherwise immediately?

I'd argue that Australia was fairly unlike America in the 1960s, in that conservatism reigned by a lot of the 1960s here, significantly in our communication legal guidelines. Australia's 'liberation' - if you may call it that - largely got here after 1970...

The arrival OF Educational Solid-STATE KITSETS

So, how did your common child with more than average curiosity get into electronics, and more notably into optical communication, forty years ago?

In the mid-1960s, when the cost of germanium transistors had fallen to a reasonable level, digital development kits have been made accessible to stimulate the minds of future radio hams and experimenters. These kits were unlike the single-project meeting outfits then accessible from Heathkit and Lafayette. Their accent was on schooling, providing a set of digital components to be arranged in several configurations on an insulating 'breadboard' with spring clips and connecting wires. With the help of a guidebook explaining the operation of every arrangement, greater than twenty different circuits could be constructed, including Morse code oscillators, easy radio receivers, excessive-acquire audio amplifiers and low-power radio transmitters. At this time, we additionally had the bonus of a neighborhood Melbourne magazine for young experimenters, 'Transistor Kits' printed by Colin Witchell - extra not too long ago of 'Talking Electronics' journal fame - from a tiny store in Church Street, Brighton. For individuals who were interested, the mysteries of electronics held the important thing to a new world of technical possibilities - and lots of Colin's tasks concerned gentle detectors in some artistic means...

An outdated good friend from my major school days, Rowland Legg, acquired a Philips twenty-in-one package for the Christmas of 1965, and I was given an analogous Japanese 'Eleco' kit on the same day. My mother and father had apparently noted my early aptitude for building crystal radio receivers, so they'd determined to 'push the envelope'. The pure outcome was that over the following five years Rowland and i spent many afternoons together, unravelling the mysteries of digital amplification. Whatever time wasn't spent indoors with the kits or on different digital initiatives, we spent up on our rooves erecting lengthy wire radio antennas, or making an attempt to get them up on progressively greater timber or supporting masts.

During these antenna-elevating sessions, Rowland Legg and I discovered that we could see one another's properties from vantage factors on our rooves. In an try and set up a signalling system, we spent many evenings up ladders with kerosene lanterns, using a black card to cut off the light and send messages to one another in Morse code. The standard Morse alphabet appeared slightly complex, so Row devised considered one of his personal, following a logical mathematical progression:

This Also proved to be too advanced to memorise and, with gentle flashes, too difficult to relate to any typed list! We looked for a extra facile means of communication, with sufficient security not to draw consideration from the licensing authorities.

OUR FIRST OPTICAL COMMS - JUNE 1968

In 1967 I turned aware of the potential for transmitting speech over modulated mild beams when i bought a duplicate of an ancient, leather-based-sure ebook 'Science For All' (1884), containing William Ackroyd's account of Bell and Tainter's then-new 'photophone'. Another early influence was a ebook printed in 1921, 'The Boy Electrician', which gave constructional details of selenium gentle delicate cells, Tesla coils, audio transmission by multi-flip induction loops (which I constructed) and even the small print of a small X-ray machine (which, thank God, I did not assemble).

Like many teenagers of the late 1960s, I experimented with modulated light communication using amplifier-driven torch globes or neon lamps for transmitting and CdS photoconductive cells or OCP71 germanium phototransistors for receiving. A web page from my bench notebook dated 25 May 1968 - a few weeks after my 14th birthday - shows my earliest plans for an optical system:

On the next weekend, 1st June 1968, a highschool friend named Howard McCallum and that i set up the deliberate modulated gentle system with an incandescent torch bulb for transmitting and a CdS LDR in collection with a 9 Volt battery and headphones for receiving, collimating a mild beam between the two with magnifying lenses. The results over a distance of four metres had been loud, however very distorted with frequency doubling effects - a result of our preliminary attempts to transmit without DC bias on the filament lamp!

Soon afterwards, a Mr A G Murrell of Penola, South Australia, published the main points of his simple 'photophone' within the 'A Reader Built It' page of March 1969's 'Electronics Australia' (pps. 91-93). The undertaking's simplicity made it a pretty proposition for younger individuals, and plenty of Australian experimenters of my era will remember it:

The geographical setting for our own reconstruction of Murrell's device was less than salubrious. In reality, it was probably the most eccentric radio 'shacks' that I've ever encountered. At the bottom of my good friend Rowland Legg's again yard, a big picket packing crate originally used for transport a Volkswagen car to Australia was set up by Row's father, Ern Legg, as a tiny electronics room - 'the tin shed' as we referred to as it. To keep out the drafts, its partitions had been papered with out-dated promoting posters for Melbourne's weekly scandal-rag, 'The reality', obtained from our kindly local newsagent across the nook in Tooronga Road. It was the form of newspaper that no person would admit to buying, though the newsagent assured us of its wonderful native circulation. From each angle in Rowland's radio shed, headlines in an unlimited typeface assaulted the eye: "SHOCK BIKIE Film" - "BLACK PANTIES Murder" - "CATHOLIC FATHER Wants Sex Surgery"! The piece de resistance among these posters was tactfully hidden behind a cupboard door, its wording being one thing like - "UNWED Mother TELLS Court: 'HE SHAGGED ME; THEN WE HAD INTERCOURSE' !!" - and I'm still questioning concerning the implications of that!

In these inglorious surroundings, and in the backroom of my home proven within the photo at the top of this net web page, we spent many weekends constructing electronics initiatives of each conceivable kind. Together, we quickly had a model of Murrell's photophone transmitter built into the optics of a small ex-WW2 Aldis signalling lamp with a concave parabolic mirror of about 7.5 cm diameter. The Aldis housing was eventually mounted (with yards of PVC tape!) on a sizzling water overflow pipe protruding from the tiled roof at 1 Cole Street, Rowland's house.

However, my very own preliminary optical comms assessments with the Murrell gear had been finished round March 1970 from the the yard radio shack then occupied by Hughie Paton, VK3ZEP, at 49 Havelock Road, throughout that home's again garden, down its back driveway and throughout Torring Road to the glassed-in front veranda of our family dwelling (to the left of the entrance door in the photograph below) - a distance of about 50 metres. The system provided a link in one course only. These exams had been logged on open reel audio tape, in order that I might check my audio quality with out the need for an assistant. My mom occasionally got here onto our veranda to listen to the outcome and talk her reaction by waving via the windows. The tape still signifies the fairly good audio high quality that one might get with a restricted depth of modulation. It also demonstrates the very high hiss stage of the OCP71, the precise machine used on that day being seen in the picture on the appropriate.

I was out in the open air on the back of the Havelock Road property, across Torring Road from my home, shown above, with the Aldis lamp's glass parabolic reflector optics on a telescope tripod. This had a three volt 300 mA torch globe at its focus. I later discovered that the high frequency response of the system was inversely proportional to the thermal inertia of the filament. The very best excessive frequency response was obtained with lamps of decrease rated most present, a thin filament with thick lead-out wires to dissipate the heat shortly.

The transmitter gear, consisting of a microphone pre-amp, 3 watt transistorised audio energy amp with transformer output, and two measurement D torch cell battery supplies (one for the amp and one as DC bias for the lamp) all sat in a carton between the legs of the tripod. On the receiving end, a 7.5 cm diameter magnifying lens focussed the image of the Aldis lamp onto an OCP71. I can not remember whether or not the OCP71 was instantly linked to the microphone input of the Sanyo 5" open reel tape recorder (photovoltaic configuration), or whether or not it was working into the road input by the same old two-transistor pre-amp (photoconductive association). I seem to remember that both had been tried at totally different instances, with a minimal difference of outcomes. An occasional temporary buzz can be heard when widespread Australian blowflies happened to fly through the beam, modulating the light with their wing beats. Within the latter a part of the extract, a gentle shower of rain occurred, the droplets falling by means of the beam producing a type of tender 'plip-plap' sound. The effect of waving one's hand by way of the beam was also demonstrated. Whenever the beam was interrupted the hiss level rose substantially - an effect for which, even now, I have no rationalization, besides that it clearly had one thing to do with the impedance of the phototransistor supply rising as the sunshine enter fell.

The audio tape of the test was made on a heat, fairly overcast Saturday afternoon. In the background, Ghera Harris (1896 - 1991) and her architect daughter Berenice Harris (1925 - 2002), who owned the Havelock Road property, will be heard washing dishes after lunch and planting hop bushes within the backyard. The primary World War veteran aero-engine fitter Reg Harris (1894 - 1979), Ghera's husband, was apparently planning a brand new ingredient for some house brew! The son of the household, Brian Harris (1936 - 1992), was briefly VK3ZFH within the late 1950s. Brian confirmed the author the best way to tune up a transmitter and browse an oscilloscope on the age of four, in 1958. This set Chris on the lifelong 'downward' path into technical interests... Considered one of Brian's last jobs concerned the design of among the communications gear for the Hubble house telescope. Suburban life could possibly be interesting with neighbours like the Harris household at forty nine Havelock Road!

On this ten-minute mp3 extract from the unique 45-minute tape, the author at the age of fifty one in 2005 introduces his squeaky-voiced 16-12 months-previous self in 1970. Computer nerds did not exist then - WE were RADIO nerds! Anyone fascinated can download the audio log file of this March 1970 take a look at as an mp3 file here:

(Currently broken - coming quickly) March1970.mp3

One curious side of the germanium phototransistor OCP71 was its excessive infra-pink sensitivity. It was doable to transmit audio, as one can hear on the tape above, with the torch bulb running at a voltage so low that it had no perceptible visible output. The germanium detector could resolve the modulated HEAT from the lamp filament. Its work operate as a photodetector was very low, which meant that it was inherently topic to giant quantities of thermal noise, much more than silicon and very a lot more than photomultipliers:

On 1 September 1970 we extended the vary of the system to transmit audio from Rowland's electronics room ('the tin shed') at 1 Cole Street to my home at 6 Torring Road, East Hawthorn, where I stood atop a ladder next to our backyard bungalow with an an optical unit product of tin cans, like Murrell's within the picture above. The audio hyperlink covered about 600 metres, and that i could clearly see the sunshine focussed on the OCP71 in the receiver tube. In truth, I might aim the receiver by having the focussed spot disappear behind the square sensitive material contained in the phototransistor. The hiss stage from the OCP71 germanium phototransistor was gross, the 7.5 cm diameter of our optics was inadequate for the vary and the trebles were restricted by the thermal inertia of the torch bulb, however this was our first actual 'mild beam DX'. On the age of 16, the pleasure of listening to these results prompted my arms to shake, so that my optical receiver's aim was erratic, however I managed to log the entire contact on an audio tape in two 'bursts', with a pause to telephone Rowland to inform him that each one was being obtained, midway. Initially, Rowland and his buddy Neil Florence were simply relaying the 7:30 pm news broadcast from the Melbourne broadcast station 3AK with its items on the Vietnam War, and floods in New Zealand. As I slowly managed to align the receiver, the transmission step by step rose out of the noise - although not by much! Later, typical teenage music of the day could be heard - The Crystals singing 'After which He Kissed Me' (with Rowland attempting to interpolate a 1970-vintage type of 'karaoke') and the document of 'Lay Down' sung by Melanie Safka. When Rowland switched to his carbon microphone to announce 'Hello, Chris... that is being transmitted on the primary day of Spring, the first of September 1970 - and if you can't hear this in spite of everything this bother I'll must kill you!', I might easily have fallen off the ladder in amazement. We'd lastly devised a means of circumventing the illegal usage of a radio transmitter - although to be practical, with this appalling signal-to-noise ratio the success was only marginal:

http://www.bluehaze.com.au/modlight/1Sept1970.mp3

I've rarely skilled more thrill from experimental work than I did on that night 35 years ago, and i need hardly add that instantly afterwards, adrenalin-charged, I ran non-cease to Rowland's to report our success! The three of us posed for a photo around that time, with me holding a pair of 1920s-vintage headphones which we incessantly used. The image might be titled 'hear evil, SEE evil, DO EVIL' - but mostly, our backgrounds have been simply too 'Eastern Suburbs' and discreetly shy for any of that!

OPTICAL COMMS IN SOUTH AUSTRALIA - 1968 TO 1972

Elsewhere, different Australian experimenters had been attempting the potential of atmospheric optical communication more significantly. In Adelaide throughout 1968, my future collaborator Mike Groth (presently VK7MJ, then VK5ZMG) and a lab assistant good friend, Stewart Powell, built a pair of optical communication items in the suburb of Hammersmith utilizing torch globes and OAP12 germanium photodiodes. They had a maximum vary of about 1 km, and, as Mike puts it, "terrible fidelity". Mike's spare time for optical checks was restricted on the time by the calls for of doing the third 12 months of a BSc - with a brand new spouse.

However, in 1969 Mike Groth did his Honours in Adelaide, and one of many course projects in that 12 months involved modulated light. He explained the subsequent occasions to me in a letter dated 28 February 1988:

"The challenge concerned the evaluation of modulated gentle as a means of transmitting geomagnetic information over quick distances to avoid wire hyperlinks to the remote sensors, which tended to introduce hum loops if not rigorously balanced. This was a golden opportunity to study the speculation behind optical hyperlinks and search again via the literature at the University Library. It grew to become obvious that it would be no drawback to transmit knowledge over a few hundred metres on a transparent night time using the new infra-purple diodes as sources, but the reliability of the hyperlink was unknown, especially because it was for use at a field station in the Adelaide hills, the place fogs and mists had been frequent. I constructed an infra-purple hyperlink at 930 nm which measured the path loss over the winter and spring of 1969, and my estimates of the path losses [in the 1987 'Amateur Radio' article 'Photophones Revisited'] had been based on this information.

Optical communications fell into the background for the subsequent sixteen years, as I spent 1970 in New Guinea teaching and moved to New Zealand in 1971 to do postgraduate work on the University of Otago in Dunedin. I returned to Australia at the tip of 1979, but was fairly inactive in experimentation till 1985, when i determined to write down my experiences as a evaluation of the prospects of optical hyperlinks. The trouble took almost 18 months..."

Mike's resultant article published in 1987 with revisions from 2005 may be discovered at:

Essentially the most powerful novice atmospheric optical communication tests in Australia around 1970 have been executed by an Adelaide staff and reported in an extraordinary Tasmanian journal. The Hobart-based mostly 'Electronics Exchange Bulletin' was published round this time by the Tasmanian team of Leo Gunther VK7RG and Rodney Reynolds VK7ZAR (now VK3AAR). Their extraordinary journal inspired articles by local experimenters on every conceivable subject related to electronics and communications. Through the pages of 'EEB' between August 1968 and October 1972, two college college students, Kingsley Burlinson VK6ZEA and Robert Averay VK5ZGE described their experiments in modulating fluorescent and mercury vapour gas discharge lamps, reaching atmospheric ranges in excess of 3.5 miles (about 5 km), simply outside Adelaide in valleys shielded from the city lights.

Though their gear was massively bulky by trendy requirements, Burlinson and Averay pursued a novel line by driving their gas discharge lamps with audio-modulated 10 KHz pulse width modulation from 'class D' switching output transistors, thereby avoiding linearity and modulation effectivity issues. In this manner, the efficiency was just like that of the celebrated (on the time) class-D British Sinclair 'X-20' transistorised audio amplifier of the mid-60s. I was not aware of the reviews of these exams till the start of 1976, otherwise I might have skipped the following few steps in my very own assessments...

OUR Switch TO Gas DISCHARGE LAMPS

A search for higher modulated gentle sources than incandescents drew my consideration to neon lamps. Gas ionisation is a much quicker course of than the incandescent heating of a filament. In those days, neons had been cheaply obtainable in any dimension from a pea lamp to a full size "beehive" bulb from one marvellous source. Melbourne experimenters energetic within the 1960s will remember Waltham's Trading Company in Elizabeth Street, Melbourne. At the end of a narrow stairway main down from the pavement was a relatively grubby Aladdin's cave of tables laden with cable, conflict surplus junk (each battle besides Vietnam), containers of valves and khaki-painted objects of indeterminate origin. Cash-strapped adolescents milled about with down-turned eyes, slowly sifting row after row of cartons and crates full of technical cast-offs of each description. I acquired several boxes of neons there, together with varied other gas discharge gadgets to attempt. Many years later, I found that a few of these contained traces of radioactive material to assist ionisation. Let the buyer beware!

I initially wired a neon lamp into the anode circuit of a Philips battery triode from the 1920s, kind B406. The orangey-pink glow around the cathode on these neons is reasonably intense, though it doesn't even strategy the intensity of trendy LEDs or lasers. After setting up the neon modulator I was rewarded with near perfect audio from my OCP71 for the primary time. Fortunately, a parental veto on building mains-fed energy supplies ended on my fifteenth birthday (eight March 1969), so the required 300 volt rail was no problem.

SIDETRACK INTO 'MECHANICAL' Television - 1971

I used the neon modulator and OCP71 as the premise for a simple television system in 1970, by including a couple of Nipkow scanning discs to the outfit. This diverted me from the modulated gentle communication checks for a couple of years as I delved into all of the pre-struggle Baird television texts to deliver the images to a suitable standard. This introduced me into collaboration with the late Dan Van Elkan (b.1952 - d.1986, call signal VK3UI) and Tony Sanderson (b.1945, VK3AML), now the moderator of the 'bluehaze' net site. They had been the 'ringleaders' of a larrikin group of amateurs operating dwelling built AM transmitters on the 160 metre band, then occupying 1800 KHz to 1860 KHz. These guys' amateur radio interests were unique and individualistic. The vast majority of conservative hf (shortwave) operators had a slim and obsessive emphasis on 'communication quality' modulation, usually 300 Hz - 3 KHz, clipped, non-linear and cruddy. Dan and Tony had been both hi-fi fanatics. Their transmitters were relatively broadbanded and immeasurably low in distortion, exceeding the audio specifications of many broadcasters. AKG or Western Electric microphones and broadcast-quality audio peak limiters with twin time constants were concerned. The design and construction of their modulation transformers and amplitude modulators was nothing in need of an art. Their 160 metre receivers, additionally, employed biased, low distortion envelope detectors working by carefully designed audio amps into enormous speaker programs with vented enclosures. The content material and audio quality of their in-depth conversations on communications technology made listening to their transmissions an absolute pleasure. Even their standard of audio compression served to convey the listener into the acoustic setting of their properties - an underestimated facet of making a realistic auditory illusion. Their exercise ruffled fairly a few newbie operators' feathers at the time... which solely increased my youthful admiration for each of them.

I really met Dan (3UI) - dare I admit it - on the air on thirty first December 1969, as the result of a brief dalliance I had with pirate radio transmissions on 1.Eight MHz. He lived in Hawthorn solely a mile from my residence, close to the nook of Glenferrie and Riversdale Roads. Naturally he was among the first to listen to my feeble and unstable transmissions, and he inspired me to experiment further to gain the information to get the novice 'ticket'. We have been each given a 'stop it or else' ultimatum by an over-zealous radio inspector recognized to the locals as 'Uncle Ugh' (many will still know who I mean), but Dan and i grew to become great buddies consequently. Dan was about two years older than I, and was extremely influential on the instant future course of my life. As the yr 1970 progressed, I discovered myself spending more time with novice radio pals and less with Rowland and the old style mob. Rowland ultimately carved out a very successful profession in the Victorian Police Force (ironical, isn't it?), and that i imagine he still does, but his early curiosity in electronics declined...

In subsequent many years, the amateurs have legislated themselves into 'band plans' with 'accepted modes' and 'accepted bandwidths' for varied frequency segments. Consequently - and I will categorical an opinion here - they've systematically eliminated the legal basis for the kind of justifiable experimentation that 3AML and 3UI used to undertake. Many 'hams' are now operators of commercial 'black field' transceivers which can elegantly provide single channel telephone high quality (or worse) on each out there band for $4000+, however which often can't be correctly adapted to some other mode or form of experiment. From my perspective, the result is that novice radio has had progressively less attraction as a interest, and I'm sure I'm not alone in expressing this opinion. For my money, for those who legislate in opposition to experiment you kill the one attraction that novice radio ever had. So lengthy because the emissions do not unfold beyond the newbie band edges, what is the problem? Anyone who pushes the hoary old argument that "bandspace is at a premium, so transmissions should be of the minimum possible bandwidth" must be deaf and blind to the steadily declining stage of newbie band utilization over the past fifteen years. Who is to say what beginner radio should entail, so lengthy as it supplies coaching and encourages experiment? Many people wonder why I've never bothered to pursue an beginner radio license. I hope that I've justified my position. Thank God for options with more freedom of content and bandwidth, like gentle beam communication!

Dan (3UI) and i eventually arranged take a look at transmissions of narrow band tv using mechanical disc scanners over his 160 metre transmitter early in 1972. The following couple of years gave all of us a superb grounding in the rules of light detection and modulation, video amplification and optics.

Eventually, with D B Pitt and others within the United Kingdom, we formed the Narrow Band Tv Association, nonetheless in active operation and now represented on a web site:

http://www.nbtv.org

One particularly variety donation to this mechanical Tv scanner challenge was offered by the late Kevin Duff, VK3CV (b.1927 - d.1996). Kev worked in telecine at Melbourne's government Tv station, ABV channel 2 in Elsternwick, working an archaic monochrome Marconi 35mm movie scanner. This was initially said to have been designed for the 405 line British service and used at BBC Tv's original studio on the Alexandra Palace in London. By 1972 it was only getting used for half-hour a day, at about 4:00 pm, to broadcast 35 mm movie episodes of the youngsters's serial "The Cisco Kid" - the one common program materials that they had which still used that gauge of movie. The EMI 6097 photomultiplers in this Marconi telecine had been written off as soon as they developed spots on their photocathodes - Kev called them 'dynode spots' - however they were still fairly serviceable for mild detection. In 1972 Kevin saved some of these from the dustbin for us.

Dan (3UI) and that i tailored the EMI 6097 photomultiplier to be used in our experimental digital camera and for the modulated gentle receivers. Its sensitivity was such an unlimited quantum leap from the OCP71 that it opened an entire new world of technical prospects to us. Further photomultipliers had been acquired from the late A H 'Mac' McKibbin, VK3YEO, who used 931A's for slow scan tv scanners in these days.

MOD Light ON 160 METRES - FIRST CROSSBAND Tests 1974

In the course of the Autumn of 1975, I constructed a modulated light communication hyperlink that was used briefly between two members of the 160 metre AM group, Paul Higgins (then VK3BEK, now VK3EN) and Dave Stewart (VK3ASE). Both had radio shacks at first ground stage, going through one another throughout suburban Glenhuntly with an uninterrupted line-of-sight, and separated by about seven-hundred metres. The modulator from my Baird mechanical tv receiver by then used a 6L6 beam pentode in series with the neon, which was re-mounted at the focus of a rough 30 cm moulded glass parabolic reflector provided by Tony (3AML). The reflector had initially been used in a site visitors sign.

This optical transmitter was placed on Paul's balcony in Glenhuntly Road, on the corner of Clarke Avenue. The receiver was placed within the attic window of Dave's QTH in Burrindi Road, Caulfield South. It used a thirteen cm diameter magnifying lens focussing onto a 0.5mm focal plane aperture, with a 931A photomultiplier catching the transmitted light behind it.

Using Dave's 160 metre amateur transmitter as the return link - the final word in split frequency operation - Paul managed to carry on a crossband contact through the neon lamp. Sig/noise was poor owing to the very poor spectral match between the orange neon and the blue delicate photomultiplier. The neon was pushed beyond its present scores, so that its bulb was rapidly blackened by cathode sputtering, but not less than it might be totally modulated. I used to be later capable of measure the bandwidth and was astounded to seek out that the neon could possibly be modulated to around 500 KHz, which was a terrific improvement on the torch bulbs. Distortion was extreme. We were pushing the modulation fairly arduous, owing to the poor sig/noise ratio. A pattern of the audio log of the contact is at present out there on Dave's web site. The results had been sufficiently good for us to realise that we were heading in the right direction for additional enchancment. The contact was logged on tape and an extract might be heard on VK3ASE's web site:

[ Actually, Dave appears to have eliminated this one for now. (Tony, VK3AML) ]

The principle downside to be overcome was the low characteristic intensity of the neon discharge. We additionally wanted a source with vital blue output, to match the spectral response of the photomultipliers, which may solely detect violet, blue or inexperienced mild. Development was accelerated by the involvement of one other member of the 160 metre cross-band contact clique, John Eggington (then VK3ZGJ, now VK3EGG) whom I met at the top of 1975.

BREAKING THE ONE-MILE BARRIER - DEC. 1975.

At my dwelling, I used to be lucky in having an elevated place close to the top of a hill in East Hawthorn, South of Camberwell Junction and quite close to the high copper dome of 'Our Lady Of Victories' catholic church. Standing on our roof, the view of downtown Melbourne to the West and of the suburbs around to the North was unobstructed, encompassing all of Hawthorn, Kew, Toorak, Kooyong as well as components of Malvern, Richmond, Abbotsford and Northcote in an unbroken a hundred and twenty diploma arc. Just on my aspect of the Yarra, on the top of a rise in clear view, was VK3ZGJ, together with his shack going through me on the third floor rear of an old Victorian mansion at 29 Shakespeare Grove, West Hawthorn. The topography was splendid for optical communication assessments, precisely two miles (about 3.5 km) on an East-West path.

In the early 1970s, before I met him, John (VK3ZGJ) constructed a collection modulator for fluorescent lamps, consisting of a number of 807 output valves in parallel, with the fluoro within the anode return. He additionally built a portable mild dependent resistor (LDR) receiver with a FET preamplifier. The LDR operated with bias right into a load of 10 megohms or extra, and with its low noise preamplifier it had much better sensitivity and spectral match to a fluorescent lamp's output than my old OCP71. You needed to arrange for the image of the fluoro to focus precisely onto the hole between the conductive combs on the LDR's delicate floor. This concerned peering on the LDR through a 'spy-gap' in the optical mounting whereas deftly manipulating the receiver's alignment.

LDRs have a very sluggish response, rolling off not less than 6dB per octave above 50 Hz. Treble enhance may only partly appropriate the issue, as John discovered. My photomultipliers provided the reply to John's receiver issues, just as his fluoro transmitter modulator permitted advances over my feeble neon. We pooled resources over the following 5 months.

Late in December 1975, John and i obtained the communication system working between our homes, utilizing a vertically mounted fluorescent lamp on the roof which "broadcast" mild in all directions. The modulator consisted of several (4, I feel) type 807 beam pentodes feeding the forty watt fluoro of their anode circuit. A rail voltage of about 600 volts DC was applied to the lamp. We didn't use a lamp starter or a ballast choke to operate our fluoros. Instead, we had a novel starting association consisting of a band of aluminium foil wrapped across the glass close to the cathode finish of the tube, which was connected to the secondary of an automotive spark coil. To begin the discharge, you'd apply the 600 volt rail, then energise the ring around the cathode by applying a battery briefly to the spark coil main. The high-voltage spikes utilized to the glass near the cathode began a barely perceptible glow discharge inside that end of the tube, which might immediately spread the whole size of the tube beneath the affect of the 600 volts DC rail. Standing present was diverse by altering the value of the cathode resistor on the series 807's. The filaments at both end of the tube had been by no means heated up with this association, which appeared to extend the working life of the tube.

Our first forty watt 'fluoro' check transmission got here simply after the Christmas of 1975. It was in one path solely, with John transmitting and me receiving. 3ZGJ managed to arrange some quite elaborate music programs interrupted by announcements, one in all which I recorded on tape. Within the early 1970s a British group, calling themselves "Radio Love" (hey man, actual groovy) had proposed a system of native mild beam broadcasting. This seems to have been an attempt to avoid the heavy hand of British officialdom, which at that time had compelled a number of independent broadcasters (eg 'Radio Caroline') to transmit from ships anchored in International waters off the English coast. Nothing was heard from the group after the publication of its initial plans, together with the diagram under which neatly summarised their ideas:

John 3ZGJ, in imitation of this 'Radio Love', jokingly introduced his programs as being transmitted from "Radio Hush". The name was a bit much less poofy, but nonetheless retained the essential atmosphere of the authorized fringe-dweller! Later we duplicated the system to offer full duplex communication (simultaneous transmit and receive) in both directions. A typical extract from these fluorescent light transmissions in the final week of December 1975 can be heard on this tape, recorded from the photomultiplier output at my finish of the hyperlink, two miles from the transmitter:

RadioHushDec75.mp3

We used no reflector or collimator with these fluorescent lamps. The photomultiplier receivers only managed to attain 15 dB sig/noise over the 3.5 km range with this arrangement. Fluorescent lamps had greater than their justifiable share of problems as a modulated source. Their phosphor coating had time lag, the persistence limiting the higher modulated frequency to about 5 KHz. The time lag was not constant with the wavelength of the emitted mild. The purple phosphor components had very long persistence, while the blue phosphor was much faster. The system's higher audio frequency limit therefore varied with the spectral response of the detector, but with a blue-sensitive photomultiplier it was greater than sufficient for audio.

The fluorescent lamp discharge would wander and 'snake' inside the tube, particularly at swap-on, interfering with the modulation. For some purpose which we could by no means clarify, the output on the cathode finish of the fluoro, and solely on the cathode finish, was modulated by an erratic whine at about 400 Hz, presumably attributable to the discharge hopping about from one a part of the coiled cathode filament to a different. This all the time set a definite limit to the sig/noise achievable, even the place there was loads of gentle sign to demodulate.

The audio frequency response of the fluorescent tube was unexpectedly uneven, an effect undoubtedly caused by acoustic resonances of the modulated mercury plasma column within its tubular enclosure. One might definitely hear a faint acoustic radiation from the tube while it was in operation, as these acoustic waves actually penetrated the glass walls. Certain modulation frequencies, clearly associated to 'organ pipe' resonances throughout the fluoro tube, would trigger the mercury discharge to extinguish, or to interrupt up right into a collection of spaced glow discharges alongside the tube. I later found that this downside had been noted by N C Beese, who wrote a chapter on "Light Sources for Optical Communication" within the e book 'Infrared Physics' (Pergamon Press Ltd., London, 1961, Vol. 1, pps 5 - 16). To quote from Beese (pps thirteen - 14):

"Enclosed arc lamps operated on a.c. energy within the audio-frequency range, or on d.c. and modulated by a.c. currents could cause sound vibrations to be produced throughout the arc chamber. They're attributable to thermally induced variations in gasoline pressure that result from adjustments in current density in the arc. At sure crucial frequencies, resonance of appreciable intensity is constructed up by reflection from the bulb walls. The size and shape of the bulb, type of fuel or vapour filling, temperature and operating circumstances decide the frequency of the plasma oscillations which are just like standing sound waves within the discharge. Ordinarily this phenomenon isn't observed because lamps are operated on d.c. or low frequency a.c. with adequate ballast to ensure stable operation. In lengthy tubes the discharges assume a constricted, snakelike look at the important frequencies and are caused by sound power mirrored from the ends of the bulb. In a spherical bulb the sound waves unfold to the bulb walls and are then focussed back upon the arc to supply instability at the electrodes [...]"

Beese goes on to analyse a Xenon discharge lamp 1.5 cm in diameter and 13.3 cm lengthy with three Amp d.c. current utilized with a 2 amp a.c. modulation utilized. He noted:

"[...] the arc confirmed violent distortions at 2250 Hz however was quiescent at 2000 Hz and 2500 Hz. With 5 A d.c. and 3 A a.c. modulation at 2500 Hz, the discharge once more showed pronounced disturbances, however was stable at 2300 and 2700 Hz. The instability may begin at both electrode, whereupon the discharge constricts into a thin luminous ribbon with sinusoidal form, and the voltage will increase due to elevated arc size. [Instability at] harmonics of the basic frequency might even be observed[...]

"[...] In a spherical bulb with electrodes on the centre [like a Xenon arc] acoustical resonance occurs when the bulb diameter is equal to 1-half [of the acoustic modulation] wavelength [...] An arc centred in a spherical bulb will actually 'blow itself out' by its personal sound waves if any of the sturdy resonance frequencies are applied to the lamp for an appreciable time [...]

[...] In a low pressure discharge lamp [eg fluorescent], assuming a mean fuel temperature of 250 levels Centigrade, the velocity of sound in mercury vapour was 19,000 cm/sec, calculated by Laplace's system. At 600 Hz, the wavelength equals 31.7 cm [...] Maximum disturbance or turbulence on the electrodes happens at a half wavelength from the nodes that are on the ends and centre of the lamp."

Owing to the low depth and prolonged source space of the fluorescent lamp, the radiated flux might by no means be properly collimated. These limitations, and the frequency response irregularities clearly indicated that our gentle transmitter wanted a change of approach.

At this point, Rodney Reynolds VK3AAR drew our consideration to EEB's publication of the sooner work of Burlinson and Averay, whose modulator circuit and mercury arc source was a completely novel strategy. Their optics, however, had been quite crude, not nearly directive sufficient for our work in suburban Melbourne, where street lights and illuminated advertising signs proliferate.

FROM FLUORESCENT LAMP TO MERCURY ARC

We needed the next intensity supply, no fluorescent coating, and an output wealthy in blue mild to match the photomultipliers. From the electrical supplier Arthur J Veall in Bridge Road, Richmond, John and that i obtained some Philips high pressure mercury arc lamps in January 1976, of the type used for factory lighting. Most of these had inconvenient fluorescent coatings, apart from the smaller lamps below one hundred watts output, which unfortunately had frosted glass envelopes. We purchased a few HP80 mercury lamps rated at eighty watts. Driving these was a real downside. The fluorescent lamps that we previously used had been low present, high voltage gadgets, modulated simply with commonplace output valves. By comparability, the high pressure mercury arcs ran at medium voltages (30 to one hundred volts) however with high present (0.5 to three amp standing present), they usually exhibited a severe damaging resistance characteristic - their voltage drop decreased sharply with increasing current flow.

Other issues arose owing to the instability of the mercury arc. Its hanging voltage diverse broadly with ambient temperature, and its standing present diversified with time as heat induced the mercury to vaporise and the arc pressure elevated. At its peak, the quartz arc tube needed to withstand internal pressures of around 30 atmospheres, so it was a system that you just had to deal with with nice respect!

The slightest overmodulation peak would extinguish the discharge, and the entire system then had to cool earlier than it could be re-struck at a moderately low voltage. The lamp polarity additionally had to be reversed at frequent intervals with a DPDT change to keep away from premature cathode failure by ionic bombardment. The lamps have been designed for AC operation, where the effects of cathode heating were shared 50 occasions per second by each electrodes. Despite utilizing DC bias on the tubes, for which they weren't designed, we managed to get a few hundred hours out of them, which was Ok when the lamps only price $7 apiece. Today they cost about ten occasions that.

The modulator needed to steer the current passing to the arc. Quiescent class-A amplifier conditions demanded that no less than half of the provision rail must be dropped by the sequence modulator, the remainder being dropped by the mercury arc. The adverse resistance of the arc was our stumbling block. As the current via the arc elevated its voltage drop decreased. This dragged the voltage applied to the collection control machine up in the direction of the rail voltage as current reached a maximum. With 200 volts-on the rail and a peak current approaching 5 Amps, no transistor commonly out there in 1975 may handle the job. They couldn't take the excessive peak currents simultaneously with the excessive emitter-collector voltage that this load with its unfavorable resistance would present to the output transistor in class A. We had some costly pyrotechnic shows of 2N3055 and BUX80 transistor failure owing to this secondary breakdown level being exceeded. Like many foolhardly experimenters, we not often used fast-blow fuses in our gear. Youthful arrogance, I suppose!

The elegant answer, suggested many years later by Rod Reynolds (VK3AAR), was to run the management transistor in parallel with the arc, with a single series resistor up to the availability rail from both. In that configuration, the transistor passed maximum voltage at minimal present, and vice versa, so that a much smaller transistor might be used than in the collection modulator configuration, the tradeoff being solely a decreased total energy efficiency. However, on the age of 21 in 1975 - and John was 20 - neither of us considered that...

Instead, we used a category-A vacuum tube amplifier of elephantine proportions in sequence with the arc. The facility provide was capable of 200 Volts DC at three Amps, and the reservoir capacitors totalled 1500 µF at a 350 Volt ranking! For the modulator, 4 hefty 6080 or 6AS7 high current, low gm triode regulators had been placed in parallel with low-worth balancing resistors of their cathode returns. The anodes had a nasty behavior of glowing a dull crimson when the arc was initiated, as it took a few minutes for the mercury discharge to run as much as its traditional 100 Volts drop. During that point, the most important part of the provision rail was utilized to the valves, which had been sent past their dissipation restrict. Under these circumstances, one may see alarming high resistance 'spots' scintillating with a shiny and sparkling yellow mild on the large oxidised cathode surfaces of the 6080s. I all the time anticipated a 'bang' but in some way I was all the time lucky...

Each 6080 filament consumed 2.5 Amps at 6.Three Volts. The valve heaters alone consumed a complete of sixty three watts! It was all brute power, ignorance, bulk power and heat! Fan cooling was obligatory. We used a domestic fan of 1928 vintage with a bum sleeve bearing, which rattled consistently throughout QSO's. Nevertheless the system worked remarkably effectively, and due to Melbourne's many disposals shops, it could possibly be built for a few dollars. Not so now! Electronic disposals shops of that sort at the moment are mostly a thing of the past.

This mercury arc transmitter was reasonably unstable in operation, vulnerable to permitting the arc to drift right into a sluggish thermal runaway. To correct this tendency, one had to trace the current of the arc by manipulating the grid bias of the output tubes - therefore the explanation for the metering of arc current and voltage within the modulator circuit, above. The temperature of the quartz bulb was the most important variable, as this and the arc's standing present had been intently associated. The bulb temperature managed the mercury vapour strain within the quartz phial, and therefore the electrical resistance of the arc. With insufficient common current flow, the heat of the arc would be too low to maintain the mercury vapour stress, causing the voltage throughout the arc to slowly fall, in turn leading to an extra fall of current. If the standing present was too high, the arc would grow to be so hot that it not only turned harmful, however was impossible to modulate totally. One couldn't leave the gadget unattended for greater than about 5 minutes. Arc present would vary, significantly in the time simply after 'swap on', because the log report beneath clearly indicates. Warm-up would take round 15 to 30 minutes earlier than the system finally settled into static current circumstances, with the heat pumped into the arc electrically equalling the heat lost to the surrounding environment. The average standing present assorted with the audio program content material and with the symmetry of the audio waveform utilized.

A typical example is offered by the following log that I took of the arc transmitter's electrical standing, measured by means of the night of Saturday 31st January 1976. Operation on that evening was greatest described as 'intermittent'! Eventually, in response to 3ZGJ's insistence that I add a adverse peak clipper to my modulator, I fitted a diode to the grids of the output tubes to do the job!:

6:05 pm

80 V @ 0.42 Amp, 33.6 Watts

REMARKS: Test TRANSMISSION Only earlier than nightfall. Arc has been on for 45 mins of warm-up, approx 60% peak modulation, with music from 78 rpm discs (avoiding copyright issues!). Ambient temp. 350 C. VK3ZGJ not listening but. Power and heater transformers are each sizzling to touch, but Ok.

PHOTOMULTIPLIER RECEIVERS - 1975/76

The receivers used in this system have been also designed around components obtainable from disposals sources. Photomultipliers are superbly delicate, and never practically so fragile or tough to arrange as some so-known as consultants recommend. They require a smooth one thousand volt supply at about 5 or 10 mA - easily organized with an previous valve radio transformer working into a voltage doubler. The hand-held photomultiplier receiver was a slightly hazardous gadget whenever you had been manipulating it at night from the highest of an earthed steel ladder! Particularly so when the 1000 volt provide had eight µF of oil-stuffed block reservoir capacitor behind it (ugh!) and was fed to the receiver by flat 240 volt twin lead flex. It became much more harmful when rain was falling! In my youth I had scant appreciation of my own mortality. Few teenagers ever do!

The previous disposals photomultipliers typified by the 931A had virtually no response to purple mild, so that they could not be used with LEDs or HeNe lasers. More recent photomultipliers with gallium arsenide photocathodes have good crimson sensitivity, but at prices exceeding $1000 they're out of my league.

Overall results with this cumbersome link system have been remarkably good. Considering the potential theoretical problems of thermal lag and ionisation time, the mercury lamp's limit of 50% modulation at 10 KHz was quite usable, totally acceptable for direct amplitude modulation at audio frequencies. Piles of signal have been accessible. The output was terrific. At evening, with one's eyes darkish-adapted, you needed to protect your eyes with darkish glasses or welding goggles. But data and narrow band video modulation revealed wild part shifts above 5KHz. I suspect that the mercury plasma, when thermally modulated at an audio rate, was making an attempt to broaden and shrink towards its quartz tube housing with each modu1ation cycle, interacting with the arc cavity in a fancy collection of acoustic resonances. Indeed, one might actually hear the modulation coming from the arc, notably at high audio frequencies, by putting your ear down near the quartz bulb. One undesirable aspect effect was that certain modulation frequencies near the arc cavity's acoustic resonance extinguished the discharge. Sibilant sounds were especially risky. In reply to a query concerning the modulator I remember saying over the link "it is working completely!" At the other finish, all John heard was "it's working perf - Click". The arc extinguished itself, right on cue!

Over the four months up to April 1976 the system was in operation up to six nights a week between VK3ZGJ and my QTH at Camberwell Junction. It was unaffected by all but the heaviest fogs and rain showers. On very hot nights, low frequency noise and fast flutter fade have been evident, but infrequently obtrusive. The impact was attributable to hot air cells rising from the heat floor into the cooler night time sky. This triggered a heat shimmer, with a consequent scintillation of the acquired light. We considered using FM subcarrier modulation to clip off the scintillation on the obtained mild modulation, but the mercury arc lamp hadn't a sufficiently quick rise time to assist this. Subsequently I discovered that using longer wavelengths - purple or infra-red mild - tremendously reduce scintillation. FM technique may be desirable for hyperlinks of over 5 miles vary, however for shorter distances amplitude modulation's higher available base bandwidth in all probability makes it the preferred mode.

The old mercury arc/ photomultiplier system may have had appreciable lengthy range potential however we by no means had the chance to test it. The heavy energy calls for of the terminal equipment demanded a mains provide at both ends of the link. We by no means efficiently tried DX away from our respective properties and two miles (3.5 km) was the best distance tested. We could typically obtain 45 dB sig/noise over that distance.

Communication ceased in the course of the day owing to the scattered blue gentle of the sky. The photomultipliers have been saturated by it. Interference filters could have been used to extract a dominant spectral emission line of the mercury discharge from the final background radiation, however I used to be unable to acquire one until the 1980s. They're expensive and suitable just for parallel rays of mild. There are better methods of achieving good sig/noise during the day, using a monochromatic supply and a wavelength away from the peak output of the sun.

However, I might energy up the photomultiplier with a relatively low voltage - about 500 volts - simply before sunset, to catch John's first distinct phrases because the ambient mild light. On sizzling evenings, the signal was at all times lower than anticipated. I could not account for this, until one night I climbed the ladder to adjust the receiver and seen a wierd odor. The receiver's optical cavity was filled with smoke! I had forgotten that John was nearly directly West of me, and because the summer solstice handed, the place at which the solar set on the horizon was transferring North on successive evenings. That evening, it was setting directly behind John's home - so my thirteen cm receiver lens was focussing the sun's picture on the again of the receiver housing, which was manufactured from black-painted particle board. Awk! We were so fortunate not to have set fireplace to all the things - however I stored the incident a secret from my of us. Things that my dad and mom didn't know could not damage them! With some embarrassment, I fitted the optics with a removable lens cowl.

One potential use of the mercury arc transmitter that LED's, Luxeons and most low power lasers in all probability might by no means rival lies in non-line-of-sight (NLOS) linking by reflection off clouds. Some years ago I acquired a pair of 1 metre diameter searchlight mirrors to attempt the thought. The mercury arc can be mounted in one, photomultiplier in the other, both tracking the identical cloud. Street lights right this moment use the identical kind of mercury discharge because the modulated supply, so that 100 Hz hum background plus harmonics would be unavoidable in an urban atmosphere. I've yet to try the idea, as direct linking is undoubtedly extra dependable and efficient, and has subsequently been my essential line of analysis.

OPTICAL COMMUNICATION'S 'Comic CUTS' - 1976

Almost as quickly because the optical hyperlink between John VK3ZGJ and myself was established, he relayed me to 144 MHz on the cross-band discussions radiated by VK3AML on 160 metres, several occasions each week. For all practical functions, I used to be on an intercom in 3ZGJ's shack, and there was no legislation against that. Legally and technically, I used to be a supervised (ie, pull the plug out if he misbehaves) visitor who simply occurred to be present near 3ZGJ's microphone!

Through the early part of 1976, there have been many cross-band contacts on 160 metres with me "on lightbeam" which appeared to add technical novelty to the proceedings. The discussions have been absolutely open to any subject, offered that the due decorum of the medium was maintained. On some occasions, it wasn't! One evening we by some means drifted onto the topic of funerals, morticians, Egyptian mummification and the 'dear departed'. This contact, with many others from that 'gentle beam hyperlink' period, was logged on audio tape by Tony VK3AML and by other listeners, and extracts could be downloaded from Tony's 'bluehaze' internet site ("Multimedia Page 3") as "Death By Light Beam" (mp3).

Over time, the content different from creditable technical depth to immature ratbag ramblings. I never stored monitor of all of the log tapes, and i never know when bits of it will re-emerge on the weekly replays that have been maintained by VK3ASE - and others on Internet - haunting me ever since...

Most of our problems with the outdated gasoline discharge lamp system have been more comical than theoretical, and most had been completely unforseen. Moths, for instance, were irresistibly drawn to the extremely-violet output of the mercury arc. Bogong moths, large Emperor Gum moths, Christmas beetles, flying ants - the complete field and dice! They swarmed the transmitter arc on hot nights, attenuating the beam flux and suiciding in opposition to the hot lamp. Every couple of hours I'd need to scrape their smouldering our bodies out of the lamp reflector. Their fluttering wings modulated the transmitted beam, and the light they reflected formed a suggestions path into the adjacent, constantly working, duplex optical receiver. If I had my transmitter microphone operating close to the monitor loudspeaker, the presence of an Emperor Gum moth (which, I should clarify for non-locals, are the scale of a small dinner plate) was signalled by ear-splitting suggestions warbling at their wing-beat frequency of about 15 Hz. This was often adopted by our neighbours lobbing pebbles onto the tin roof of my radio shack to get me to 'flip my bloody noise down'! As we continuously operated between midnight and 3 a.m., I can hardly blame them. We known as the effect "moth-again" .

Another unexpected fault took ages to detect. After a seemingly harmless rainstorm, the mercury lamps would stop to function. The lamp itself appeared perfectly Ok, even on shut examination. We lastly found that water had drained down the bulb into to lamp's Edison screw socket. There, the DC bias arrange an electroplating motion, dissolving the wires connecting the bottom with the bulb. The problem was fairly simply solved by mounting the lamps upside-down.

Our line-of-sight path, skimming the tops of suburban Hawthorn's buildings, backyard foliage and energy lines made continuous contact between the terminal stations the topic of continuing concern - and some unusual QRM. The situation worsened in windy weather as there was a tall gum tree very shut the the optical path, in a garden about a half mile distant from 3ZGJ. In a Northerly wind this huge nuisance had branches that will sway again and forth into the beam path, slicing words and phrases out of our conversations. Eventually the damn factor grew to become such a drag that we made half-critical plans about sporting balaclavas on an illicit midnight raid on its owner's backyard with ropes and a pruning saw! Another suggestion involved borrowing a surgical cutting laser from Melbourne University's medical college, and trimming the foliage from a distance - the gap of 3ZGJ's shack veranda, the truth is. A few years afterward, I took a look at the identical path with binoculars, and located that the nuisance tree, by then with several others, had grown to fully block the optical path. L.O.S. paths could be impermanent! The rising of tall trees in suburban streets and gardens is now much more fashionable than it was thirty years ago. It may be unfortunate for optical communication enthusiasts, but it surely offers an aesthetically pleasing means of supporting h.f. wire antennas. (I'm humming a chorus of Monty Python's "Always look on the brilliant facet of life" whereas I'm typing this)!

The color of the mercury lamp also provided problems. It appeared the same as another avenue gentle in Melbourne. One night, we drove up to 1 Tree Hill within the Dandenong Ranges outdoors Melbourne in an unroadworthy Austin with the portable LDR receiver, optimistically hoping to set a distance document. At the top of the old lookout tower there (eliminated in 1981) we looked down on the lights of Melbourne, which have been as quite a few because the grains of sand on a beach. With John's optical receiver we searched the various lights visible in the final route of Hawthorn for about half an hour, vainly making an attempt to listen to audio modulation among the grunts and farts of mains-fed