?? An Introduction To Amateur Television.
By Ian Waters
What Is Amateur Television?
Amateur Television is one of the specialist modes of communication within the hobby of Amateur Radio. It takes two forms, Slow Scan and Fast Scan.
In slow scan still colour images producing modulating frequencies within the audio band are transmitted either using FM at VHF/UHF or SSB on HF. In the latter case worldwide contacts are possible. Pictures are generated by a video camera and processed using a computer with suitable software. Received images are processed and displayed by the computer.
In fast scan moving colour images with standards similar to broadcasting in the operator?s country are used. As modulating frequencies extend to 5 MHz, transmission must be in the UHF/SHF bands. Amplitude modulation is employed on 70 cm, where due to bandwidth constraints only monochrome pictures may be used. Frequency modulation with colour and a sound sub-carrier is employed on 23 cm and on microwaves. Ranges of typically 50 km are possible with greater distances during lifts. In many parts of the country repeaters operating in the 23 cm and 3 cm bands, or sometimes with input on one and output on the other band, have been installed at elevated locations. Unlike communications repeaters, mainly intended for use by mobiles, ATV repeaters provide better coverage and increased range for fixed stations. With all stations in a group beaming at the repeater, a picture transmitted by any one station may be seen by all the others.
It is anticipated that the digital compression and modulation techniques now used in broadcasting will be adapted to amateur use leading to conservation of bandwidth with improved range and picture quality. This article concentrates on fast scan.
Those not familiar with ATV may wonder what a QSO is like and what ATV operators get up to. It is usual to use one or more video cameras to take pictures, usually in colour, of the shack in which the operator may be seen speaking to his contact. Anything being discussed, perhaps a component, a piece of equipment or circuit diagram may be brought into close up. The only limits on content are those imposed by the licence. Amateur made video tapes, perhaps taken by the operator during a visit or on holiday are often transmitted, although commercially produced tapes or the re-transmission of broadcast programmes are not allowed. Some operators have a camera installed on their antenna that can be panned round by a rotator to give a panoramic view of the district. It is usual for the sound side of the QSO to be duplex, with the outgoing sound carried on a sub-carrier on the vision channel while return sound is on 2 m using the ATV talkback frequency of 144.750 MHz although other frequencies are used in some areas. Occasionally full duplex vision and sound is employed with the reverse circuit being on another band. As mentioned, in many areas operation is both direct station to station and via a repeater. Repeater groups often nominate specific times each week when stations try to be active and join in a net. A particularly good use of TV is when two or more operators are designing some piece of equipment or tracing a fault. One may instruct the other, via the talkback channel, to connect an oscilloscope or other test gear to a particular point while observing the result shown over the air, so analysing the problem.
So you think this sounds interesting and would like to have a go. How to go about it depends rather a lot on where you live. If you are fortunate to live in the coverage area of a repeater, see fig 1, the first thing is to listen on the talkback frequency, get some idea of local activity and make contact with some ATV operators. You will find them friendly and helpful and very willing to assist a newcomer to join their ranks.
Most repeater groups have coverage maps that will enable you to see if a signal is likely to be available at your QTH. However as these are usually generated by path prediction software backed up by some verification measurements, they can give a good general indication but should not be taken as the absolute truth. This is because they are based on topographic data and take no account of obstructions along the path such as buildings and trees that can add significant extra attenuation. If the map shows you to be in the coverage area and you have a good unobstructed view in the direction of the repeater you will probably be OK. Equally there are many cases in which he software suggests an inadequate signal but satisfactory results can be obtained. 23 cm repeaters generally have a better coverage than those on 3 cm. In cases of doubt many repeater groups have portable equipment and will be willing to visit you and carry out on site tests. It is far better to carry out preliminary investigations before making or buying equipment and installing it, which could lead to disappointment if you happen to be living in a black hole!
If you do not live in a repeater area all is not lost. There may well be activity on a direct station-to-station basis within range of you. Again listen on the talkback frequency and ask questions on the air to see what may be happening, are there any stations and on what band(s) are they active? If you find that there are possible contacts and you can get access to a computer with path prediction software, try entering your national grid reference and then theirs and see what the path looks like, whether it is obstructed or not and what path loss is calculated for the band in question. Local knowledge and observations will enable you to get a feel for whether there is likely to be significant extra attenuation due to obstructions. Knowing the estimated path loss you can calculate the probable picture quality you may receive as follows: On the 23 cm band take the transmitter power (in dBm), add the effective transmitting antenna gain (gain dBd - feeder loss) deduct the path loss, then add the effective receiving antenna gain (gain dBd -feeder loss). This gives the power at the receiver input in dBm. A typical ATV receiver with a front end noise figure of 1 dB and a bandwidth of 14 MHz will give a just resolvable picture (P1) with an input of about -99dBm. It will require an increase in signal of some 34 dB i.e. an input of about -65 dBm to give a top quality (P5) noise free signal.
The antenna you will require varies enormously with your circumstances. If you are fortunate to be say 10 km from a 23 cm repeater with an unobstructed path you may be able to receive P5 with a dipole literally only 22 cm long a metre or so off the ground. If you are further away and/or have an obstructed path you may require a panel array with 22 dBd or more of gain. Antennas may be either made or bought, designs for building a range of panel antennas have been published in CQ-TV (ref 1) while some Yagi arrays may be purchased from the Severnside ATV group (ref 2). Generally it is best to erect an antenna with as much gain as possible as high and in the clear as possible. If you only intend to operate via a repeater the antenna may be aligned and locked, if a range of contacts is possible a rotator will be necessary. Remember that high gain antennas at these frequencies are very directive; there must be some compromise over height as cable attenuation is high. For shorter runs UR 67 type cable may be used, but for longer runs it is preferable to employ better quality LDF4-50 with a flexible tail of UR67 if required for a rotator. If you have a weak signal and/ or are forced to use a long cable run you may consider a mast head pre-amplifier which will improve the signal/noise by the cable attenuation. Masthead pre-amplifiers however complicate things when it comes to transmitting.
Here again you have a buy/ make option the necessary boards to assemble a receiver may be bought from a number of suppliers these are listed in ref 3. Alternatively you may wish to make a receiver following one of the several designs that have been published. These are listed in ref 4. Although it is not an ideal solution, perhaps the quickest and easiest route is to adapt a satellite receiver indoor unit such as the Amstrad SRX200 that provided the i.f. amplification and demodulation in the satellite system. The i.f. tuning covers the 23 cm band. Units of this sort are available at rallies for a few pounds. If using a satellite unit several points need to be remembered. 1. These units output D.C. on their r.f. input sockets. This was used to feed the LNB on the dish. If you do not wish to use it to drive a preamplifier it must be disconnected otherwise the antenna would short it out and damage the power supply. 2. An indoor unit alone does not have enough gain for ATV reception except with an extremely strong signal. You will need a low noise preamplifier with a noise figure around 1 dB and gain of about 40 dB. Again these can either be made or bought. (Ref 5). 3. While it is possible to tune a satellite receiver to the ATV channel using its tuning push buttons, it is better to disconnect the original tuning arrangement and substitute a simple potentiometer tuning control. It is also necessary for 6.0 MHz sound sub-carrier demodulation to be selected. Details of these aspects are given in ref 6. 4. As the deviation used for satellite transmissions is about twice that standardised for ATV, the unit?s i.f. bandwidth will be too wide. This leads to some reduction in signal/noise and less immunity to adjacent channel interference i.e. radar or amateur communication stations. The level of the demodulated video will also be low but this can usually be accommodated by the monitor contrast control. Having made these points, many stations use satellite receivers and they are certainly a good way to get started.
You may either use a video monitor, such as those supplied for use with older computers which outputted standard 625 line video, or a television set equipped with a SCART connector to which composite video and sound from your receiver are connected. Have a dedicated monitor; don?t be tempted to use the domestic telly it will lead to strife!
With the above items plus your 2 metre transceiver you are now equipped to join the fun with ATV at least as a receiving station. After gaining some experience you may wish progress to transmitting. How to do this will be the subject of a future article.