Beginners Guide to Amateur Television (ATV)
This article is intended as a basic guide for getting started in ATV, it is also worth having a chat with one of the growing numbers of ATV enthusiasts within MKARS.
What can I receive?
The three main bands used in the UK are 23cm (1.3GHz), 13cm (2.4Ghz) and (3cm) 10GHz we are lucky enough to be served by all three in the MK area. The repeaters are as follows:
GB3TG 3cm repeater sited in Brick hill
GB3TV 23cm sited in Dunstable
GB3MV 23cm sited in Northampton
What ATV band would suit me?
It depends on your location, ideally you need line of sight to the repeater for maximum signal. The first and cheapest check to determine if your location is suited is by using one of the computer plotting programs available on the internet, in their simplest form they show a cross-section of the land and in some cases can also calculate signal loss. Such plots can only be used as a guide as unfortunately, trees and buildings not shown could stifle the signal. In other cases paths that are blocked by obstructions may well receive signals reflected off buildings or trees. It may take some time to familiarise yourself with the use of plotting software so why not ask around the club for some guidance on this?
Each band has its own advantages and disadvantages, 10Ghz is quick and easy to get started using modified satellite equipment but it is line of sight only although signals can be reflected or "bounced" off buildings. 2.4GHz is also line of sight but its attenuation in free air is not so great and power amps for 2.4GHz are cheaper then 10GHz. The most useful frequency for built up areas is 23cm as it has less attenuation in free air and can pass through objects (but attenuation will be high). The final advantage is "fringing" which is the ability of the signal to bend around objects and the horizon to a limited degree.
When testing always try the aerial exactly where it is to be fitted as a difference of a few feet in any direction (even up or down) can have an enormous effect on signal levels.
|▲ The MKARS ATV station constructed by Calvin (M1EPM) and Steve (G6ALU)||▲ GB3FV at Wisbech received by Calvin (M1EPM) in Milton Keynes during lift conditions|
What equipment do I need?
Equipment used will vary depending on the band required and how strong the signal is in your area; the following is a guide for the average set-up.
A complete receiver can be purchased from www.13cm.co.uk or If you prefer you can modify an old analogue satellite receiver. Some of these can be a bit on the deaf side but experiments done by club members show the Amstrad SRX200 and 400 boxes seem to have a fair amount of gain. As signals for most people will be very weak a pre amp will be needed (mounted at the mast head), not only will this amplify the signal but it will also have a good output drive that will overcome coax loss (remember at these frequencies coax cable can be quite lossy). This can be obtained from Bob Platts or the RSGB Microwave Committee. There are a few ready made aerials available but for an area of average signal level one can be home brewed as seen in the picture, this was made by James M0BZF.
Here again a complete receiver can be obtained from www.13cm.co.uk an aerial from Masthead Antennas or Sandpiper and a pre amp from the RSGB Microwave Committee.
Here an L.N.B. (Low noise block) as used in your satellite dish (but with the local oscillator retuned) is used with or without a dish depending on signal level. To down convert the 10GHz signal to 1.3GHz (23cm), so this can be run into a satellite receiver. The output level of an L.N.B. is quite high so a pre amp is not required.
23cm and 13cm transmit:
www.13cms.co.uk has available ready made units for both these frequencies but they have low output power. You will almost certainly need a power amplifier, a range of which are available from G.H. engineering. Remember to keep the coax run from TX / power amp to aerial as short as possible as it is easy to lose half your signal or more.
|▲ Homebrew 23cm portable setup by Calvin (M1EPM/P)||▲ A close up of the display|
This normally consists of a microwave oscillator (often a gunn diode in a tuned cavity, such as used in alarm movement detectors) beaming out a signal, a horn fitted to the front of this will greatly increase the gain. To modulate this 10GHZ signal the voltage to the unit is modulated with the video and sound signals superimposed on it, a kit supplied by Bob Platts or RSGB Microwave Committee can be used for this
Camera and monitor:
A complete set up Tx and RX should cost you no more than a normal rig. Obviously you will also need to have a TV with a video input and some sort of camera, an old camcorder would be best but you can get colour cameras for as little as £20 from rallies.
What about coax feeder?
RG58 or RG213 are unsuitable as they are lossy at ATV frequencies - the coax to use would be H 1 00, Westflex W 1 03 or better
What about plugs and sockets?
Because of the high frequencies involved high quality low loss plug and sockets are required normally SMA and N type. These tend to be on the pricey side if purchased new but second hand ones on patch leads can very often be found at rallies cheaply.
ATV protocol, how do you make a QSO?
In a QSO the party sending the picture sends his sound on a sub carrier so his sound will be produced from the television or monitor. If you transmit ATV the mode for the log is F3F. The person receiving the picture "talks back" via normal radio usually on 2M 144.750MHz.
Getting up and running and then to squeeze the maximum signal out of the set up is enjoyable, rewarding and at times challenging. ATV QSO's are friendly and informal. Who was it that said, "A picture is worth a thousand words"?
Where do I get ATV equipment from?
Below is a list of suppliers and their web sites, they are some of the more popular ones but its also worth doing your own search on the internet, check the rallies, and ask around the club for recommendations.
Steve (G6ALU) rx and tx controller, information on satellite receiver modifications
Antennas 13cm and 23cm
Masthead antennas (13cm and 23cm aerials)
Westlake (coax and connectors)
RSGB Microwave committee (13& 23cm pre amps no filter fitted)
B.A.T.C. (British amateur television club)
Thanks to Calvin (M1EPM) for contributing this article.
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