Back to main
Last modified 2016-01-20
ATV (Amateur TeleVision) in Stockholm
There are a couple of other hams in different stages of getting QRV as well, plus a few others unrelated to us, whose whereabouts are unknown.
EquipmentMost of our equipment consists of Comtech RX and TX modules, which are pretty cheap to buy and easy to assemble. Since the TX
has a measly 50 mW output, we also have separate power amplifiers to raise the output to something between 15-20 W. We all use
multi-element yagis with at least 15 dBd gain, which has proven to be quite sufficient for simplex contacts between the members of
our little group.
Care must be taken when planning and installing the antenna system though. Keep in mind that this is wideband communication in our
lowest microwave band, and you must use the best feeder and connectors possible for your installation. If you fail just a little bit in this
department, then you'll probably not see much, and maybe no one can see you.
See also this area for some usable hardware.
Both video and audio is FM-modulated, just like analog satellite TV transmissions. In fact, an old analog satellite receiver may be the very
first step into receiving ATV-signals in the 23 cm band. It should however be noted that these units are fairly deaf as they are intended to
be connected to a powerful LNB in a satellite dish. A good preamp is therefore needed unless you're watching fairly strong signals.
Much of the early ATV-equipment for 23 cm used AM-modulated video, but the more modern FM-video means higher video quality.
InterferenceSome time around the winter 2014-2015, SM0WLL started having big trouble receiving my transmissions. I myself experienced some
interference on his too, but not as serious. Well, after some investigation it stood clear that the problem was the ever expansion of cellphone
base stations. To clarify, the Comtech receiver modules are broadband devices, capable of receiving from below 900 up to over 2000 MHz.
The 23 cm band covers 1240 to 1300 MHz in most countries. Now, if you have a strong cellphone base station nearby, transmitting on, say
900 or 1800 MHz, the risk for your receiver being seriously affected or even totally blocked is very high.
So, what can we do about this? Well, a bandpass filter tuned for the 23 cm band will do the trick for sure. When inserted before the receiver at
SM0WLL's site, my picture when from barely visible through the interference to full "studio" P5 quality! Such filters can be homebrewed or
purchased from online vendors. A good ready-built filter from Germany will set you back about 50 to 55 Euro. This is money very well spent.
Time and frequencyFor simplex contacts, we use 1255 MHz with horizontal polarization. We are mostly QRV late in the evenings. If you want a sked on ATV, then
try calling any of us on 144.750 MHz FM (ATV Talkback) or on the R6x repeater.
Reporting systemThe usual S-report system does not apply. Instead, the P-system (for picture quality) is used.
PicturesPlease note that the off-air photos does not do reality any justice.
It's difficult to take good pictures of a flickering TV screen .