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Last modified 2024-05-27
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Back to main Last modified 2024-05-27 |
ATV (Amateur TeleVision) in Stockholm
EquipmentMost of our equipment consists of Comtech RX and TX modules, which are pretty cheap to buy and easy to assemble. Since the TXhas 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 someinterference 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 powerful 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, thentry calling any of us on 144.750 MHz FM (ATV Talkback) or on the R6x repeater. 2.4 GHz (13 cm)SM0VPJ and I are QRV on this band with max 100 mW input to the antenna system. Sadly, this power limit was invoked by the swedish authoritiesin the nineties (or possibly the late eighties). This of course means a limitation in the general range, but with line-of-sight and decent antennas, the range can be quite usable. 5.7 GHz (6 cm)Since april 2016, I (SM0OFV) am also QRV on the 6 cm band (5.7 GHz). The equipment consists of transmitter and receiver units built forR/C models, mainly drones, helicopters and planes. This type of equipment is very cheap and easy to use. Among my 5.7 Gig stuff is a 1.5 W transmitter and a diversity receiver. For the moment I use these while out and about i.e. "hilltopping". If future circumstances allow, I will also install fixed gear at home. As of july 2019, SM0VPJ is also QRV on this exciting band. 10 GHz (3 cm)In the spring of 2020, SM0VPJ and I performed local tests of modified, existing electronics for use in this band. Picture quality was found to beexcellent. Range tests and 2-way QSO are on our agenda for the summer of 2021. We will however not be able to have this equipment fully ready for the 2020 IARU Region 1 ATV Contest. 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 . Click to enlarge...
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