Quick And Easy Way To Receive ATV
In strong signal areas (Northeast Baltimore), it is possible to receive the BRATS ATV repeater by using a cable-ready television. To do so, tune to cable channel 60 (i.e. Cable mode not Network mode). Then disconnect the cable from the back of the television and connect the TV to a UHF antenna. If you don’t have a UHF antenna, use any VHF antenna or at the worst, use a 5 to 10 foot length of wire. When you are using an antenna but set to CABLE CHANNEL 60 you pick up the 439.250 Mhz. repeater output. (This is because cable channels are not bound by other services and simply transmit their signals on the wire one channel after another.) The cable converter picks off the specified numbered channel and outputs the signal to channel 3 or 4. If you live in Northeast Baltimore, and have not tried this yet, shame on you! It is a real treat to watch the ATV net on Thursday nights at 9:00 pm. or the Answer Men net on Saturday at 1:20 pm. where Heru W3WVV can answer questions using a white board and some times provide live show-and-tell of some new antenna or other interesting device.
Outside of the normal nets or other activity, you can also monitor the evening ATV beacon at 7:45 pm to 8:00 pm.(EST) (8:45 pm to 9:00 pm EDT)
More On ATV Receiving
The low edge of UHF channel 14 is 470 Mhz. which is about 6 channels above the ATV repeater. If you could just twist the TV tuning knob counter-clockwise hard enough, you would be there! That’s really not far fetched because in the 1970’s we used Blonder-Tongue tuners which were originally designed to allow reception of UHF channels by VHF-only televisions. These little units had enough overlap at the low end to even cover down to the BRATS ATV repeater frequency of 439.250 MHZ. The most sought after BHT-111 even had a pre-amplifier built in. (Any Blonder-Tongue unit can be used if you are in a good signal area.) You can still find comparable units at hamfest for a couple of dollars. These use 300 twin lead from the output terminal to the TV VHF antenna terminal. (75 ohm coax can be used but you will need 75/300 ohm transformers on both ends because coax was so infrequently used for that application.) You should use a 75/300 ohm transformer and coax to the antenna because of the weaker strength of ATV signals and the fact that twin lead more easily picks up interference.
Later, Science Workshops out of Bethpage, NY marketed a wallet-sized surplus varactor diode tuner that when voltage-tuned made for a hotter down-converter. The output went into the 45 MHZ. IF jack on any standard television. The downside was that in order to provide precise tuning, a tight vernier potentiometer excluded all but the bottom-most UHF channels. One also had to be careful to use an isolation transformer between the TV and AC power to avoid making a hot microphone or other shock hazard. One can imagine the surprise when tongue or wet lips meet 117VAC from the metal microphone screen during an ATV QSO. Some hams still have these and have adapted them to other uses. According to Science Workshops, the tuning range was 50 MHZ. to 1 GHZ. (We didn’t have 911 MHZ. ATV then to find out how high they would really go for TV.) A back issue of the BRATS Milliwatt has plans for a spectrum analyzer based on one. The cost for the varacter diode tuner was $5.00 each. It could be packaged for antenna mounting since the only controls were a power OFF/ON switch and a potentiometer to control the tuning. A source for the 3-18VDC tuning voltage up to the antenna and the new necessity for an antenna-mounted transmit/receive relay would be the only real complications to resolve.
Another technique is to use a thumb-wheel tuned VCR to tune down to the ATV frequency at 70 cm. A unit like that would probably be at least 5 years old. (Digital tuner VCRs don’t usually give this flexibility.) It is possible that the thumb-wheel VCRs can also be tuned up to 911 MHZ. Remember that sensitivity falls off on the high end, results might be poor. In either direction, the tuning is not precise…there are easier ways to do the job.
Yes, there are commercial ATV down converters available. These are particularly popular because they have tight tuning and a pre-amplifier. If you are purchasing an ATV down converter, you might consider 911 MHZ. or whatever alternate output frequency your ATV repeater supports. This gives the advantage of seeing your ATV signal come back from the repeater (assuming you transmit in another band) instead of relying on a transmission line sampler.
ATV Antennas
The antenna is the most important part of an ATV system. ATV veterans in Baltimore advise putting half of one’s ATV investment into the antenna system. Vertical polarization is used in the BRATS ATV repeaters. One reason is that this gives some automatic rejection of the horizontal broadcast UHF TV transmissions in the area. Omni-directional antennas are rarely used for ATV, except at the repeater site since the combination of very high frequency and low strength signals make directional antennas a necessity. As in other areas of amateur radio, antenna quality and placement are among the most important success factors. A rule of thumb is that 450 Mhz. antennas should be up 40 feet or more, however, at a minimum, optimize the ATV antenna over other antennas if you must make a choice. If you can’t do 40 feet, just do what you can. Fortunately, top quality ATV antennas are available from commercial sources or are described in adequate detail that they can be home-built.
The most popular commercial antenna in Baltimore for ATV is the Rutland FO22-ATV antenna. The boom is 14 feet long yet the antenna weighs about 5 pounds. At 15.8 dB gain, it handles the maximum legal power and costs under $150. The 48 element J-Beam Multi-beam imported by Spectrum International had been very popular prior to the FO22. This short 6 foot boom yagi claims 15.7 DB gain and allows vertical polarization mounting. However the better gain 88 element version tends to droop when mounted for vertical polarization unless some extra mounting effort is expended. Ernie WB3DVL reported that in England, the Multi-beam is used extensively for UHF broadcast reception. Instead of omni-directional or broad patterned ones, those folks often use separate multi-beams for each channel desired. Coax must be soldered to a terminal on the antenna as opposed to some of the other brands which include a female N-connector. The antenna is harder to find now but still is popular.
Building antennas for 450, 900, and 1200 is actually rather practical. A good source of information for a 902 Mhz. loop yagi is the November 1985 QST magazine. The loop yagi has very good gain, is forgiving in construction details, and allows boom length flexibility. 6 feet of boom at 900 Mhz. is suggested while shorter or longer boom lengths are acceptable to meet turning radius requirements or additional gain requirements. The risks are that too short a boom will result in lower gain while longer booms will be more difficult to line up the elements. Thicker wire or tubing for elements will result in a wider bandwidth for the antenna which can also result in more weight. W3ZQI built his out of a 4 foot piece of copper water pipe with #10 wire loops. WA3DVL built a 6 foot version using aluminum siding loops that was very lightweight for rotor requirements…watch out that too thin loops might not be heavy-bird proof!
A dish antenna is not much value at 450 or 900 Mhz. unless 8 feet in diameter or more. On 1200 Mhz., use of a dish is more promising. Regarding a dish, one might wonder if a dish is cut for a specific frequency? Actually a dish is a usable dish if it is a parabola (the way they are supposed to be made). Any determination of frequency is done through placement of the horn in the middle of the dish. Change of signal direction can be done either by moving the entire dish structure or just moving the horn.
Some folks still use rotors for their ATV antennas but the trend in Baltimore MD is been toward installing ATV antennas in fixed position, aimed at the ATV repeater. (One less thing to buy and have go wrong!)
Feedlines And Connectors
ATV is best treated as weak-signal work. ATV stations typically have transmit power of 10 watts or less and a rather wide (several megabytes) AM video bandwidth. The wide bandwidth leaves plenty of room for extraneous signals to adversely affect one’s transmitted signal or output from the ATV repeater.
It is recommended by many experts that only N connectors or better be used for transmit and receive on ATV. N connectors maintain a constant impedance across the connection. Perhaps the most important reason is that N connectors are also waterproof which would be particularly important on outside installations. The difference in cost is not worth the amount of time one would spend later to upgrade from PL-259s to N connectors. Note, many of the ATV antennas and down-converters are shipped with female N connectors. (An interesting side note, there have been a number of instances of commercial equipment for close to 1 GHZ. yet using RCA phono plugs and PL-259s…I’ll stick with the N-connectors.)
For receive, where a long feedline run is expected, consider a remote pre-amplifier or a remote down-converter. Receiving 45 MHZ. or channel 3 at the ham shack is less demanding of your feedline than 450 MHZ. or 911 MHZ. I prefer the simplicity of providing power to a remote pre-amplifier by a separate set of wires while others say “what can be simpler than having the necessary 12-24VDC share the feedline?”. There are good arguments for both approaches.
For transmit where a long feedline run is expected, consider a remote exciter and power amplifier. You can then use inexpensive 75 ohm RG59 for your video up to a remote transmitter.
Old RG58 is one of the cheapest, and most convenient dummy loads in an ATV station. A watt or so at 450 MHZ. into a good run of old RG58 is almost undetectable at the other end. At 420 MHZ., new RG58 has 10.4 DB attenuation per 100 feet. A 15 foot drop of not-new RG58 at 1289 MHZ. provided Bob W3WCQ (SK) with the 6 DB (4 to 1) drop needed in a new application of his. Remember too, the power handling capabilities of coax drops sharply as frequency is increased. RG8 solid-dielectric coax is capable of handling 2000 watts at 20 MHZ. but is only good for 680 at 20 MHZ. The point here is how important good quality, new feedline is to the intended transmit or receive signal. (Overall, ATV and RG58 don’t play well together.)
While discussing feedline, here is the W3WCQ method for testing coaxial cable. For the desired frequency (regardless of the age of the coax), test the standing wave ration (SWR) at the input end of an un-terminated run of coax. If the resulting SWR is greater than 7 to 1, then the coax is okay. If the resulting SWR is less than 7 to 1, then do not use it at that frequency. A “do not use” result for a given frequency might allow use at a lower frequency.
Start with 9913 coax or better and keep whatever you use as short as possible. Note, by saying 9913 or better, the point is “minimum loss quality equivalent to 9913 coax”. In reality, 9913 is okay for indoor or protected use but not for outdoor use because it weathers poorly.
The above material is a summarization of articles written by Neil W3ZQI (ex-WA3ZQI) for the BRATS Milliwatt–various Summer 1995 issues with technical credit by various folks especially Bob W3WCQ (SK) and Heru W3WVV.
