In strong signal areas (northern half of Pinellas County, west Pasco County and northwest Hillsborough County), it is possible to receive the ATV repeater by using a cable-ready television or VCR or a cable converter box with a standard TV or VCR. Tune to cable channel 57 (i.e. standard cable not broadcast mode). Then disconnect the cable from the back of the television, VCR or cable box and connect a UHF antenna. If you are using a UHF TV antenna, you must rotate it so the elements are vertical. If you don't have a UHF antenna, try a VHF antenna or a 5 to 10 foot length of wire. A preamplifier may be necessary to improve reception. Keep the transmission line short. If you must use small diameter coax, RG-6 has lower losses than RG-59. Do not use 300 ohm twin lead for the transmission line because it is more susceptable to interference. When you are using an antenna and tuned to CABLE CHANNEL 57 you receive the 421.25 MHz. repeater output. (This is because cable channels are not bound by other services and simply transmit their signals on the cable one channel after another). A cable converter box downconverts the specified numbered channel to channel 2, 3 or 4. Give it a try!
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 televsions. These little units had enough overlap at the low end to even cover down to the ATV repeater frequency of 421.25 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 ohm twin lead from the output terminal to the TV VHF antenna terminal, (75 ohm RG-6 can be used but you will need 75/300 ohm transformers on both ends because coaxial cable was so infrequently used for that application.)
Later, Science Workshops out of Bethpage, NY marketed a wallet-sized surplus varactor diode tuner that was voltage-tuned and a better down-converter. The output was connected to the 45 MHz IF input on any standard television. The disadvantage was the critical tuning required, a precision vernier potentiometer excluded all but the lowest UHF channels. One also had to be careful to use an isolation transformer between the TV and AC line to avoid the shock hazard caused by the "hot" chassis. 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. The cost for the varacter diode tuner was $5.00 each. It can be packaged for antenna mounting with a transmit/receive relay. A power switch in the shack controls the relay. The 3-18VDC tuning voltages can be fed up the transmission line using blocking capacitors at both ends. A precision potentiometer is used for tuning.
The older VCRs with the rotary channel switch and thumbwheel fine tuning may be able to receive the ATV repeater.
There are commercial ATV downconverters available. These are particularly popular because they have tight tuning and a pre-amplifier.
The antenna and transmission line is the most important part of an ATV system because it affects both transmission and reception. ATV veterans recommend investing half of your ATV budget into the antenna system. Vertical polarization is used in the ATV repeater. This provides some automatic rejection of the horizontal broadcast UHF TV transmissions in the area but is more susceptible to noise from amateur and commercial transmissions that are also vertical. Omnidirectional antennas are rarely used for ATV, except at the repeater site. The very high frequency and weak signals make high gain directional antennas necessary. Antenna quality and placement are most important. Top quality ATV antennas are available from commercial sources or are described in adequate detail that they can be home-built. Install it as high as possible. Yagi antennas that are sold for use with FM do not have enough bandwidth for ATV work. They tend to exhibit higher VSWRs at the edges of the ATV channel. The extra reflections can cause phase distortion which changes colors. Compared to FM voice an ATV receiver's sensitivity is reduced by 26 dB due to the increase in bandwidth.
The most popular commercial antenna for ATV is the Rutland Arrays 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. 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. Other excellent antennas include the KLM 440-16X and the M-Squared 440-18 and 432-9WL.
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. Six feet of boom at 900 MHz is suggested while shorter or longer boom lengths are acceptable to meet turning radius or additional gain requirements. A boom that is too short will yield lower gain and it will be more difficult to line up the elements on longer booms. Thick wire or tubing for elements will result in a wider bandwidth for the antenna which can also result in more weight. WA3ZQI 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. Keep in mind that if the loops are too thin, then they may be flattened by a tired bird looking for a place to rest is wings.
A rotor will allow you to use your antenna for simplex when not using the repeater.
Amateur television is weak-signal work. Transmitters typically put out 10 watts or less that is shared amongst the 6 MHz bandwidth of AM video. This wide bandwidth leaves plenty of room for extraneous signals to adversely affect one's tranmitted 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. They maintain a constant impedance across the connection and are waterproof. 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.
For receive, where a long feedline run is expected, consider an antenna-mounted preamplifier or a remote downconverter. Transmission line losses are much less at the IF of 45 MHz or channel 3.
For transmit, where a long feedline run is expected, consider an antenna-mounted exciter and power amplifier. You can then use inexpensive 75 ohm RG-6 for your video to the remote transmitter.
Start with Belden 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 WA3ZQI for the BRATS Milliwatt--various Summer 1995 issues with technical credit by various folks especially Bob W3WCQ and Heru W3WVV.
Thanks to the Baltimore Radio Amateur Television Society for use of this material.
Return to the TBATS ATV Page.
Last update: 15 July 1997