With more and more amateurs using the 70, 33 and 23 cm bands here due to the code free Tech license and the loss of 2 MHz of the 220 MHz band, it is important for every ATVer to know and understand the unique practices that have been developed to allow all of us to enjoy our segment of the hobby.
The ATV frequencies are coordinated by SCRRBA (So. Calif Repeater and Remote Base Assn.) and were determined to be the best technical fit given all the users of each band. SCRRBA hosts band plan meetings with the best technical representatives from each mode of interest. Band plans are only adopted when all representatives agree. I have been a member of the SCRRBA Technical Committee for many years and am the primary coordinator for the 23 cm band and ATV.
On the 70 cm band, there are over 400 ATV transmitters in the area that could possibly come on at the same time on either of the 2 available channels. Only two channels are available in any given area for ATV given the bandwidth. For many years the interference possibilities between ATVers as well as other band users have been minimized by the use of a 2 meter FM simplex coordination frequency and a cooperative attitude. Therefore it is extremely important that anyone transmitting on 434.0 ATV monitor and announce their presence on 146.430 and those on 426.25 use 144.345. The frequencies were chosen for non 3rd harmonic interference. 439.500 simplex is also coordinated for ATV intercom use, or coordinating ATV transmissions at 33 cm and higher. 439.5 would capture 434 video reception and to a lesser extent 426.25.
434.0 ATV uses vertical polarization to minimize interference to 432 weak signal users who are horizontal. However since ATV sideband power density is typically more than 30 dBc down 1 MHz each side of the video carrier (see ARRL Handbook pg 20-3 Fig 6B), probability of interference to weak signal or satellite users is very low depending on distance, power and video gain setting.
Tests were made with a satellite prototype flight in the 70's to prove the compatibility. However ATVers must accept occasional interference from satellite users uplinking 435-438 MHz within our passband when ever a bird is in view. We have asked them to use 432 for their terrestrial contacts rather than stay on 435. Also we must accept the Navy ship radar that puts the horizontal lines in the pictures.
Other mode users can work out local interference problems on a case by case basis with ATVers on the TASMA coordinated channels of 146.430 or 144.345. Conversely satellite users use 144.144 USB and weak signal people use 144.170 USB to coordinate.
All available FM repeater and link channels in the band have been full for many years now. It is very important that ATV transmitters observe the 25 kHz frequency tolerance to prevent interference to the FM hilltop receivers. FM coordinations allow +/- 200 kHz protection for ATV sound and color sidebands around our two channels. Given the sound deviation, color spectrum energy, and 25 kHz frequency tolerance, 200 kHz just makes it.
Your ATV transmitter frequency should be checked with a counter periodically along with the sound subcarrier. Make the video carrier frequency check with no video plugged in, the AM modulation can make the counter reading erratic. The oscillator trimmer does not directly rubber the frequency but will change it slightly. It should be peaked at the emitter of the first doubler with at dc Voltmeter rather than for a frequency. The crystal tolerance is +/- 22 kHz at the output frequency.
The 4.500 MHz sound subcarrier should be within 2 kHz after warm up of 1 minute. No audio or video should be plugged in for this measurement. The counter should use a high impedance probe on the top of the sound injection pot. Carefully turn the slug in the sound VCO with an insulated tuning tool if necessary.
You can also cause interference from over modulation. First make sure you have set the pedestal pot in the transmitter for 60% of max power out of the last amplifier with no video connected. This sets the proper video to sync ratio at it's constant value. Then plug in the video source and set the video gain control to the point just before white saturation. This should be done with something in the scene that is well lit and white against a darker back ground. You have cranked it up too far if the white area seems to smear into the darker areas. Also the sync buzz in the audio will increase as you go into white limiting or cut off while splattering the band.
You may also have some sync buzz if the sync tip is over driving the amp into saturation. Drive reduction should never be done by detuning the transmitter. That could cause instabilities and degrade out of band harmonics. RF peaking is always done with a dc Voltmeter on the emitter of the stage following the trimmers you are tuning. Also the pedestal pot is set to full output and no video is connected while peaking.
Maximum legal power is 50 Watts P.E.P. The blanking pedestal pot is not to be thought as a drive adjustment since it does not have any affect on peak sync power. There is a RF level pot on the end of the TXA5-70 exciter board in the TC70-10 Transceiver or on other transmiters, a piece of small 50 Ohm coax, such as RG174, as an attenuator (16 ft = 3 dB) can be used. Do not reduce applied voltage below 12.0 Volts.
Note that a Bird Wattmeter reading is only valid for setting the sync and blanking power pedistal pot values. It does not read true average power of the complex video modulated RF. The average meter response is only valid for sine waves up to 50 kHz, and has no repeatable or comparable value under video modulation.
ATV is double sideband due to it's low cost and no affect on performance with a standard TV set. VSB, reduced power, traps, etc., however may be required on a case by case basis of interference to users on the lower sideband below 1.25 MHz. To date, I know of only a few who have had to do much past proper set up and alignment of their gear. Each case is different with its own value of necessary rejection dependent upon distance, antenna gain, power and actual sideband energy. So a set value of LSB sound and color dB down from the video carrier peak envelope power is meaningless in the real world.
Vestigial Sideband
A VSB filter attenuates up to 5% of the LSB transmitted power, namely the color and sound subcarrier, which is not used by the TV receiver. VSB commercial specs say -60 dBc for the LSB color and sound subcarrier, but this would mean a very expensive filter in the antenna line along with it's insertion loss. ATV transmitters output the sound at up to -15 dBc and the color at -22 dBc. A DCI 435-6 VSB filter in the antenna line will cut that down another 30 to 40 dB if necessary for $299 and are available through P. C. Electronics. Insertion loss is only about 1 dB. Except for very expensive commercial amplifiers that are truely linear, the LSB is just reinserted due to the amplifiers intermod when VSB is generated at low level. A filter is necessary on the output of any commercially made amateur amp in the antenna line to get VSB. 434.0
This is the most popular channel. It is the common input for all but the Palomar repeater, and some simplex. So anyone transmitting on this frequency will probably key up one or more repeaters regardless of power or beam heading. The best way to know if you are interfering with another existing transmission using simplex or one of the repeaters you may or may not be able to receive, is to use 146.43. An omni antenna is suggested for 2 meters rather than a beam for this reason. Most of the ATV repeaters also repeat 146.43 on the sound subcarrier output mixed with the sound subcarrier input so that those not in 2 meter simplex range can still communicate.
Simplex can also be run on 434.0 since we only have two channels available, but 426.25 simplex is encouraged for all local, point to point, R/C, emergency services groups, long contacts, or any thing that might tie up the repeaters significantly enough to prevent others from using the frequency.
If someone has been on for a while with no obvious special activity going on, and you want to transmit, then 146.43 is the method be used to contact that person and to courteously ask for your turn or better yet, join in. There is nothing wrong with long transmissions, we need to demonstrate usage of what we have, but be prepared to share some time with others when reasonably requested. Remember to ID every 10 minutes and at the end, preferably with a call card in the video. 426.25
This frequency is strictly simplex and shared with FM voice links. Both antenna polarities are used. Some repeaters have capability of switching to this as an input for special events like a remote base, but must accept the interference and overload from the strong link transmitters on the same or adjacent hilltops. Point to point FM links are not placed within 1 MHz of the video carrier and 200 kHz of the color and sound subcarriers.
Rag chewers (viewers), RACES, emergency services groups, Radio Control models, special events and any activity not requiring a dedicated repeater are encouraged to use this channel and coordinate on 144.34 fm simplex. This is especially important for Emergency operations - helicopter, portable, or mobile remote damage accessment, hazardous materials and preparedness drills - to coordinate on 144.345 while on 426.25 ATV since in a real emergency, there could be many groups trying to get on at the same time. 33 cm
Due to Teletrac's Automatic Vehicle Monitoring coming on in the area, the band has been changed here to 913.25 simplex and 919.25 repeater output or input. This could be eliminated or changed with entry of more licensed users from other services that have priority over amateur. I do not recommend investing in new transmitters for this band, but a downconverter for receiving the 2 repeaters or for better receiving the new part 15 no license required TV transmitters might be considered.
The new part 15 wireless TV broadcasters are legal if they have a FCC ID number on them and the fixed antenna is used and has not been altered. Hams can of course alter them and use a beam to increase the range past the 100 ft range (FCC limit is 50,000 microvolts per meter of antenna measured at 3 meter distance). If a part 15 user interferes with amateurs, the part 15 user must shut down. If the amateur interferes with the part 15 neighbor the neighbor has to tolerate it. Tell that to the 300 lb ex-linebacker next door who just shelled out $100. Good luck! 23 cm
8 ATV repeaters output on this band. Simplex on 1265 is shared with satellite and wideband experimental. 1289.25 is also simplex and shared with wideband experimental. 144.345 or 439.500 should be used if running simplex only on 1265 or 1289.25. Novices are allowed simplex operation on 1289.25 MHz.
1289.25 is also used by public service and emergency groups as an output for special event or emergency repeaters. These non-dedicated temporary (operation not to exceed the length of the event) portable repeaters operate on a non-interference basis with 426.25 as an input typically. If a portable repeater is being used, then the 2 meter frequency that corresponds to the 70 cm channel is used.
Some of the ATV repeaters are also linked. Eventual plans of the ATN (Amateur Television Network) are to cover from Las Vegas to San Diego and Santa Barbara.
FM ATV is only on 1265.0 simplex and must be non hill top and no interference to FM voice repeater inputs at 1270 MHz. This segment is the only one wide enough to accept the occupied bandwidth with some limitations. To stay within the 10 MHz slot, maximum sync deviation must not exceed 4 MHz and no sound subcarrier injected - use two meters or 439.5 MHz for voice instead. 13 cm and up.
These frequencies must also be coordinated through SCRRBA. 2417 is for FM ATV links and 2441 MHz for simplex and repeater input. 10.250 gHz is the design center for Gunnplexer and DRO type ATV systems and 10.400 for repeater input.
These standards and practices were developed on a technical basis rather than by a vocal few or large group. The band plans are reviewed periodically by the SCRRBA technical committee. If you have any questions or discussion about coordination or practices contact me on one of the nets or call on the telephone: 626- 4474565 Mon-Thu 8 am to 5:30 pm or email [email protected] 73, Tom O'Hara, W6ORG ©2/99
Reprinted by permission of W6ORG
copyright ©1996, 1999 W6ORG all right reserved.
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Changes last made on: Sunday Feb 21, 1999
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