434.0 and 439.25 MHz ATV
Technical Band Plan Considerations

As more and more FM voice repeaters come on the air in any given area below 444 MHz, ATVers get more interference and consider moving down the band. Cross band repeat with 426.25 input and 33 or 23 cm output is a good alternative if most have good line of sight and close enough to the repeater site. However the 70 cm band still goes much farther than the higher bands given the same power and antenna gains - 900 MHz is 1/2 and 1200 band is 1/3 - not to mention the higher coax loss and cost of equipment. The optimum ATV repeater is still 439.25 or 434.0 input and 421.25 MHz output for widest coverage and least cost to the users.

Forget the ARRL 70 cm band plan, it was never a technical band plan but was adopted as such by the ARRL Directors back in 1979 after a survey by the VHF / UHF Advisory Committee of what some people happened to be using at that time. It has never stood technical review since then. There are so many different local band plans now, I doubt one national band plan could be agreed upon and impossible to change to. One of the glaring errors is the overlap of the 439.25 MHz ATV passband to 444.0 MHz and FM voice repeaters down to 442 MHz. This has been a big source of friction between ATVers and FM frequency coordinators. Some local band plans have been devised solely by FM voice repeater coordinators with little or no consideration or contact with other modes.

I suggest each area have a band plan meeting with 1 or 2 (no more) of the best technically qualified representatives from each amateur mode to work out an agreed upon technical band plan as we did here under SCRRBA (So. Calif. frequency coordination group) sponsorship for the 70cm band in the 1970's. SCRRBA does repeater and link coordination as well as maintains the band plans but the band plans are engineered and determined by consensus of a technical committee representing weak signal, satellite, digital, experimental as well as FM voice and ATV. Unless each mode has an equal vote and the band plan can only be adopted unanamously, it will end up unfair and fail no matter how well intentioned a frequency coordination council may be.

Using 434.0 or 439.25 lvsb for ATV can be a win win situation for FM repeaters as well as ATVers. Both will have room to operate with minimal interference. FM repeaters or what ever modes the local band plan works out can go down to 441.0 if 439.25 is used for ATV or 438.7 if 434.0 is used. ATVers do have to change the local oscillator to the high side in their downconverters to receive the lower vestigial sideband. The A line people really only have 439.25 or must forgo transmitting the normal sound subcarrier if on 434.0. Asking individual ATVers to go to the expense of a VSB filter ($400+) is not practical as it limits them to simplex or one frequency.

We here (SCRRBA) are satisfied with the legal reasoning we have given to use either of these two ATV video carrier frequencies given the spirit and purpose of the FCC rule of preventing interference from terrestrial repeaters within 431-433 and 435-438 MHz segment. Each area will have to make their own minds up but those who just want to eliminate ATV from the band, or have had a personal bad experience with an ATVer, or enjoy debating or strictly interpreting the FCC Rules will just have to spend the time battling it out with local ATVers - unfortuneatly it seems that is more of a hobby for some than enjoying all the many modes of Amateur Radio and finding ways to work it out.

Let me restate the legal reasoning, because SCRRBA did not arrive at it lightly. Also we feel to put in a request for a rule change just to clarify the repeater prohibited frequency segments would stir up the pot much like it has done on coordination and other mode remailers. Most of us did not feel we wanted to put the time in trying allay the interference fears by educating and responding to so many people across the country. Hopefully, as each area works out their local all mode band plans, the true technical characteristics and proof would come out to everyone concerned, just as we had to do here with technical information and on air demonstrations with our local weak signal and satellite people. There is no way we could travel around the country with a spectrum analyzer and gear to make the tests to so many, and no amount of written technical proof seems to over come the fear of the interference possibility.

We moved down to 434.0 in Southern California in the early 1970's to allow more FM voice channels which were growing very rapidly at the time. Southern California 70cm FM voice repeaters go down all the way to 440.0 and that segment has been fully coordinated since the early 1980's - over 600 coordinated voice repeaters, most of which are on high sites greater than 4000 feet above the coastal plain. At the same time we did not want to exchange interfering with one mode only to have problems with another. Given the spectrum power density of an ATV signal being 95% +/- 1 MHz from the carrier, the 434 frequency had little energy in the weak signal or satellite bands and on the air tests confirmed it to the satisfaction of all 3 parties. So we feel we have satisfied the prime directive of 97.101(a) of good amateur engineering and practice and 97.101(d) where the use of 434.0 does not cause interference to the two modes who wanted protection from repeater users with the exclusion.

In addition, the ATV repeaters use horizontal sync detectors (15734 Hz) to key up the repeater. Therefore the ATV repeater does not intentionally key up and repeat in the presence of any other mode - just its intended AM video modulated carrier at 434.0 MHz.

The received energy at the repeater is below the 26 dB in the 431.0-433.0 and 435.0 to 438.0 MHz segments per the definition of bandwidth per 97.3(a)(8) when using the 434.0 MHz video carrier frequency. The sideband energy from camera video is very low and random +/- 1 MHz from the video carrier. If you have access to a spectrum analyzer, watch a broadcast TV upper side band and you will notice the levels are way down. A photo can be found in the 1995-2000 ARRL Handbook page 12.48 figure 12.61 also.

The vertical and horizontal sync is the only video component that is constant and according to the Television Engineering Handbook - 1992 - Benson - Fig. 5-11, the horizontal sync harmonics sideband energy is more than 40 dBc down +/- 1 MHz from the video carrier. So if you are transmitting 100 watts pep AM ATV from a Mirage amp, the sync energy in the weak signal and satellite segments is no worse than transmitting a 10 milliwatt 10 micro second wide pulse every 63.5 microseconds.

In addition, a narrow band receiver is like a low pass filter to the video since at any given frequency, the amplitude of the video pulse depends on the rise time as the camera horizontal sweep goes across and sees a white to black, or vice versa transition. A full white to black vertical line in the picture is going to be a rare occurance, more likely smaller amplitude changes in the shades of grey. Since the video amplitude change is not a continuous sine wave, but occurs at the 15.7 kHz sync rates in time, this pulse is further attenuated by the narrow IF and audio filters of a multimode voice/cm receiver. Try tuning across the band with a SSB or CW receiver as an ATVer is transmitting. I doubt you will note anything outside of +/- 100 kHz of the video carrier, in fact I can only remember one case of interference here in 20 years where the ATVer was less than a half mile away from the 432 receiver and they worked it out on two meters. Some ATVers have multimode voice rigs and come on during contests just to give the 432 gang some points or work satilites.

In any given area, the local hams must decide to go to 434.0 or 439.25 Lvsb - you cannot run both 434 and 439.25 upper or lower VSB since the video will overlap. Therefore the decision will have to be what two ATV frequencies will be part of the local band plan. They need to be at least 8 MHz apart to prevent interference in the weak on channel vs. strong adjacent channel case. This is why broadcast TV skips a channel on VHF and 2 channels on UHF. Cable TV does not have that problem because they go to great lengths to equalize and filter all the channels to the same amplitude.

If more than one repeater is to be in the area on 70 cm, then sharing a common input can be done and coordinate on the two meter talk back frequency. Often, the two meter frequency is repeated back - at about 1/2 the sound subcarrier input level - on the ATV repeater sound subcarrier output. Areas using 434.0 typically use 146.430 and those using 439.25 are on 144.340 simplex to avoid the 2 meter third harmonic affecting the received video. Here in So. California, a person coming on in the southern part of Los Angeles County could possibly key up 5 ATV repeaters at one time if running an omni on 434.0 MHz as well being linked to Las Vegas, San Diego and Santa Barbara - we have to co-operate with each other. See the Southern California ATV Repeater Map, ARRL Repeater Directory ATV section and ATV groups link on our ATV application Notes web page.

When using 434.0 and 439.25 lvsb, ATVers must accept the fact that local hams up-linking to a satellite will occasionally wipe out the video depending on beam headings. But most satellites are only in view for up to 15 minutes on a pass, and track in both azimuth and elevation so they just have to be patient and wait. We even have some ATVers who put the satellite tracking map from their computers on the air so we can see when the pass will be over. As a consequence, many ATVers here enjoy working satellites (myself included) and vice versa.

Those using 439.25 MHz ATV can use horizontal polarization to give up to 20 dB of cross polarization loss to the vertically polarized FM repeaters. Conversely, since 432 MHz is horizontal, those using 434.0 MHz ATV should be vertical.

There is plenty of spectrum for all of us if those concerned are willing to work out the local band plan on a technical basis with all mode users. Southern California has the highest communications density of hams and the unusual geography of a high mountain range full of repeater sites that cover 100 miles up and down the coastal plain. If we can work it out here, other areas should be able to figure out engineering solutions also.

73, Tom O'Hara W6ORG (posted 12/15/1999 and updated 5/2004)
ARRL Technical Advisor for Spectrum Management and ATV.
SCRRBA Technical Committee member and 23 cm and ATV bands manager.

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