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for inband ATV Repeaters By Tom O'Hara W6ORG - Email:  Retired owner of P.C. Electronics, The Leader in Amateur Television Equipment for 50 years - history Last update January 1, 2015 This app note discusses the characteristics of vestigial sideband filters for ATV repeaters and compares 8 pole vs. 10 pole filters. |
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For instance, the unwanted lower sideband sound energy on a 100 watt ATV transmitter would go from about 1/3 of a milliwatt to less than1/100th of a milliwatt. Neither of these power levels sound very high but for reference, 1/3 mw after coax loss and antenna gain is about the same level as the Part 15 license free TV transmitters that get up to 500 ft. However, into a FM voice receiver you have a 23 dB lower noise floor, so with the 10 pole, you should not interfere with a narrow band receiver greater than 500 ft away and the 8 pole about a mile and a half line of sight. The other benefit of the 10 pole over the 8 pole is the greater attenuation which might be required if you cannot get enough antenna vertical separation on the repeater tower or find you have desense after going from 10 watts to 100 or 150 watts, but cannot move the antennas any more. The 10 pole can also give a few more dB's attenuation to FM voice systems that are just outside the video passband. Cost of the 10 pole is about $50 more than the 8 pole VSB filter. ATV repeater desense, especially a 70cm inband, has been a problem for many. Even with good VSB filters some repeaters still have problems with key up cycling with the ID video, noisy video, or the receiver AGC being pumped up and seeming to be insensitive because of it. We made a surprising discovery while testing out a DCI VSB filter that it repeated its 434 MHz passband at the 3rd harmonic with only a few dB of loss. I then checked the International Crystal FL407 and the Spectrum International PSF-434-ATV and found that they also had 3rd harmonic responses with none better than 12 dB down. All cavity type of filters will repeat their pass band characteristics on the odd harmonics. The odd harmonic energy out of your final amplifier may pass through the VSB filter or duplexer with very little attenuation which will overload the receiver front end. You could also interfere with other systems at the same or close by comm site if their frequency is near your odd harmonics. The cure is to add a low pass filter in the transmit and/or receive coax line as applicable. A simple low cost and effective transmit low pass filter is to use one side of a multiband antenna triplexer like the Comet CFX-4210C and at the receiver, the Mini Circuit Lab NLP-550. VSB Filter Sources:
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For inband 70cm ATV repeaters, especially those transmitting on 421.25, the DCI 10 pole VSB filter might be prefered over the standard 8 pole. While the insertion loss is only abut .3 dB more for the 10 pole (1.2 dB), the attenuation slope is much faster which will attenuate the unwanted lower side band sound subcarrier by 23 more dB (42 dB attenuation vs 65 dB) which could be significant if there is an another service user on or within 100 kHz of that frequency at the same comm site or within a mile or so line of sight.