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post #11941 of 11943 Old Today, 08:17 AM
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Originally Posted by eacalhoun View Post
This is a simple technical personal interest question - just to see if I correctly understand the "co-habitation" of adjacent channels. For example - in the Charlotte market, WCNC and WBTV currently are on adjacent channels, 22 and 23. This works because their sites are within close proximity at the Dallas tower farm. Theoretically, could WJZY and WMYT (also at the Dallas farm) occupy channels 21 and 24, or 24 and 25, or 20 and 21? For the purpose of my example in my perfect RF world, no stations within 200 miles occupy channels 20, 21, 24 or 25. If my thinking is on the right track - so far - how many miles out does that "site proximity" end where interference kicks-in and channels CANNOT be adjacent? Or - as an alternate example - can adjacent channels exist at both Dallas and in NE Charlotte at WSOC and WCCB's sites?

Thanks - tried to make that example as simple as possible.
Your theory is correct by "co-habitation". Due to the tightness of the mask filter digital stations can do that where analogs couldn't. Also, the first adjacent channels need to have about the same power levels so the receiver doesn't try to lock onto a station with a much greater power level, say, one station is 100,000 watts and another is 1 millions. I was trying to remember off the top of my head what the on channel separation distance is and if I remember correctly it is 150 miles, but if you can get a waiver or can prove you will not cause additional interference, you can be closer.

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post #11942 of 11943 Old Today, 09:57 AM
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To expand on what foxeng has said, distance separations are dead. Whether a station is permissible is now governed by modeling the stations' coverage and interference and determining whether you're within a rounding tolerance of 0% population. It's not quite that simple, but that's the basic premise.

So yes, adjacency is permitted. To provide a good example, in Los Angeles, there are full powered signals on 31/32/33/34/35/36. They can be all in a row like that because they're all very close together on Mount Wilson or Mount Harvard and at roughly the same power, height, and antenna pattern. At the receiver, then, all the signals look to be roughly the same strength and are easy to pick out. By contrast, also in Los Angeles, KVCR-26 is the only full-power station on a different mountain. As such, the big full-power stations cannot be on adjacent channels. However, there are LPTV/Class A stations on adjacent channels on Mount Wilson and Mount Harvard because they're weak enough that they do not cause interference to KVCR.

So getting closer to home, you'll find that on the NAB Clearinghouse, the FCC's final channel assignment tool has assigned WCCB to channel 18 next to WSOC on 19, and that's because they're at similar location, height, power, and antenna pattern. Similarly, you'll see WCNC on 24 and WJZY on 25 for the same reason. You can't have full powers adjacent to each other at the different locations, but note that WCCB is on 27 and WGTB-CD is on 28 despite being at the different locations. WGTB-CD is weak enough that it does not step on WCCB's signal despite the distance.

Does that help?

- Trip


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post #11943 of 11943 Old Today, 10:39 AM
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Thanks FoxEng and Trip for the explanations - yeah, that helps me understand it more. Particularly interesting is FoxEng's point on the how the power levels need to be very close - that's something I had a hunch about in the back of my mind when I posted my question, but failed to mention. The "exception" is also interesting - using Trip's example of WCCB and WGTB given the distances between each site, yet on adjacent channels. Would part of this also be that WGTB (which I believe is LPTV) does not have the same interference protections as full power TV stations?

Eric Calhoun

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