2nd-Adjacent FM spacing questions

[Steve Green NEPA]

There might ensue some 'duh' eMojis here, but I've been busy.
According to Radio-Locator dater, the WRDR 95.1 translator (27 watts) is a few blocks away from WPLJ 95.5's stick. 'W236CH' also sends a lot of those watts in the direction of WPLJ's tower.
So one question is, 'Whatever happened to that nail-biting 10.7 mHz concern about 2nd-adjacent interference within 12 miles of places like the ESB? From what I recall, outliers like 96.7 and 103.9 have had to adhere to such clearance. True, those two are Class A's and have more power. So what is the maximum power that new translators 30 feet away from a 2nd-adjacent Class B are actually allowed?
Is this waiver of spacing requirements some sort of recent annulment?
An LPFM up this way was issued a letter, skull and crossbones masthead and all, advising that the licensed LPFM (granted 10 watts) was 3300 feet inside the protected contour of a 2nd-adjacent Class B (a 'major incursion' was the wording). The Class B contour offered 41 or 42 miles of clearance,
The LPFM had three options to override and toss out the comlaint. It picked the third one -- change to another frequency.
(As a footnote, the Class B, unaware of, or uneasy with, the options the LPFM had and just to be on the safe side, I guess, wound up speding some $$$ putting a translator on the original LPFM frequency. Lol -- the coverage of that new facility was completely within the coverage of the parent class B !)
See, in a few apartments I've used that very 10.7 thing to cancel out noisy stations from adjacent apartments. You don't even have to turn up the volume on your radio. Naturally, the stfling can only work when the offensive station is on or above 98.9. And the existence of 10.8 nullings, advertent or otherwise, on many receivers, is too close for FCC comfort.
Why is this mathematical quirk not a concern for other close NYC Class B's. such as 105.9 ? The WRDR translator certainly sends the same power towards them as it does toward WPLJ. Both those stations and a hundred other Class B's should have that 41-mile protected contour holstered, no?
It's understandable why 96.7 and 103.9 abide by the rule ; they want their licensed wattage to remain up full.
Forgetting for the moment why a Jersey shore station wants a translator that serves Times Square : Is there an FCC TPO figure or an engineering calculation that obviates the second adjacent spacing rules? And is the end-around sweep a recent thing?

 

[fybush]

First things first - the "10.7" thing and the "second adjacent" thing are two separate... things.

The "10.7" thing is intermediate-frequency spacing. Until recently, most FM radios used a 10.7 MHz IF stage - they downconverted 96.7, let's say, to 10.7 MHz and then demodulated it. If there's an equally strong signal 10.7 (actually 10.6 or 10.8 MHz) above or below the frequency you're trying to demodulate - in this case, 107.5 or 107.7 - both signals can mix and cause interference. That's why the FCC has spacing rules (73.207 and also 73.213 and 73.215) that enforce strict mileage-based spacing between full-power stations on IF channels. Those rules also apply to translator stations of 100 watts or more... which is why you'll often see translators at 99 watts, the maximum power at which IF spacing doesn't apply. (I have translators near me in Rochester on 92.1 and 96.1 that are limited to 99 watts to protect full-power stations on 102.7 and 106.7, for instance.)

Second-adjacent spacing is, obviously, stations that are two channels away from full-power stations, like your example of 95.1 near a full class B 95.5. As you correctly note, there's mileage-based spacing for full-power stations to other full-power stations on second adjacent channels (all set out in 73.207), as well as some grandfathered exceptions for pre-1964 allocations and, in 73.215, rules for "short-spacing" based on protection of actual contours instead of mileage separation.

And so yes, of course, it doesn't immediately make sense that there are translators that nestle right up next door to second-adjacent full-power signals - that 96.1 near me, for instance, is literally on the same tower as a full 50,000-watt class B 96.5, WCMF.

Enter the ratio!

The FCC's translator rules allow for waivers of second- and third-adjacent-channel interference with a detailed showing that no actual interference will occur over any population (defined as inhabited housing or offices or a regularly-traveled roadway). "Interference," in this case, is defined as a translator signal that is more than 40 dB below the predicted signal level of the full-power station on the second- or third-adjacent channel.

If you're putting your translator at the edge of a class B station's protected (54 dBu) contour, that means you have to keep your 94 dBu (54 dBu + 40 dB) contour away from all population to show protection. You can do that in a few ways - you can mount the antenna very high on a tall tower so that less of that very strong interfering signal even gets to the ground. You can use a directional antenna to provide a null if there's a populated area in only one direction (I've done that to avoid individual houses!) Or you can use a multiple-bay transmitting antenna to reduce downward radiation, including a showing of the vertical antenna pattern to demonstrate no interference.

But it's actually much easier to simply co-locate (or nearly co-locate) with the station you're trying to protect. Take your 95.1 example - here's the actual exhibit from their 2016 application:

THE APPLICANT ACKNOWLEDGES SECOND-ADJACENT CHANNEL OVERLAP WITH FACILITY ID 73887, CLASS B, WPLJ, NEW YORK, NY. THE AMOUNT OF SIGNAL FROM WPLJ ARRIVING AT THE APPLICANT'S PROPOSED TRANSMITTER SITE IS 111.5 DBU. USING THE 40 DB UNDESIRED-TO-DESIRED RATIO METHOD FOR DETERMINING SECOND OR THIRD ADJACENT CHANNEL INTERFERENCE, THE PERTINENT INTERFERING CONTOUR IS 151.5 DBU. ACCORDING TO THE FCC ONLINE COMPUTER PROGRAM 'FM AND TV PROPAGATION CURVES CALCULATIONS', THE INTERFERING CONTOUR EXTENDS APPROXIMATELY 1 METER FROM THE ANTENNA LOCATION PROPOSED IN THE INSTANT APPLICATION. SINCE THE APPLICANT HAS PROPOSED TO MOUNT ON AN EXISTING COMMUNICATIONS TOWER ON TOP OF THE TRUMP TOWER AND SINCE THE VERTICAL DISTANCE FROM THE APPLICANT'S PROPOSED ANTENNA TO THE ROOF-LINE OF THE TRUMP WORLD TOWER IS 8 METERS AND THE DISTANCE TO THE INTERFERING CONTOUR IS 1 METER, IT IS IMPOSSIBLE FOR THE INTERFERING CONTOUR TO REACH ANY POPULATION. THUS THIS APPLICATION IS COMPLIANT WITH 47 CFR SECTION 74.1204 WITH RESPECT TO WPLJ.

Does that start to make some sense?