Newsletter for Broadcasters
Issue #6

The 3rd Adjacent LPFM Question

Some advocates of the Low Power FM Service have been known to state that thousands of new LPFM stations could be created if only the FCC would drop the requirement the LPFM stations protect full power stations on the third adjacent  frequencies.

Looking at the country as a whole, this may actually have been true at one time. However, they would almost all be in rural areas, since very few new low power stations could be built anywhere in the top 50 markets, and none at all in the top 25. And with over 10,000 translator applications to protect from the last filing window, the opportunities for new LPFM applications have shrunk dramatically.

I have looked at a number of LPFM cases, and I have yet to find one that would be helped substantially by eliminating the third adjacent requirement. Certainly, eliminating the requirement could allow some proposals to go forward, but in my experience, they would be rare indeed in communities of any size.

But there is a big push to eliminate the third adjacent requirement, and it is supported by the very influential Senator from Arizona, John McCain. So let's have a look at the practical aspects of this issue as it affects a commercial station operator.

Of course, all radio stations require an audience to support them, regardless of whether they are commercial or noncommercial in nature. Because LPFM stations serve only small areas, they require fairly dense populations to support them. This means that the most likely location for you to encounter a new LPFM proposal is in a community that you serve and sell time in.  Even so, the current rules protect you from interference from the LPFM service on the same basis that you're protected from interference from full power stations, which is to say on your frequency and the three adjoining ones. The proposal to drop the third adjacent protection means that you would be protected on your frequency and the two adjoining ones instead.

So how bad is this interference likely to be?  Most modern radios of any quality all are not terribly susceptible to third adjacent interference. It takes the combination of a very strong interfering signal and a weak desired signal to create any interference at all.

As an example, WBRU, a Class B in Providence, RI, receives third-adjacent interference from WSRS in Worcester, MA, also a Class B, in the area around the WSRS transmitter on Little Asnebumskit Hill in Paxton, MA.  WBRU's signal in the interference area is about 44dbu - well beyond its claimed coverage area. The interference occurs within a radius of 2km from the WSRS transmitter, where the interfering signal is 112dbu or greater. This represents a 68db ratio between the desired and undesired signals.

To verify this, I took some test equipment on the road. Spectrum analysis done in the interference areas confirms that where the ratio is less than 60db, there is no interference, and where the ratio exceeds 70db, interference is almost always present.

(The FCC's third adjacent requirements are based on a ratio of only 40db.  This means that the modern radios used in this test are significantly better than the assumptions used by the FCC.  This is not too surprising, considering that these requirements were laid down in the 1960s, and the performance of all electronics, especially radios, has improved significantly since then. In this case, we used standard factory-issue run-of-the-mill GM radios. As car radios go, these are not particularly good or particularly bad, they're just average.  Radios of lower quality, such as clock radios and some cheap table radios, would not perform as well.)

Because the interference area is the largest when the desired signal is the weakest, we can look at a worst case scenario of a Class B desired station, whose protection is based on the 54dbu contour. Based on the same interference ratio that we encountered in Paxton, we should be looking for a signal level of 122dbu from the LPFM station to interfere with the 54dbu contour of the desired station.  However, that is significantly greater than the blanketing contour of 115dbu, and way out of range of the normally used f(50,50) and f(50,10) curves used in contour and interference prediction.  This means that anywhere within the normally protected coverage area of any commercial radio station, the blanketing contour of the LPFM makes a conservative estimate for the extent of third-adjacent interference.

(Blanketing occurs when the receiver picks up a signal that is so strong that it reduces the sensitivity of the receiver across the entire FM band.  Such interference is not limited to the three adjacent channels. )

For this reason, we can largely ignore relative signal strength between the desired and interfering stations, and focus on the blanketing contour of the LPFM - an interference area with a radius of about 400'.  Given the interfering signal strength of 115dbu and the observed ratio of 68dbu from the Paxton tests, this interference area will occur when the desired signal is as little as 47dbu, which is significantly beyond the claimed coverage of almost all commercial stations. It is only beyond the 47dbu contour that the interference is likely to grow beyond the 400' blanketing radius. 

Therefore, it appears that for car radios of a reasonable quality, the effect of interference from any new LPFM station is likely to be limited to at most the blanketing contour, a radius of about 400 feet.

And one political factor to consider: If the proponents of LPFM get the third adjacent rule dropped, you can bet that translator advocates will look for similar treatment. Translators can be up to 250W, and their blanketing coverage is as much as 650'.

The bottom line:

There would clearly be some cases of interference from third adjacent LPFMs, but it would not be widespread.

Next: Translators and LPFM

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