Ever Wonder Why Some FM Translators Are Using So Many Bays?

[Schroedingers Cat]

I know I was puzzled a first as to why many translators are using so many bays. You would think that with 250 watts ERP maximum, you would need to have have one, maybe two bays, right? Most of them are just about as high on the tower as much higher ERP antennas. So it can't be the downward RF radiation levels, unless the combination of the other transmitting antennas put it over the edge. Many are using 4-6 bays. I have gone through a lot of the applications, and I have found that the main reason is second and third adjacent signals where the translator is within the protected 60, 57, or 54 dBu F(50,50) protected contour of a nearby station. They are required to have no 100, 97, or 94 dBu inverse field signal which interferes at any populated point within the powerful station's protected signal.

Believe it or not, the CLOSER to the powerful station, the better as far as U/D ratios. If you are just inside and near the protected powerful station contour, you may need 6 bays or more to not have the 100, 97, or 94 dBu signal at ground level in the vicinity of the transmitter. I believe there are exceptions when the interference is on the transmitter station property and or there are no people residing in the area where the interference occurs.

Sometimes, tower weight loading is a factor, and they can only use one bay, sometimes requiring them to be directional to avoid interference at ground level, as in some directions the powerful station has higher or lower dBu signal levels, so to meet the U/D ratio, a DA is advantageous. But considering what David has told us about stations in other countries, there should be fewer restrictions for second and third adjacent signals in the same general area. There are exceptions on NCE-FM where this is allowed (Raleigh Exceptions), and second and third adjacent stations were allowed in the same general areas in the early days of Commercial FM. Many are still grandfathered today.

 

[Michi]

Yes, the main reason is due to needing to protect second and third adjacent channel stations. In order to demonstrate a lack of interference, the facility may not provide more than a 40 dBu U/D ratio to any occupied area (for example, if the weakest second or third adjacent overlapping station reaches the translator at the 75 dBu contour, the 115 dBu contour's pattern from the translator is considered. The elevation patterns are used to determine the antenna's power along the various depression angles and as long as the field strengths do not exceed 40 dBu U/D. If the 40 dBu U/D contour does "hit the ground", as long as it hits the ground in areas where there is no occupation, then it is acceptable. In the past, I have done "donut waivers" where while there may be occupied structures fairly close to the tower and some further away, but the interfering contour only hits the ground in areas between the occupied areas. I have also done "petticoat waivers" where the translator (or LPFM) is on a tall building and because of the elevation pattern, the interfering contour does not penetrate the building and reach human heads.. instead it skirts down the sides of the buildings outside the windows and therefore does not reach any human heads.

There are many that may debate whether second or third adjacent interference is harmful, especially in this day and age. I have made the argument that there are many situations where second or third adjacent "interference" has reached occupied areas and there are no issues. The mostly unrestricted use of LPFM stations on third adjacent channels prove that third adjacent is not an issue. Second adjacent stations that were overlapped either through subsequent application filings, overlap due to "foothill effect" (stations located in areas with high terrain behind them and very low terrain in front of them, which evens out the HAAT) and grandfathered short spaced arrangements, do not seem to have many, if any complaints. Second adjacent complaints filed against LPFM stations in the past have been found to have no merit in real world testing and are mainly used as retaliation against the LPFM station. We saw that happen recently in Connecticut.

Even the FCC in the past, has acknowledged that second and third adjacent overlap is more inconvenient than it is harmful interference , when compared with co-channel and first-adjacent channel interference. (Education Information Corporation aka the "Raleigh" decision) Though the FCC did not move forward on it, I had filed a Petition for Rulemaking to allow for lower powered NCE stations inside the service contours of second and third adjacent channel stations if it can be shown that the population within the 40 dBu U/D area is less than 0.05% of the overlapped station's service contour and never more than 3,000 persons and there are substantial public interest reasons (such as no other station licensed to the community, outside of urbanized areas, blocked because of urbanized areas and local new-entrant ownership). I filed an application in the NCE window with a waiver request to test this. It will probably get denied, but it is still pending for now.