LBA asks: There has been a lot of activity recently among FM broadcasters, what with adding IBOC digital, more transmitting power and station upgrades for better coverage. Knowing your “RF sensitivity”, where is all this going for our spectrum?

One day not too long ago I happened to have my spectrum analyzer connected to the outside discone broadband antenna.  Out of curiosity I scanned the entire first GigaHertz of the RF spectrum, curious to see who had the strongest RF signal into my house.  It turned out to be a pair of FM broadcasters.  So I then focused in on the FM broadcast band spectrum.

I quickly discovered two FMs who jointly held the top signals.   I measured received signal levels from each station of -16 dBm.  These two are the strongest measured off-premises signals at my residence between 0 and 1,000 MHz, and probably in the entire RF spectrum.   When that signal level is developed across a 75 ohm receiver input, it produces more than 43,000 microvolts!

Few, if any, non-broadcast broadband receiver front ends are going to handle a signal of that magnitude without folding into compression.  It’s no wonder that my sensitive monitoring receivers on outdoor antennas curled up and died whenever they were tuned to within a few tens of Mhz of the FM broadcast band.  That grossly excessive received signal level is really needed only for receiving broadcasts on electric toothbrushes!

I then went to the FCC Media Bureau database to determine just what transmission conditions these two stations were authorized.  And I discovered, after some data analysis, that great changes had recently occurred in the FM authorizations in my market.

First, both of these “top signal stations” are now authorized 50 kW ERP.  Both are atop the same six hundred foot tower whose primary use is as an AM (ND) antenna.  The tower is located on flat land, line of sight to my driveway, three miles due south, and in the middle of a long-established residential area.  But there is no rational purpose in using this great a power level in my area, since VHF signals are always blocked by existing terrain before they can decrease to the noise level at far distances.

The primary FM had always been a twin of the co-sited AM, but it appeared that the FM had recently received a power increase authorization.  The second FM had been first established at an mountaintop broadcast site but had moved down to the ground and in doing so picked up about another 10-13 dB in authorized ERP.

Two more established FMs had moved their locations to another pair of AM towers, these only four miles distant from my house, and also had been granted 50 kW ERP each.  So the game became clear to me:  relocate to a tower on the ground, receive big ERP increases, and be able to sell air time to your clients on the basis that they are buying onto a “dominating 50 kW signal!”

Also, almost all of the other local FMs had separately moved to the premiere established broadcast hill and all picked up additional ERP; the average now from that hill (about ten miles away from my house and also line of sight) is about 30 kW.

But one dirty little secret remains.  The local FM NCEs don’t have that gross power level.  They kick out with an average 2 kW ERP.  AND NO LISTENERS EVER COMPLAIN THAT THE NCEs  “CAN’T BE HEARD!”

So the commercial broadcasters are all pumped up with “superpower” ERP authorizations, and non-broadcast VHF receivers all over the region are being squished.  And for what?  What’s the point?  Where’s the ecological regard for “Mother RF Spectrum?”

We have lots of RF power in the region producing bone-crusher signals that go nowhere.  Lots of primary electrical power being consumed for transmitters and A/Cs in a region noted for continuing power insufficiency.  Lots of imported oil, domestic coal and natural gas being burned to generate lots of electricity.  Lots of carbon dioxide and nitrogen oxides spewing into the atmosphere.  But just what do we get for all this waste?  The Curmudgeon can’t see the point in it.

Media Bureau, do you have any idea what you are doing?

The “fix” to my receiver desensitization problem was fairly easy, even if it came at my own expense.   A commercial FM band-stop filter, working ahead of a broadband distribution amplifier and the multiple monitoring receivers, solved the problem.  The filter has a minimum of 22 dB attenuation from 88 to 108 MHz, and a notch of 45 dB at one of the -16 dBm carriers.  My monitoring receivers can handle FM band signals in the range of -40 to -50 dBm without folding.  I can hear VHF aircraft band once again!  The receiver blanketing is essentially gone!

For many years I unquestioningly believed that “bigger outdoor antennas were better antennas.”  Finally I measured actual received signal levels.  And I discovered that the real problem was far too much RF in the air over my residence, not too little!

“Let’s keep the universe safe for RF!”

The Old RF Curmudgeon

About The Author

LBA Group, Inc. has 50 years of experience in providing RF asset solutions and risk management for industrial and telecommunications infrastructure assets. The group is comprised of LBA Technology, a leading manufacturer and integrator of radio frequency systems, lightning protection and EMC equipment for broadcast, industrial and government users worldwide; the professional consultancy Lawrence Behr Associates and LBA University, providing on-site and online professional training. The companies are based in Greenville, N.C., USA.

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