One day not long ago, I had my spectrum analyzer hooked up to the discone external broadband antenna. Out of curiosity, I scanned the entire first GigaHertz of the RF spectrum, curious to see who had the strongest RF signal in my house. It turned out to be a couple of FM stations. So I concentrated on the spectrum of the FM broadcast band.
I quickly discovered two FMs that carried the higher signals together. I measured the received signal levels from each station to -16 dBm. These two are the strongest measured off-premises signals at my residence between 0 and 1,000 MHz, and probably across the entire RF spectrum. When that signal level is developed through a 75 ohm receiver input, it produces over 43,000 microvolts!
Few, if any, non-transmitting broadband receiver fronts are going to handle a signal of that magnitude without bending to compression. It’s no wonder that my sensitive monitoring receivers on outdoor antennas kink and die every time they are tuned to a few tens of Mhz from the FM transmission band. That extremely excessive received signal level is really necessary only for receiving transmissions on electric toothbrushes!
I then went to the FCC Office of Media database to determine what broadcast conditions were authorized for these two stations. And I found out, after some data analysis, that there had been big changes recently in FM authorizations in my market.
First of all, these two “overhead signaling stations” are now licensed with 50 kW ERP. Both are on top of the same six hundred foot tower whose primary use is as an AM (ND) antenna. The tower is located on level ground, in line of sight from my driveway, three miles to the south, and in the middle of a long-established residential area. But there is no rational purpose in using this high power level in my area, as VHF signals are always blocked by existing terrain before they can decrease to noise level at far distances.
The main FM had always been a twin of the collocated AM, but it appeared that the FM had recently received a power boost clearance. The second FM was first established at a mountaintop broadcast site, but had moved to the ground and in doing so collected another 10-13 dB in authorized ERP.
Two more established FMs had moved their locations to another pair of AM towers, these just four miles away from my house, and they had also been awarded a 50 kW ERP each. So the game was clear to me: relocate to a tower on the ground, receive huge increases from ERP, and can sell airtime to your customers on the basis that they are buying a “dominant 50 kW signal.”
In addition, almost all other local FMs had separately moved to the premiere established broadcast zone and all acquired an additional ERP; the average now from that hill (about ten miles from my house and also from line of sight) is about 30 kW.
But a dirty little secret remains. Local NCE FMs do not have that raw power level. They start with an average ERP of 2 kW. AND NO LISTENER WILL EVER COMPLAIN THAT THE NCE “CANNOT BE HEARD!”
Thus, commercial broadcasters are excited about “superpower” ERP clearances, and non-broadcast VHF receivers across the region are being crushed. And for what? What is the point? Where is the ecological respect for the “mother RF spectrum”?
We have a lot of RF power in the region that produces bone crushing signals that are going nowhere. Much primary electrical power is consumed for transmitters and A / C in a region known for continued power shortages. A lot of imported oil, domestic coal, and natural gas are burned to generate a lot of electricity. Large amount of carbon dioxide and nitrogen oxides released into the atmosphere. But what do we get for all this waste? The Curmudgeon doesn’t see the point.
Media Bureau, do you have any idea what you’re doing?
The “fix” to my receiver desensitization problem was pretty easy, even if it was on my own. A commercial FM band stop filter, which worked before a broadband distribution amplifier and multiple monitoring receivers, solved the problem. The filter has a minimum attenuation of 22 dB from 88 to 108 MHz and a 45 dB notch on one of the -16 dBm carriers. My monitoring receivers can handle FM band signals in the -40 to -50 dBm range without bending. I can hear the VHF aircraft band one more time! Receiver protection is essentially gone!
For many years I believed without question that “bigger outdoor antennas were better antennas.” Finally I measured the actual levels of received signal. And I discovered that the real problem was too much RF in the air over my residence, not too little!
