[BC] 9th Harmonic from FM transmitter...

[Burt I. Weiner]

I have had experience with the 9th harmonic from a FM transmitter 
wiping out a cell phone site.  One case in particular the station had 
two transmitters, a main and a backup, running the same power into 
the same antenna for 6 KW ERP - not at the same time, of 
course.  There was a 3-cavity bandpass filter between the output of 
the transfer switch and the antenna.  Transmitter A caused the 
problem while transmitter B did not cause the problem.  (Transmitter 
A also caused the problem when running into a dummy load.)  This told 
me that it was not the fault of overload at the cell site's 
equipment.  A spectrum analysis sampled at the output of the bandpass 
filter showed the 9th harmonic below the noise floor, which was at 
approximately 110 dB below carrier peak.  Further examination with a 
small loop type probe showed that the transmitter had a severe case 
of cabinet radiation.

The amplifiers in this transmitter were switching FET's and there was 
a slot between the circuit board where the transistors were soldered 
and the heat sink that held the transistor's bodies.  The back door 
of the transmitter had minimal shielding, which was more intended to 
hold the foam air filter in place.  There was no finger stock, he 
back doors were painted and the only connection between the back 
doors and the chassis was a small black wire about 4 or 5" long.  In 
other words, there was no shielding to speak of.  Replacing the 
various cables between modules with double-shielded cable helped 
somewhat but not enough.  The cell site abandoned the frequency 
because of the interference and the station chose not to pursue the 
issue any farther.  By the way, the cell site was 1.7 miles away in 
the direction that the transmitter's back face, through a steel 
re-enforced concrete block building.

So, you need to check three things:

1. What is the level of the 9th harmonic going into the transmission 
line?  Depending on transmitter power output, the FCC may require 80 
dB or more attenuation relative to the peak carrier value.  Your 
responsibility may not end just because you find that it's 80 dB 
below the peak carrier.

2. Is it possible that the cell site equipment is being overloaded 
and generating the 9th harmonic?

3. Cabinet radiation.  While the antenna sample may show that the 
transmitter harmonic content is well within requirements, is there 
cabinet radiation that may be the culprit?

There are a lot of things to consider when making FM broadcast 
harmonic measurements, but I hope this gives you a little guidance 
regarding this particular issue.

Burt

>From: Ron Cole <rondcole at gmail.com>
>
>Has anyone one had an issue with 9th harmonic interference with 
>Cellular networks?
>
>It looks like it may be more common with with stations that are 
>running low-level combining for IBOC.
>that operate on 92.1 - 94.3 Mhz.
>
>Ron Cole