[BC] FM Harmonic Measurements

[Warren Shulz]

Len,

You are right on the antenna performance.  The transmitter is a very low
impedance current source only at carrier frequency and non-termination
bizarre impendance at all other frequencies.  The antenna return is a
low impedance current source with who knows what pickup performance.  

In the two-way radio world you put a circulator on the transmitter to
solve  these issues. But, it's not economical to do a circulator at 10
Kw.  Another thing we seen at Sears Tower FM installations was a
band-pass filter at carrier F is wild as your move up to octaves beyond
carrier.  Even though you have a BPF you cannot eliminate the
transmitter's LPF.  

On site station leakage back into the PA are non-terminated standing
waves from stray antenna pickup.  It always amazes me to see 800 + volts
of transmitter carrier RF going to the antenna and measuring in
millivolts stray return signals from near by antenna's cross coupling.
80 dB is a big spread. 

Warren Shulz
WLS CGO

-----Original Message-----

 From: Len Watson

I've been through one of these and a couple of things can bite you in 
the backside.

First, if you're measuring while feeding an antenna, the sample point on

the line makes all the difference since the SWR will be different for 
any harmonic compared to the fundamental (evens through the roof and 
odds relatively low but still varying).  That was proven with a slotted 
line.  We could almost pick our carrier:harmonic ratio by sliding the 
probe.  Into a good dummy, it's more constant - if the dummy will 
respond up that high.

Second is the rejection/pass characteristics of the antenna since the 
commission isn't concerned with what's in the line but what's leaving 
the antenna.  That's the real fun part since you never really know what 
the antenna do at 9f.  The directionality definitely changes - like 
feeding 33 megs (about 9f) into an 80 meter dipole and getting an 
endfire.  I only mention that because it means that you can measure the 
carrier:harmonic ratio in different places and get different results.  
In this case, the complaintant (is that the word?) had "shopped around" 
for a location where the signal was less than 80 dbc and used that as 
his benchmark!  That happened to be on a hill about 1000 yards from the 
tower.  Anywhere else - including at the base of his tower - everything 
but the fundamental was in the noise.

The station GM took a stand and, for whatever reason, they went away.  
He also rebilled the complaintant for my time.  I don't think he ever 
saw his money.

Len Watson