[BC] FM Harmonic Measurements...

[Burt I. Weiner]

While it's nice to have a calibrated probe, and I use the same one 
you refer to, the real problem is the load.  The antenna should be a 
great load at the fundamental carrier frequency, but at the harmonic 
frequencies all bet's are off.  A dummy load designed to work at FM 
broadcast band frequencies may exhibit higher VSWR or leakage at 
harmonic frequencies.

I always measure out to the 10th harmonic, sometimes 
farther.  Obviously, when making harmonic measurements you need to 
attenuate the fundamental carrier frequency as much as possible.  For 
this I use a Microwave Filter Company High Pass Filter model 
5-KHP.  This is a fairly expensive filter, about 
twenty-dollars!  It's specified impedance is 75 Ohms, but I've swept 
mine in a 50-Ohm system and found that it attenuates the FM band by 
about 50 dB and is flat within 2 dB from around 120 MHz to out past 1 
GHz with an insertion loss above 120 MHz of only a few dB.

All that aside, I look for the presence of any harmonic energy and 
measure the best I can given the circumstances of the particular 
system I'm measuring.  Generally, I do not see much in the way of 
harmonic content out past the 3rd harmonic, although I have seen a 
few transmitters that magically reappear at the 9th harmonic only.  I 
make a sweep at a 1 GHz span with the center frequency at 500 
MHz.  On Mt. Wilson this can be loads of fun considering the amount 
of stuffs coming back down the antenna.  Another issues that makes 
this problematic is that the deviation multiplies by the harmonic 
number, which means the 9th harmonic deviates 675 kHz, so in order to 
do this you need to use a RBW that probably will not allow you to see 
down as far as necessary.  I often use 10 kHz RBW, peak store and let 
it sweep for 15 to 20 minutes.  If the particular station will allow 
it, I set up the analyzer for each harmonic with a narrow sweep and 1 
kHz bandwidth and have them pull the output of the stereo generator 
for about 2 seconds.  This gives me enough time to make a measurement 
of the un-modulated signal at the harmonic frequencies.

Accurately measuring harmonics can be tricky, and as I mentioned 
earlier, if you are having harmonic interference issues you should 
consider the possibility of cabinet radiation.

Burt

At 06:00 AM 2/18/2010, broadcast-request at radiolists.net wrote
> > To insure compliance you need a calibrated line probe and compensation
> > chart for the 9 octave frequency change.  Dielectric products makes a
> > directional probe that can be calibrated from FM band to 9th harmonic.
> > You cannot connect an analyzer to a random probe and get usable results.
> > Best would be with a tracking generator  establish the sensitivity curve
> > for the directional probe.  This is not a trivial measurement to get
> > accurate results.  Its one thing to get an estimate and a whole other
> > matter to
> > Obtain FCC qualified - 80 dBc results.
> >
> > Warren Shulz
> > WLS CGO