[BC] 9th FM harmonic causing interference

[Mike McCarthy]

I had this very same issue occur to a co-located cell site 200ft. down
from the main 7 bay FM. It was Nextel.

We have the very coupler Warren refers on a CEC 816. The scans showed the
emissions were -100dBc (after compensation and filter loss calculation) at
899Mhz. After showing him the scans and calaculations showing there is at
least 40dB of added pattern isolation from their antennas and our vertical
pattern, he still insisted it was our problem. The Nextel tech was
absolutely sure we were pounding his system.  Both his and my SA showed a
nasty emission at our 9th harmonic.

Finally, I called the studios and had the operator switch to the much
lower power back-up SITE some 15 miles away. (Why not just do it and not
bother staff you ask???...Simple. I want the staff operators to practice
this procedure...) Then shut power off to the 816 insuring that all RF
generation was terminated.

After shutting down the site, I asked the tech if he still showed the
problem. He said nothing changed.  His pilot channel was still being
hammered. I then waved him over to our building and showed a completely
quiecsent transmitter. Kodak momment.

I received a call a few days later from the tech and the shop foreman
advising they have egg on their face. Turns out it was a cell site a mile
away emitting stuff on a channel outside their passband.  I told them no
problem.  It's an easy mistake to make when one starts debugging IX.

MM

> 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
>