[BC] Harmonics...

[Burt I. Weiner]

When I'm doing a NRSC Proof and I find harmonics or spurs higher than 
allowable I'll rotate my FIM for a null to determine the direction 
the undesired signal is arriving from.  Quite often I'll find it 
coming from a direction other than the station.  I'll include a 
comment in the report that the source is "off axis" from the xxx 
antenna.  Overhead lines are a real source of problems.  I try to 
stay away from overhead power and phone lines but sometimes you don't 
have that option, particularly in older areas.

I recently had a problem where the third harmonic was about 13 dB too 
high.  This was definitely coming out of the antenna yet the 
transmitter into a dummy showed the harmonics almost 
non-existent.  The problem turned out to be a corroded connection in 
an ATU of a D.A.


Burt

At 10:11 AM 1/3/2006, you wrote:
>From: "tosenkowsky at prodigy.net" <tosenkowsky at prodigy.net>
>Subject: Re: [BC] Closest-frequency AM diplexes
>To: broadcast at radiolists.net
>Message-ID: <380-22006123154224328 at M2W130.mail2web.com>
>Content-Type: text/plain; charset=iso-8859-1
>
> >( if I measure 87db at the tx output, why is it 54db in the field ? )
>
>External influences. I used to perform NRSC measurements for an AM DA
>in CT. If I measured in a McDonald's parking lot I measured third
>harmonic about 30 db down. Went 1/4 mile beyond, measured 85 db down.
>
>Classic example from years ago. A local ham complained to the FCC
>that an AM station had harmonics in the ham band. The station CE
>said he did a proof and harmonics were barely audible on a shortwave
>receiver at the station. FCC inspector arrives at the ham's residence
>near the station and measures harmonics about 20 db down. Inspector
>goes to the station, tells the CE and GM there are harmonic issues.
>CE says "Show me". Inspector goes to the van, no harmonics. Goes back
>to the house. Harmonics. Culprit: Neighbor's downspout/gutter had a
>poor connection. Diode effect. Instant harmonics.
>
>I would look for loose connections, poor bonding, etc. Track it down
>with an FIM. I had a Kahn AM Stereo install. A lot of noise in
>stereo, not in mono. Not showing up on scope, mod monitor etc. but
>was very audible in stereo within 1/4 mile of the station. I cured it
>by placing a wire over the vibration dampener on the A/C duct system.
>Vibrating ductwork caused L-R phase noise!
>
>Tom Osenkowsky, CPBE

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at earthlink.net
K6OQK