[BC] 9th FM harmonic causing interference

[Dan Dickey]

> Although not harmonically related

I was holding off on this thread but it seems ok to tell this story about 
when interference isn't really interference. In my case it was an AM 
transmitter interfering with approach operations. This is not what you want 
to hear when the local international airport tower calls to complain that 
pilots can hear your station but not the ATC operator. At least one 747 had 
to abort a landing and fly around again. Turning the transmitter on and off 
confirmed the source of the "interference".

It turned out that one of the tower controllers had refused to give up his 
1940's era headset which had an old carbon microphone. It was rectifying RF 
from the new high power AM station and became an "crystal detector". 
Whenever he keyed the transmitter to say something to the pilot what the 
pilot heard was disco music. It was amusing after we found the problem and 
convinced the controller to give up his 40 year old headset. But before we 
found the problem it was causing my heart to race a bit.

Regards,
Dan