[TowerTalk] Subharmonics Again!

[Ward Silver]

> The key is a diode connected to something near a 1/2-wave
> length on the lower frequency band.

A common source of diodes connected to just such a means of excitation and 
re-radiation is rotator control boxes.  The current-steering rectifiers in 
the control boxes (I'm most familiar with the Tailtwister box) are connected 
directly to several of the conductors in the control cable.  Lots of RF is 
present on those cables.  The fix is to bypass each diode with a 0.01 uF 
disc ceramic (or something like that) by soldering the capacitor directly 
across the diode.  The T2X box has eight diodes - it takes about 10 minutes. 
You can also bypass each line to RF ground at the barrier strip at which the 
control cable is connected.  One symptom of this source is that when you 
push the brake release button to turn the rotator, the interference changes 
character (because the diodes are either cutoff or fully conducting and not 
acting as mixers).  In my case, the RF was from a local BC station a couple 
of miles away, so it doesn't have to be all that strong to cause trouble.  A 
local 1.5 kW transmitter will pump those diodes just fine.

73, Ward N0AX