Topband: Rotor Birdies

[Ward Silver]

This is not rotator bearing rectification, it is the unbypassed rectifiers in the rotator control box.  The control cable acts as the antenna, bringing strong local RF into the control box, where the rectifiers turn it into birdies that are cheerfully re-radiated by the same control cable. (This can cause huge problems for SO2R or multi-op stations, by the way.)  

The reason it goes away when you turn the brake on is that large (compared to the RF) voltages are applied to the rectifiers, either turning them full on or full off so that they are not acting non-linearly except during the 60 Hz zero crossings.  If you listen carefully, the noise doesn't go away, it just changes to a low-level 120 Hz buzz from the rectification only occurring for short periods around the zero crossings.  

Bypassing the control cable connections to ground works, as does adding a .01 uF capacitor across each of the control box rectifiers.  

73, Ward N0AX

> I will add one more  weird one - I would  get several of those signals on 
> 160 randomly and I searched all my wires and fences etc with a portable rx 
> ( was surprised at a couple guy wires but that was not what I was hearing ).
> 
> One day I accidentally hit the brake switch on my Ham-M and the birdies went 
> away . From then on I could just switch it a few times and that would fix it 
> for a day or so.  Apparently rectification in the rotator bearings - or in 
> the brake. When I rebuilt all my antennas a year ago I added a flexible 
> cable tying the antenna mast to the tower top  and that eliminated the 
> issue.
> 
> 73 Hank K7HP
> 
> > Often, the mixing is being done near where the AM
> station is transmitting and is
> > very hard to track down.
> >
> > 73 Tree N6TR
> > tree at kkn.net