[Boatanchors] Electronic TR Switch?

[mikea]

On Sun, May 01, 2005 at 09:23:27PM +0000, eldim at att.net wrote:
> Hello Eddy, Steve and Crew,
> Frankly speaking, I can't figure out how a ANTENNA SWITCH can be
> responsible for TVI. I'm puzzled with that one. I used a B&W Electronic
> Switch for years without any known problems. Unfortunately, I traded it
> off years ago-so I can't try and duplicate a similiar incident. I've
> always been taught that unwanted harmonics are generated by a mis-tuned
> transmitter or transceiver or a exciter that is poorly designed and dosen't
> have parasitic suppressors. Let's hear from the rest of the grouind with
> your take on this comment.

Well, if the switch were a perfect passive switch, that'd be fine:         
it would just act like a piece of copper, swinging from receiver to        
transmitter and back.                                                      

But it's an _active_ device, with a vacuum tube in it, and if the grid is
driven positive, then it's no longer linear: it stops acting like a piece
of copper. Instead, it changes those pretty sine waves to something non-
sinusoidal, with all those rich, chunky second and third harmonics that the
local TV-watchers have come to know and love.

That's all it takes, and some (not all, but *SOME*) of the tube-type TR 
switches are known for exactly this behavior, as the original note stated:

     From: StephenTetorka at cs.com
     Subject: Re: [Boatanchors] Electronic TR Switch?
     To: gswynar at durham.net, boatanchors at mailman.qth.net
     Cc:
     X-Mailer: 7.0 for Windows sub 8001
     
     Good Observation about TVI.
     
     The 1970 ARRL handbook - which does not have my solid state version - does
     clearly say " the preceding t.r. switches generate harmonics when their grid
     circuits are driven positive, and these harmonics can cause TVI...."
     
     In order to work, the grid are driven positive when the transmitter is 'on'!!
     
     R,
     Steve

That's pretty clear, I would think.

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin