Topband: Detuning TX ant + QSK

Jim Thomson jim.thom at
Wed Nov 28 10:22:25 EST 2018

###  another way to speed up the release of a vac relay is to wire a 
resistor, with a value of aprx  2-3  times that of the vac relay coil 
resistance, in series with the rvs connected  back emf diode.
Doesn’t matter which leg of the diode, that the resistor is inserted in. 
End result is the vac relay will  release a lot faster, vs just a back emf 
diode alone.   I use that trick on the input vac relay on my hb amps.
Output vac relay  gets just the diode.  Then the input relay releases  1st, 
and the output relay releases   last.  And both are sped up with an 
overvoltage. ....+  drop resistor.  If you are V limited for an overvoltage,
you can also wire a small value lytic  across the drop resistor.     Another 
method, if you are  V limited is to use the  12 vdc  version of the GH-1, 
instead of the  26.5 V version.    14-18 vdc   + drop resistor
will easily speed up the 12 vdc version of the  GH-1.

Jim   VE7RF

-----Original Message----- 
From: Richard (Rick) Karlquist
Sent: Wednesday, November 28, 2018 7:02 AM
To: jkaufmann at ; 'Jim Thomson' ; 'TopBand List'
Subject: Re: Topband: Detuning TX ant + QSK

On 11/28/2018 5:18 AM, John Kaufmann wrote:
> Jim,
> I use Gigavac GH-1 vacuum relays for detuning my TX antennas when 
> receiving.  The contact closure/release time is 6 msec.  You have to dial 
> in enough QSK delay in your radio so that you don't hot-switch the relays.
> 73, John W1FV

A quick note on the GH-1:

1.  In theory, there is no limit to speeding up closure.
OTOH, release can only be speeded up to the extent of collapsing
the magnetic field by omitting the usual reverse voltage
protection diode.  After that, the mechanical response

2.  This works neatly with the GH-1 being energized on
transmit, where you want the quickest response.

3.  The limiting factor to speeding up this model is
contact bounce.  You can only apply a modest amount
of speed up before you are limited by contact bounce.
With other models, of course, YMMV.

Rick N6RK 

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