Topband: [Fwd: Re: +12V T/R sequencing of ICE 184A preamp]

[Herb Schoenbohm]

Brian Machesney wrote:
> I am planning to try out the ICE 184A combination matching transformer and
> preamp that I bought several years ago. I have no dedicated RX antenna now.
>
> I understand that it might be wise to turn off the preamp when I transmit by
> sequencing the +12V supply. Does anyone on the list have enough experience
> with this unit to know if that's really necessary?


Brian,

If the pre-amp is located, I presume where the Beverage or  flag, 
pennant, etc are located, at least a half wave away and out of the 
induction field of the TX antenna your pre-amp should be able to be 
rugged enough to withstand the legal limit in a 1/4 wave vertical 1/2 
lamda away.  If it can't you need a better device.  But rather than 
guess just place a #47 bulb across the RX feed where it connects to the 
pre-amp and if it just barely glows or doesn't glow at all, you are most 
likely OK.  If it glows brightly or burns out then most pre-amps need 
additional protection. MFJ uses small lamps in series with the center 
conductor in the front end of the phasers for protection.  However, I 
believe that W8JI recommended mod for the MFJ is to remove them with the 
result in a bit lower signal loss of 1.2 db or so. He has some other 
mods for these devices but in your case it all depends on the proximity 
to the TX antenna.  No matter what it is...if you are to close and 
excessive votages are induce the antenna probably will be a mediocre 
performer due to pattern distortion and introduce noise unless of course 
you  isolate the antenna during RX.  But IMHO all these switching 
voltages and cables are a flirtation with other negative issues such as 
critical mission failures like during contests or when some rare DX is 
peaking at his sunrise and the very important problem of common mode pickup. 
This can all be be avoided by keeping things as simple and straight forward 
as possible.

I hope this information is of some help to you.

Herb KV4FZ