[amsat-bb] Re: pre amp help..

Edward R. Cole kl7uw at acsalaska.net
Wed Feb 16 23:02:12 PST 2011

What Greg is describing is an RF sensed high-power 
attenuator.  Downeast makes two products that can work to do this:

One is the AOS-144 ( $80.00) which switches RF thru a 50w attenuator 
to a factory adjusted level from 10 up to 100mw output for driving a 
transverter.  The receive side is passed thru to a common SO-239 
connector which is connected to the radio.  To use it solely for a 
preamp a 50-ohm resistor can be tied across the BNC transmit output 
connector.  This would be used in conjunction with TR relay for 
unswitched preamps like the P144VDG.

They also make the SSBT ( $65.00) safety switch bias-tee which might 
be more appropriate.  It provides dc voltage to power the preamp over 
the coax line and senses RF for protection up to 50w.  Again one 
would need TR relays for bypassing the preamp if you want to also use 
the antenna for transmitting.

But if you just want protection for a dedicated receive preamp either 
would work without need for a relay.

I know that 100w was mentioned, but these probably would withstand a 
momentary application of 100w such as the turn-on "burp" that some radios have.

73, Ed - KL7UW

At 08:56 PM 2/16/2011, Greg D. wrote:

>As others, SSB Electronics model that is RF switched.  I *have* 
>transmitted 100+ watts into it, though not on purpose, and it works 
>just fine.  I got the RF switched model as a "just in case" bit of 
>insurance, and am glad I did.
>It's been reported here a few years ago  that sometimes there's a 
>little "burp" of RF that squirts out when a transceiver is powered 
>up.  Seems unlikely , but if so, maybe that did it?
>If I recall, the discussion was about people frying their AO-40 
>S-band down converters by accidentally transmitting into them.  An 
>somewhat less certain recollection is that someone came up with a 
>simple circuit that would clamp the RF to a more forgiving, if not 
>totally safe level.  As an alternative to an RF switched preamp 
>model, perhaps a stroll through the archives might net a different solution.
>Good luck,
>Greg  KO6TH
> > From: k7trkradio at charter.net
> > To: amsat-bb at amsat.org
> > Date: Wed, 16 Feb 2011 13:59:41 -0800
> > Subject: [amsat-bb]  pre amp help..
> >
> > I just sent my ARR pre amp back to ARR for repair for the 3rd time !
> >
> > (pretty sure I did not tx through it )
> >
> > What are people using for pre amps. I only see the Mirage as an 
> alternative.
> > Any other choices. I'm kind of disappointed in ARR products.
> > Is there a pre amp that you can tx through preventing accidental frying?
> >
> > Any thoughts appreciated
> >
> > 73, Ted, K7TRK
> > _______________________________________________
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73, Ed - KL7UW, WD2XSH/45
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 144-1.4kw*, 432-100w, 1296-testing*, 3400-winter?
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