[amsat-bb] Re: AMSAT-BB Digest, Vol 7, Issue 65

Burns Fisher burns at fisher.cc
Wed Feb 8 12:33:11 PST 2012

Here are a couple things that were less than obvious to me about the ARR
70CM switched preamp.

If you are going to power it with phantom power (i.e. up the feed line) you
need to add a -C to the part number so they will add in the required choke
from the feed line to the power input.  But note that once you do this, you
can't just use either phantom or external power.  In my case, I got the
choke ("I might need it someday") but used the external power. Oops...a
nice short.  It turns out that unless your transceiver has a high-pass
filter (or just a cap!) you are probably going to be shorting out back
through the feed line.  When you do that, you may blow the choke and have
to remove it anyway, depending on how much current got through it. (I'm not
sure why they did not put a diode in, but hey...)

The other thing is that I was going to get one of their phantom power
injectors also.  The problem there is that they only work with
receive-only.   So unless you can find one elsewhere, phantom power may be
useful mainly if you transceiver supplies it (sigh, mine does not).

And finally, another inobvious thing:  The power connector has a third pin
which, if you ground it, switches to transmit mode.  So if you hook this up
to the transciever, that's a way of getting around the "minimum power to
switch" spec.  (I think minimum power to switch is maybe 5W; my transceiver
goes down to 5W, but with a 75' feed line, it's not gong to be 5W by the
time it gets to the pre amp.

The SSB preamp (or at least the 2M one that I have) comes with the choke,
diode, etc and has a lower minimum switching power.  It is more expensive,

I hope this helps!

Burns, W2BFJ

> ------------------------------
> Message: 6
> Date: Tue, 7 Feb 2012 22:56:04 -0500
> From: "Mike Ryan" <mryan301 at comcast.net>
> To: <amsat-bb at amsat.org>
> Subject: [amsat-bb] Re: Receive Pre-Amps for 70cm
> Message-ID: <89830E8A0C73460A9F511377C4656703 at MikeClone>
> Content-Type: text/plain; format=flowed; charset="UTF-8";
>        reply-type=original
> I second the comment about getting a switched one. Unless you have a
> dedicated RX or have disabled the TX, you will certainly pop a RX only amp.
> Its just a matter of when. Happened to me twice before admitting my
> clumsiness.
> I've had very good luck with the ARR switched amps. I have two for 70 cm
> (one for the shack and one for my Arrow) and a 2M version as well. If the
> unit is to be in the attic, you can save some money by going for the inside
> 25 watt switched model (SP432VDG). Just don?t exceed the 25 watts. Also be
> aware that the FETs in these amps are sensitive, even on a switched model.
> Make sure your rig is completed powered off and your body grounded (to
> bleed
> off static electricity) before connecting or disconnecting antenna
> connectors.   Its tempting to save a trip to the shack by leaving the rig
> on
> and do a quick swap, but don?t do it :(
> -----
> Date: Mon, 6 Feb 2012 15:40:37 -0600
> From: "Alan P. Biddle" <APBIDDLE at UNITED.NET>
> To: "'PETER SIMONSEN'" <pureenduro at msn.com>, <amsat-bb at amsat.org>
> Subject: [amsat-bb] Re: Receive Pre-Amps for 70cm
> Message-ID: <86B0819F8E944465B5B0C98010D7AAB7 at WA4SCA>
> Content-Type: text/plain; charset="us-ascii"
> Peter,
> A 70 cm preamp is always useful! There are a lot of them out there, but my
> experience is that ARR and SSB Electronics make the best. Definitely not
> the cheapest, though.
> http://advancedreceiver.com/page2.html
> http://www.ssbusa.com/gaasfet.html
> Be certain to get one which is RF switched, and which will handle your
> rig's
> power. No matter how careful you are, you WILL transmit into it someday.
> Been there, done that.
> You will need to split your single antenna jack on the rig to two lines.
> Almost any diplexer gives 50 db+ separation, but there are a few which are
> only 30 db or so. Worth checking.
> 73s,
> Alan

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