Re: [amsat-bb] AO-40 spreadsheet

[Jim Sanford <wb4gcs@xxxxxxxxx>]

Gents and ladies:

I strongly support this idea . . ..

Just the other night, while servicing a friends handheld, I learned the
hardway that I had not disabled the APRS transmitter like I thought. 
When it came on, my signal generator went "click" and the UNLEVELLED
warning started flashing.  Since I was already feeling unwell, you can
imagine the reaction, language etc.

GOt out the manual to look for what parts I had to try and find .. ...
discovered that they have an RF fuse in the signal generator.  IT
consists of a simple diode detector, opamp wired as comparator, and a
relay which connects the generator output to a 51ohm resistor instead of
the output connector.  The click was the relay . . . .A power off reset
restored my generator to service.  Obviously, I'm grateful for Wavetek
protecting me from my self . . . 

I've seen many preamp designs which have a MMIC following the GaASFET
(WAY too much gain) then followed by a 3 or 6 or 10 db resistive
attenuator.  This will probably protect the preamp from a slight oops,
and is in fact what they do to allow RF-sensed switching without cooking
the FET.  I think the combination of something like this, combined with
the "rf fuse" can protect against anything but a >250 watt oops.

Hope this helps somebody.

73,
Jim
wb4gcs@amsat.org


hasan schiers wrote:
 > 
 > None of the downconverters are going to survive transmitting into them for
 > even the slightest amount of time. I've seen reports of them being popped by
 > doing nothing more than changing bands, or turning the radio on.
 > 
 > The best solution I've seen so far is the one Fred, W0FMS, suggests. It has
 > an MMIC fuse in the subject line and has been posted in the last day or so.
 > It uses a TCK kit, slightly modified to protect the downconverter.
 > 
 > 73
 > hasan schiers, NØAN
 > schiers@netins.net
 > 
 > ----- Original Message -----
 > From: "Jens H. Jensen" <topcat@mail1.stofanet.dk>
 > To: <amsat-bb@AMSAT.Org>
 > Sent: Saturday, May 26, 2001 6:56 AM
 > Subject: Re: [amsat-bb] AO-40 spreadsheet
 > 
 > > At 02:49 26-05-2001, you wrote:
 > >
 > > >This the reason why the SSB units seem to hear better-- they do-- but we
 > > >can improve our more reasonably priced Transystems and Drakes by adding
 > > >more low noise gain up front.
 > >
 > > Don't forget DB6NT's offers, they seem to be less overpriced than those
 > > from SSB.
 > > I wonder if they are just as good, or....?
 > > Thinking about ruggedness as well, quite important for an expensive HEMT,
 > > me thinks..
 > >
 > > Any experiences with that?
 > >
 > > 73 de OZ1LRG
 > >
 > > ----
 > > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
 > > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
 > >
 > 
 > ----
 > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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