[TowerTalk] ARR Preamp Failures

[W0UN -- John Brosnahan]

Not exactly a TOWER question--but I will make a couple of comments.

1)  Does your 6M GaAsFET preamp have back-to-back diodes on the
input for protection?   I have found that to be helpful when running 50 KW
pulses at 49.92 MHz using the ARR preamps.

2)  The failure you experienced during your recent QSO seems to be
when there was 1) no lightning and 2) no other radios transmitting.
Is that correct?

3)  If so then I would be suspicious of my own 6M TX signal getting into
the preamp.  And you say you are detecting the RF for T/R switching.
I HATE that approach--too many things can go wrong.   You need to
be back into RECEIVE mode well before you send out any TX signal.

My best guess is that you are shooting yourself in the foot with your
own TX signal.

BTW  I always put shorted 1/4 wave stubs at the antenna feedline.  This 
provides
some static drain from the antenna, it also does some even harmonic 
attenuation,
and it provides significant reduction of the HF signal levels.  I consider 
it good
design practice.  And in the case of VHF radars it also speeds up the PIN diode
T/R switching time from about 6 uS to 3 uS.

I have used literally a HUNDRED ARR preamps over the years--almost all
being the GaAs FET models with very good luck.  But if the isolation of the
TR switch is low you CAN blow the preamps with your own tansmit signal--
if you don't run back-to-back diodes on the input connector to the preamp.

--John   W0UN


At 09:41 AM 10/4/2004, Jim Brown wrote:
>Last winter, I bought an ARR mast-mount GaSFet preamp for 6m. It has now 
>failed for the
>fourth time, and I'm about to give up on it. Each time, ARR has repaired 
>it under warranty.
>Several times I described to them the proximity of the 6m antenna to other 
>antennas and
>asked what signal levels it should be able to take. The "data sheet" and 
>manual don't say.
>Finally I got a response that it is rated for 10 mW at its input, which 
>works out to 700 mV in
>50 ohms.
>
>In my installation, the preamp is at the radio (for obvious reasons, I 
>haven't trusted it
>enough to stick it up in the air) there's a 6m ICE BPF filter between the 
>preamp and the
>antenna, and I'm driving it with an ICOM 746, letting the preamp detect RF 
>for T-R
>switching. The 6m antenna is a pair of PAR loops that are a few feet from 
>a pair of 2m
>PAR loops, also getting 100 w.  I've measured the attenuation of the ICE 
>filter at >60 dB at
>2 meters (on the bench with a 50 ohm load and good test gear). It's 
>also >45 dB on 10
>meters, >55 dB on 15 meters, and > 60 dB below 10 MHz, so it ought to be 
>protecting
>against any stuff from my HF antennas, the closest of which is an 80/40 
>dipole that is
>about 15 ft away.  60 dB of attenuation would take 700 volts down to 700 
>mV, so it doesn't
>seem reasonable that the preamp should be blowing up.
>
>There is also a Polyphaser in line with the antenna, and there have been 
>no lightning
>storms that are likely to have done the damage. The most recent failure 
>happened in the
>middle of a 6m QSO during the last VHF contest.
>
>So the question is, is this a common problem?  Am I doing anything wrong? 
>Could I do
>anything better?
>
>
>Jim Brown  K9YC
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless 
>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
>any questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk at contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk