What W0UN posted is very true. I've used dozens of ARR GaAsFET preamps over
the years, and running 1500W PEP output power on the VHF bands I've never
blown a preamp yet. But I sure wouldn't rely on an RF-activated T-R
switching scheme, at any power level. Use "hard keying," with sequencing to
be sure the preamp is bypassed before the exciter can generate any power.
ARR used to sell a 4-port (or maybe more than 4?) sequencer as an
accessory...maybe they still do. Ask Jay! -WB2WIK/6
"There is no reason anyone would want a computer in their home." - Ken
Olson, Chairman & Founder, Digital Equipment Corporation (1977)
> -----Original Message-----
> From: W0UN -- John Brosnahan [SMTP:shr@swtexas.net]
> Sent: Monday, October 04, 2004 9:25 AM
> To: Jim Brown; Tower Talk List
> Subject: [BULK] - Re: [TowerTalk] ARR Preamp Failures
>
> 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@contesting.com
> >http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> _______________________________________________
>
> 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@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
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.
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