[Elecraft] XV-50 LNA problems

[Gregory P. Daly]

Bob

     I have an HP435A power meter, I'll look at how much signal shows up on
the 6 meter antenna tonight.... I expect it to be big, probably more than +20
dBm with +52 dBm comming out of the 2 meter amp. The 6 meter loop probably
works pretty good close to 3X it's design frequency, and only a couple feet
away.... I'll measure it tonight, and let you know.... it might be a good bit
of info to have in the useful ham radio trivia collection

Greg


Robert Friess <rfriess at usa.net> said:

> Hi Greg,
> 
> Because of the high power and close proximity of the antennas there is 
> probably considerable out of band power being applied to the LNA input.  It 
> is difficult to suggest what is needed to alleviate the problem.  Do you 
> have some means of measuring the power at the XV50 input when the other 
> bands are active?
> 
> 73,
> Bob, N6CM
> 
> 
> ----- Original Message ----- 
> From: "Gregory P. Daly" <gdaly at spiretech.com>
> To: <elecraft at mailman.qth.net>
> Sent: Thursday, March 31, 2005 9:55 AM
> Subject: [Elecraft] XV-50 LNA problems
> 
> 
> > Now that I have my K2 fully stuffed with goodies... DSP, NB, ATU, Battery,
> > SSB, 60M, 160M, IO... I got busy on transverters... and have all 3 
> > working...
> > but...
> >
> > I plan to use the K2 and transverters, portable, for the June VHF contest. 
> > I
> > use M2 "HO loops" for the "low gain" stack to allow quick setup when we 
> > get to
> > a hill top. By the time we've gotten "the easy ones" worked with the low 
> > gain
> > stack, the antenna monkeys have the "real" antennas up. The 50, 144, and 
> > 222
> > loops are about 2 feet apart on a common mast. The plan is to use Mirage 
> > 160
> > watt amps on each band.
> >
> > Much to my dismay, I discovered that with about 150 watts output, at 
> > 144.2,
> > into the middle loop, the PHEMT GaAs FET, in the front end of the XV-50
> > failed, gate to channel short. The first time, since my ESD discipline 
> > isn't
> > the best at home, I assumed it was static damage, and the FET was 
> > "wounded"
> > and failed after a couple days of operation. I was able to get several 
> > samples
> > of this PHEMT from Agilent, and replaced the damaged part. The second 
> > time, it
> > was obvious what had happened, the large 144 MHz signal from the nearby
> > antenna had coupled enough energy into the 50 MHz antenna to destroy the 
> > LNA FET.
> >
> > This configuration has worked in the past, with other radios. My ICOM 
> > 551's
> > receiver has survived up to 500 watts in the 2 meter loop without damage.
> >
> > This problem is soon to be worse; the spring antenna project will be a 6
> > element 6 meter yagi, on a common mast with the 2 meter and 1.25 meter
> > yagis... and up to 1500 Watts on 2 meters.
> >
> > Any suggestions about protecting the GaAs FETs from VERY LOUD out of band
> > signals, without causing nasty intermodulation products? It would be a 
> > shame
> > to build a killer transverter and waste it's excellent performance with
> > clipper diodes in the front end.
> >
> > I've been thinking about bringing out band select signals, to switch loads
> > onto the antenna ports of the unused transverters, but 12 volt coil coax
> > relays are expensive. A shorted 1/4 wave stub might work, but it's close 
> > to
> > 3/4 wave at 2 meters, so it may not be enough, an open 1/2 wave stub at 6
> > meters is getting rather long for running around in the field, the truck
> > already has too much clutter on contest weekends ;-)
> >
> > Going back to the Icom receiver is not an option, after using the fully
> > tricked out K2, listening to the 551 all weekend would be torture.
> >
> >
> > 73 DE WB7RSG
> >
> > -- 
> > Gregory P. Daly
> > http://www.mitrek.com
> >
> >
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> 
> 



-- 
Gregory P. Daly
http://www.mitrek.com