[Elecraft] XV-50 LNA problems

[Gregory P. Daly]

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