[Elecraft] XV-50 LNA problems

[Robert Friess]

Greg,

Once we know what we are dealing with then we can work out a solution.   It 
could be as simple as an external lowpass filter.  That would be nice 
because the loss would be very low.

Bob

----- Original Message ----- 
From: "Gregory P. Daly" <gdaly at spiretech.com>
To: "Robert Friess" <rfriess at usa.net>; <gdaly at mitrek.com>; 
<elecraft at mailman.qth.net>
Cc: <eric at .SYNTAX-ERROR>; <UNEXPECTED_DATA_AFTER_ADDRESS at .SYNTAX-ERROR>
Sent: Thursday, March 31, 2005 12:53 PM
Subject: Re: [Elecraft] XV-50 LNA problems


> 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
>
>
>
>