[Moon-net] PHEMT failure modes ?

[Hugh Duff]

The RX side of my 2m EME system seems to have degraded to the point 
where I can hardly hear signals off the moon anymore. It became apparent 
last fall and seems to have degraded gradually rather than abruptly 
although I only get down to the shack a couple of times a month, so I 
can't say for sure.

I'm running 2 yagis, an ATF54143 LNA at the top of the tower and a 1/4 
wave power divider with separate runs of LDF-5 from the relay down to 
the shack. A couple of months ago I pulled the LNA and power divider 
down to check them and consequently changed out the relay that I blew up 
<blush>.  The divider and LNA both appeared to be in good working order. 
I tested the gain of the LNA but I have no means to check the noise 
figure. That leads to my question;

What are the common failure modes of a PHEMT ?  Do they typically work 
or fail completely ? Is it possible for the noise figure to degrade 
while still exhibiting good gain.  Is this something that can degrade 
over time or perhaps be affected by too much leakage of TX power ?  I 
would have thought the gain would suffer too.

I realize a gradual degradation in RX performance is something more 
typical of an antenna problem...corrosion or the like, but the TX SWR is 
just about 1:1 and I'm being heard and not hearing anyone which throws 
me off that idea.  Is there a way to test the RX side from the shack ?

Thanks in advance.
73 de Hugh VA3TO
http://www.va3to.com