[Moon-net] PHEMT failure modes ?

[N6FD]

Hugh,

A couple of things might cause degradation over time for a low noise 
PHEMT.  The most likely is overstressing the transistor slightly.  This 
causes decreases in gain. 

The other possibility for 90's era transistors is hydrogen poisoning.  
This changes the operating point of the transistor until it eventually 
fails.  This is caused by outgassing of hydrogen in a hermetic package.  
The hydrogen attacks the platinum gate and causes permanent changes in 
the performance.  Most modern PHEMTs use different gate materials or 
packaging to eliminate this problem.

Erich, N6FD

Hugh Duff wrote:
> 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
>
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