[Amps] Re: Amps Digest, Vol 12, Issue 33

[Larry Kayser]

Chris:

 >Modern RF Fets are pretty robust devices and have an MTBF a couple 
orders of

>magnitude better than thermionic devices, provided the junction/channel
>temperature is kept to a sensible level, and the breakdown voltage and
>dissipation limits of the device are observed. Monitoring the drain
>currents, heatsink temperature and load VSWR is really all that is required.
>A $2 microcontroller could be used to automate the monitoring/trip
>functions......
>  
>
I am troubled by this opinion.  When I study the hardware to the R&S box 
I see evidence of great attention to details that I sense are associated 
with longevity of the devices.  I am also told that many high power RF 
solid state devices have a definite life cycle.  The issues are as I see 
it survival in the keying ON/OFF mode, some issues of linearity and 
instability below, at, and above the operating frequency.  My sense is 
that we need some serious development of protection for open or short 
antenna feedlines or relays, even to the extent of a momentary test 
every time one goes into transmit mode.  I suspect that we need some 
rather serous protection efforts as well as a way to understand what the 
protection system did when it protects the devices. 

If protection and management was simple we would already have it in 
place.  Since we have no available systems I must assume that the task 
is far beyond trivial.

>My one dread with my own station is a very close lightning strike, but then
>that would be a serious problem with many tube PAs. Fortunately we don't get
>big electrical storms in this part of the world too often. I try to remember
>to keep the
>transmitters disconnected from the antennas at times when I think we might
>get one.
>
>  
>
Broadcast stations, solid state, work right through repeated lightning 
hits on the operating tower - they survive amazngly well.  From what I 
have seen when a working transmitter does get a hit there is a real mess 
- not often but when it does it is expensive.  I sense that for amateur 
radio this is really no reason, other than for repeater service, to have 
problems of this type. 

Larry
VA3LK