[TowerTalk] Reflections, Conjugate Matching and Jim Reid's comments

[Stan Griffiths]

Steve Best said:

>     In any event, the real issue is not VSWR but rather what happens to 
>     the reflected energy?  Most on the reflector appear to believe that it 
>     is never really lost and that it does not impact system performance - 
>     like operating into an antenna with a 20:1 VSWR and not experiencing 
>     even a fraction of dB loss.  Why do ALL the antenna engineers in the 
>     world waste their time minimizing VSWR?

W7NI offers an explanation:

Because virtually ALL the antenna engineers in the world today are working
with VHF, UHF, and higher frequency antennas and feed systems where
transmission line losses are FAR greater than at HF.  Yes, there are a few
working at HF but it isn't 1 percent of them.
     
Steve Best said:

>     Ask yourself why transceiver manufacturers, even ones with built in 
>     tuners, specify a maximum VSWR load (like 3.0:1) and why the output 
>     power is lowered when operating into a high VSWR?  If all of the 
>     antenna reflected power was reflected back to the antenna, why would 
>     it make a difference?  And yes, some transceivers and tuners do burn 
>     up because of reflections.

W7NI offers an explanation:     

You are apparently talking about relatively modern, solid state
transceivers.  It is well known that solid state devices respond very poorly
to over voltages.  They short, permanently, unlike vacuum tubes which may
actually arc internally but then recover to operate again.  The presence of
a high VSWR probably creates an over voltage situation that could
permanently short the output transistors.  This is a different failure mode
than having them simply dissipate too much power and melt down.  So I think
modern transceiver protection circuits are really protecting the output
transistors from too much voltage across the junctions than from too much
power dissipation.  But I could be wrong since I am not an RF design
engineer . . .

Stan  w7ni@teleport.com    


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