[Amps] High SWR

[Scott McGrath]

Paul,

Not really of the don't care about SWR camp and I have the 
instrumentation to measure all the parameters you mention,  It's just 
that for those not versed in transmission line theory - SWR is probably 
the last parameter which one  should concern themselves with.


Paul Christensen wrote:
>> Short answer is the only thing SWR is good for is determining at what
>> point the RF voltages induced by the mismatch will damage your
>> transmitter system.    This is why modern rigs fold back transmitter
>> power when excessive SWR is detected,  This keeps the voltage developed
>> at a low enough  level to prevent damaging the finals.
>>     
>
> I think the importance VSWR in HF multiband operation lies somewhere in 
> between extreme views.    I need to know three things before I become 
> worried about VSWR:  (1) line characteristic Z; (2); the matched line loss 
> spec in dB/ft.; and (3) antenna input terminal Z range.  If I know these 
> answers, I can then compute VSWR and determine systematic loss due to a 
> combination of VSWR and the manufactures specified matched line loss in 
> dB/ft.  Also, when I know these answers, I can compute an optimum line 
> length for maximum power transfer from the transmitter into the line when 
> the line characteristic Z is not equal to the antenna input Z.
>
> What bothers me about the "who cares about VSWR" crowd is that they are 
> often ignoring total system losses as a result of unknown VSWR, unknown 
> matched line loss, and transfer loss between the Tx and line.  Even if my 
> line had zero loss, the resulting Z seen at the TX end of the line can make 
> power transfer inefficient from the rig into the line.
>
> It doesn't take much to get close to reasonable values of line VSWR (and 
> only then say "who cares about VSWR") and input Z at the Tx side of the line 
> to make the system reasonably efficient.  But I would not want to hide my 
> head in the sand and use some random length of line, and some random length 
> of antenna  -- and expect my transmatch to efficiently create a conjugate 
> match.  The system may be tuned and resonated for reactance cancellation, 
> but wouldn't it also be nice to know how much was lost in the process?  If 
> the answer is yes, then you need to know more about the antenna system 
> especially the line Z to antenna Z mis(match) and specified matched line 
> loss.
>
> Paul, W9AC 
>
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