On Oct 23, 2008, at 6:51 AM, Martin wrote:

> Nate Duehr wrote:
>
>> The SWR fold-back protection circuits in modern rigs often kill more
>> power than a high SWR/inefficient antenna system does.
>>
>> Add in a hypothetical chunk of very lossy transmission line, and  
>> you can
>> prove that sometimes at VHF/UHF and of course, higher in frequency  
>> where
>> losses really eat you alive... that a lossy piece of junk coax  
>> might eat
>> enough power going both ways, that the rig's SWR protection circuit
>> doesn't even see the reflected power....
>>
>> And...once you've set up THAT scenario, then you can compare  
>> numbers and
>> show that SOMETIMES -- switching to a better quality coax to the same
>> (bad) antenna, would result in a net power LOSS!
>>
>
> Let me preface by saying that I'm not an RF guy..
> How could the SWR fold-back protection drop the power more-so than a
> high SWR antenna? Does it see some amount of reflected power and then
> cut down the power until it sees some arbitrarily acceptable level of
> reflection?

Yes, that's it.  Many rigs attempt to back off their power  
aggressively until below 2:1 or 1.5:1 SWR is reached at the transmitter.

I can't take credit for the excellent article I based our little 20  
minute "learning" session on here locally.

This article is what I based the on-air session on, it should answer  
your question better than I can do in a quick e-mail:

http://www.cvarc.org/tech/standwave.html

--
Nate Duehr
nate@natetech.com



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