[Amps] High SWR

[Glen Zook]

Before the advent of transmitters with fixed impedance output circuits the vast majority of amateur radio operators didn't worry about SWR.  Having an SWR bridge was not high on the list of items which were found in an amateur radio station.

Basically, if your transmitter would "load" the transmission line and if you could make contacts, you didn't worry about SWR.  If you had problems "loading" the feedline or if you had problems "getting out", then you worked on the antenna until you started making contacts.

At least one of my transmitters can "handle" an SWR of at least 40:1 and I have several that can "handle" an SWR of at least 24:1.  The result is that if I happen to use one of those transmitters I just don't worry about SWR.  Yes, the higher the SWR the less efficient the system generally is.  But, like in the "goode olde dayes", the "proof is in the pudding" in that if I can easily make contacts then, frankly, I couldn't care less about improving the SWR, especially to the extent that some amateur radio operators believe that you have to achieve.

Glen, K9STH

Website:  http://k9sth.com


--- On Thu, 4/23/09, Paul Christensen <w9ac at arrl.net> 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.