[Amps] High SWR
Scott McGrath
mcgrath at fas.harvard.edu
Thu Apr 23 08:50:32 PDT 2009
SWR is an obsession with the CB set and some others CB's could not
tolerate a high SWR due to the cheap finals used the high RF voltages
would blow the junction. This effect created the oral tradition of
1:1 SWR == Antenna System Goodness (tm). It's hard to counteract what
'everybody' knows.
In reality and especially with a tube amplifier and a Pi output tank the
antenna system can be brought into resonance at a wide range of
impedances and resonance is what we are looking for and it really is
Antenna System Goodness in MOST cases. As the reflected power is
definitely real but it is best expressed as the reactive power in a
system which effectively is lost. With ladder line if long enough will
re-radiate some of this 'lost' power.
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.
- Scott N1JIN
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