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Sounds good Allan; the actual loss depends on transmission line
quality and frequency then as we chatted on the phone when I was by
Ralph's earlier. i.e the losses would be much greater with a long
run at 440MHz and 3:1 as it bounces back and forth. Maybe we should
just superconducting coax and then the answer is always 100%? Ok so
maybe not practical on that one. :)
I found the actual article on voltage, VSWR, and surge suppressors
that I had talked about during the presentation if anyone is
interested. The formula is (2) on page 4 of this document for the
techies in the club:
So on a 100W signal into 50 ohms at 1:1, the common Ohm's law
formula states 70.7V (P=(V^2)/R); solving for V.
At 3:1, the voltage isn't 70.7*3=210.9V but 173.2V...a little less
than 3X as you noted.
With 1.5kW, it's an amazing 671V. So that's why you always tune
On 9/22/2020 2:29 PM, John Haskell via
Thanks for your thoughts. Good point on the peak voltage being
higher with a non-perfect match.
The 4-11-25 "rule" works well if the line is very lossy.
I must say though that the 4-11-25 "rule" doesn't quite cut it for
low loss transmission lines.
For example, consider a lossless line with a 2:1 SWR. What
percentage of the transmitter's power is radiated? Answer: All
the power is radiated [not 90%].
What about with a 10:1 SWR? The loss is again zero! All the
power is radiated. Think open wire or ribbon line. This is an
important concept. Even with a high SWR, 100% of the power is
What really happens is the reflected power is re-reflected at the
transmitter [near 100% of it] and is not lost to dissipation in
the transmitter. The reflected power, when it reaches the
transmitter, is reflected back in the forward direction where much
will be radiated when it again reaches the antenna. Eventually
all the power is radiated in spite of an SWR greater than 1 except
for the portion dissipated in the transmission line. Just think
of the energy ping-ponging back and forth with some being radiated
when the energy hits the antenna each time.
When some line loss is present the reflected power does suffer
some loss in the transmission line on the way back and forward,
and that is what the additional loss beyond the SWR 1:1 case
represents. As long as the line has, say, less than a dB or two
of loss the additional loss caused by SWR can pretty much be