Hi all; something I noticed late last night after I re-read that
article below. And a little correction.
toggle quoted messageShow quoted text
When comparing VSWR voltages, you have to compare apples to apples.
i.e. peak voltage to peak voltage; not compare the rms (the DC
equivalent) to peak voltage. The article mentions the rms formula
and then goes on to compare it to the peak formula. Just using the
same formula below with SWR=1 is more straightforward. So...
Now you see this is even less dramatic than 3x the voltage.
Ironically it's 73% more voltage. I've seen that 73 number
On 9/22/2020 5:10 PM, RK wrote:
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 groups.io wrote:
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