A quick correction


Hi all; something I noticed late last night after I re-read that article below.  And a little correction.

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...

1:1 Vpk=sqrt(100*100*1)=100v
3:1 Vpk=sqrt(100*100*3)=173.2v

Now you see this is even less dramatic than 3x the voltage.  Ironically it's 73% more voltage.  I've seen that 73 number somewhere before.


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 high power.


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 radiated.

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 ignored.