[TowerTalk] Voltage and SWR - an ignorant question

[jimlux]

On 2/1/19 6:41 AM, N4ZR wrote:
> Last week I was in the 160 CW contest using a jury rigged antenna - my 
> 40M parasitic sloper fed with open wire line from a 4:1 allegedly 5 KW 
> balun via a long run of Buryflex to the shack.  Believe it or not, it 
> didn't work badly, but...
> 
> The swr at the amp's antenna tuner was about 4:1, but when I put 1500 
> watts on it at the end of the contest the amp would quickly fault 
> indicating an SWR of 20:1.  My guess is that something was flashing 
> over. However, on 40 meters the antenna continues to operate and take 
> full power just fine.  Its native SWR on that band is under 1.8:1.
> 
> So, my question, does higher SWR mean higher voltage peaks on the 
> feedline? Once the snow stops, I'm going to go looking, but thought any 
> advice I could get before that would help.
> 

Yes, VSWR = Voltage Standing Wave Ratio = Vpeakhigh/Vpeaklow

so with 4:1, the highest voltage is 4 times the lowest. (neither would 
be the voltage you'd get with 50 ohms, BTW)

Also, your tuning network at the amp would have changed the voltage. 
Let's, for example, assume that the Z was 200 ohms instead of 50.

TO put 1kW into 200 ohms, the voltage has to be sqrt(1000 * 200) = about 
450V.

it might be that you were seeing 12.5 ohms at the amp (also 4:1 to 50 
ohms), in which case the current would be high, but the voltage 
relatively low.    Somewhere else along the line, though, the voltage 
would be higher, peaking at the 450V - assuming the line is at least 1/4 
wavelength long.

>