[TowerTalk] Relays in RF usage UPDATE

[Chuck Counselman]

At 11:23 PM +0000 8/13/03, RCARIELLO wrote:
>I just received my copy of Worldradio September 2003.
>There is an article by Kurt N. Sterba on page 34, Aerials SWR and Voltage.
>
>Second paragraph "voltage goes up as the square root of the VSWR. So, with
>3:1 VSWR, the voltage and current are only 1.7 times greater then 
>with 1:1. A 50-Ohm perfectly matched line with 1500 watts power will 
>have a voltage of 274 volts. With 10:1 SWR the voltage will be 866 
>volts."
>...Any comments??


Yes.  As SWR changes, the ratio of reverse (reflected) power to 
forward power changes.  So does the ratio of the _net_ forward power 
(i.e., the forward power minus the reverse power), to the forward 
power.  (In the absence of transmission-line loss, the net forward 
power anywhere along the line is equal to the power delivered to the 
load, or the power delivered by the transmitter.)

Let's assume that, by "voltage," the author means not the voltage at 
the load or at the generator/transmitter, but the peak voltage in the 
standing-wave pattern on a transmission-line sufficiently long to 
show the peak.  If the load impedance varies, this peak voltage may 
or may not vary as the square root of the VSWR, depending on how the 
forward power, or the net forward power, varies.

I have an Excel 98 worksheet that calculates the peak current on a 
50-ohm transmission line, and the net forward power, given the 
forward power and the SWR.  (The peak voltage is equal to the peak 
current multiplied by 50 ohms.  Note that peak voltage does not occur 
in the same place as peak current.)  I will email this worksheet to 
anyone who asks.

73  -Chuck, W1HIS