[Antennas] VSWR on Log Periodic.

Sun, 28 Apr 2002 19:42:20 +0200

Hello Clete.

I gave a few examples on the list as to why you cannot assume that the VSWR
will be greater at the antenna than at the transmitter. That mostly involves
feed systems with another Zo than that of the tramsmitter Zo.

In general you have to at least take the phase shift produced by the length
of the feedline into account with the dampening in the feed line to get a
prediction of the VSWR at the antenna from the VSWR at the transmitter. That
is for a matched transmitter feedline system, if the feedline does not match
the transmitter Zo there are more matters to considder.

I would also like to adress the subject of VSWR.
You indicate that the VSWR can be calculated as a simple relation between
power readings
P1/P2, I hope that was just because you did not want to write the formula
down.

        SQRT(Pf) + SQRT(Pr)   Vf + Vr
VSWR =  ------------------- = -------
        SQRT(Pf) - SQRT(Pr)   Vf - Vr

Its the highest voltage met on the feed line divided bu the lowest voltage
met on the feed line. Thus the name Voltage Standing Wave Ratio.

Hope I dont sound condesending or sarcastic. English is not my mother
language so its difficult to strike the right tone for these things.

Greetings from Denmark

Jesper Wolf Jespersen

OZ8ACE

----- Original Message -----
From: "Cletus W Whitaker" <[email protected]>
To: "Jesper W Jespersen" <[email protected]>
Cc: "Harvey&Bessie" <[email protected]>; "A10382" <[email protected]>;
"Antenna reflector list" <[email protected]>
Sent: Sunday, April 28, 2002 2:38 AM
Subject: Re: [Antennas] VSWR on Log Periodic.

> de WB2CPN  South Central Pennsylvania USA  2002.04.27
>
> Everything I know I learned from some one, so this is no exception. Which
> means I'm never afraid to ask, no matter how dumb:
>
> If I take my trusty Bird ThruLine Wattmeter and measure the indicated
power
> going into the transmission line, and the power coming back, then measure
> the loss of the transmission line, why does that not tell me the level of
> power arriving at the antenna and the level of power reflected by the
antenna.
> The usual SWR formula for P1/P2 should tell me the SWR at the antenna.  Or
does
> it?  If not, why not?  My early observations indicate that the SWR at the
far
> end of a shorted transmission line is Zero.  (My General Theory of Antenna
> Activity states that the Current at the ends of a long wire or dipole
antenna
> is immeasureablly close to Zero.)  The truth is mixed in here someplace.
> As some one said, the actual SWR of the antenna (the load) is always
greater
> than what's measured at the transmitter end of the transmission line,
(safely
> assuming that all transmission lines do have loss of some value or other).
> 73  Clete
> - - -
>
> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
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