[TowerTalk] Understanding Antenna Impedance Measurements

[Jim Lux]

At 09:26 PM 11/14/2005, Jim Brown wrote:
>On Mon, 14 Nov 2005 22:45:15 -0600, Robert Chudek wrote:
>
> >So when I analyze the antenna through the feedline at 1830, the
> >readings will be valid.
>
>But you still haven't told us what the Z reading is at the low VSWR
>point, which will help us help you zero in on what's going on.  :)
>
>Also, the variation in wavelength between 1680 and 1830 is only
>about 10%, so the transformation is still not a LOT different from a
>half wavelength. Also, I wouldn't trust that your guess about the
>electrical wavelength in coax to tell you for certain what the
>antenna Z is. But the Z reading combined with the VSWR will help.


I simulated the effect of about 218 ft of RG-8X (180 deg at 1.85 MHz) with 
a load of 50+j20 ohms, at frequencies from 1.8 to 1.9 MHz.
Frequency, MHz 1.8 1.82 1.84 1.85 1.86 1.88 1.9
R at load 50 50 50 50 50 50 50
X at load 20 20 20 20 20 20 20

At the "antenna analyzer end" you get the following data:
R at input 47.74 48.87 50.05 50.66 51.27 52.53 53.82
X at input 15.68 15.98 16.21 16.29 16.36 16.43 16.41

So you can get an idea of what happens because the transmission line is 
slightly different from exactly 1/2 wavelength. (and lossy, too)