[TowerTalk] Understanding Antenna Impedance Measurements (was Foam cables and velocty factor)
jimlux at earthlink.net
Tue Nov 15 17:27:19 EST 2005
At 11:49 PM 11/14/2005, Jan Erik Holm wrote:
>Jim Brown wrote:
> > 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.
> > Jim
>How do you mean now?
>At multipels 1/2 wl electrical transmission line length the
>load Z will be seen at the other end, i e it will be transparent.
But the line's only 1/2 wl at one frequency.. at lower frequencies, it's
less than 1/2wl and at higher, it's more.
Given the electrical length of the line (at some one frequency), though,
you can figure out what the electrical length is at other frequencies, and
then apply the transmission line formulas to see what you get.
This is essentially what the port calibration of a network analyzer does.
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