Topband: Handheld Impedance Analyzer
Guy Olinger K2AV
k2av.guy at gmail.com
Tue Mar 29 09:49:10 EDT 2016
THIS is the way that you do it. The newer graphing boxes (most of them
since AIM4170) will allow you to pre-calibrate a transmission line, and
then see all results as if you were really up there.
Get an instrument (they are getting less and less expensive) that will
graph R and X (both below and above the horizontal graph axis) and see what
is really going on. Trying to extend SWR as a prime instrument is
ultimately doomed. SWR is around because in ancient days it was all that
could be afforded and built from junk box parts. Since embedded CPU's are
in everything from gas grills to washing machines to telephones to little
kid's toys, the programs and the IC chips to make the essential math really
simple have become very cheap.
Resonance is where the graph line for X crosses zero. Just use your SWR
like the oil pressure lamp in your car. Let it tell you where out of bounds
for your amp is, tell you when something got broke in your antenna because
it is not the same any more, etc. Don't try and use it for the deep stuff
or initial adjustment.
We do not have to use our hammers to fix our Swiss watch any more. Having
the box tell you the R and X values at the FAR end of a transmission is a
gift way, way too good not to be the first analytical line of attack.
73, Guy K2AV
On Tue, Mar 29, 2016 at 2:07 AM, Jim Brown <jim at audiosystemsgroup.com>
> On Mon,3/28/2016 10:29 PM, Henk PA5KT wrote:
>> You can calibrate it with your feedline connected so you do real antenna
> One point of clarification. If you make a measurement that accurately
> provides magnitude and phase, including the SIGN of the phase, then
> accurately determine the electrical length of the feedline and subtract it
> out with correct math, that measurement is just as "real" as if you took
> your analyzer to the antenna and measured there with a zero length cable.
> In other words, there is nothing "better" about measuring at the antenna if
> you made the "in the shack" measurement correctly and transformed it
> I've done it both ways -- W6GJB was on my tower working on a monoband Yagi
> for me, I set up my VNWA at the base of the tower, calibrated it to a piece
> of coax long enough to reach Glen on the tower, and measured the antenna.
> Later, I measured from the shack with a much shorter piece of coax to the
> line, and transformed the measurement up to the antenna. I got the same
> data for the antenna from both measurements.
> 73, Jim K9YC
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