On Fri, 13 Sep 2002 16:46:57 +0100 Tom Osborne <w7why@harborside.com>
writes:
>
> Hi All
>
> I have a big Globar 50 ohm resistor. If I hook it to one side of
> a "T" connector and the stub to the other side of the "T"
> connector, will it show minimum SWR at the right length of the
> stub for the band I'm making it for? Thanks and 73
> Tom W7WHY
If the resistor is a perfect 50 ohms, purely resistive, and the stub is
either 1/4 wave shorted, or 1/2 wave open, that method should
theoretically work.
However, even under those conditions, the swr will be almost 1.0
over a fairly broad range of frequencies, so the method is not
very precise.
I find that the most precise way, with equipment available to
typical hams, is to use a dip meter. The next best is to use
an antenna analyzer in the impedance measurement mode, to measure
minimum impedance at the frequency of interest. However, at least
some of the analyzers lose precision at very low values of
impedance due to "diode suckout" (an Autek term), so this method
can also be imprecise.
That reminds me of professor's story about the difference between
precision and accuracy. Assume you fire ten shots at a bullseye target:
If all of the shots are closely grouped in a small area of the
outer ring, the fire is precise, but inaccurate.
If the shots are widely dispersed over the target, but the
average of all the shots is exactly on the bullseye, the fire
is accurate but imprecise.
If all the shots hit the bullseye, the fire is accurate and precise.
If the shots are widely dispersed, and all on one side of the target,
the fire is inaccurate and imprecise.
So, if you cut a series of stubs using the swr method, there
will be significant variations in the length. The average of
all these stubs will be fairly accurate, but one individual
stub could be pretty far off frequency.
Dave Hachadorian, K6LL
Big Bear Lake, CA
