> reflection instruments make a single port measurement.
> The stub can be
> measured without a tee in line and it will measure too
> high in frequency.
I can't speak for the AIM4170, but it sure isn't that way
with a 259 or 269B, an AEA VIA, or any of the other cheap
analyzers I have.
In the typical application the user connects a stub via a T
connector. The transmission line length of that T is only
1/2 inch or so out to the flat of the male connector on the
stub. In a standard UHF connector mate virtually all of the
electrical length and impedance bump error is in the female.
Keep in mind the female is always to blame for all trouble.
A wavelength on 30 MHz is 393.5 inches. That's 360 degrees
or 1.09 inches per degree. 1/2 inch is about 0.5 degrees at
30 MHz or .05 degrees physical at 3 MHz. Let's just assume
we use a normal T and connect through a standard short
barrel to the male port of the F/M/F "T". Electrically we
might have one additional inch or 1 degree extra in the T
and double-female adaptor. We wind up with an impedance bump
of 30-some ohms about 1/2 inch long physical, or perhaps 1
degree at 30 MHz.
The stub would be electrically too long for use with a T by
one inch *because of the error of the female on the T*,
except one fact. Most analyzers are calibrated a large
fraction of an inch beyond the connector. This error is in
the correct direction to compensate for the change caused by
the T.
In practice most analyzers, if properly calibrated for zero
reactance open or short at the port, will be within 1 degree
at 30MHz. 1 degree at 30MHz is 80kHz or onlty 8kHz at 3 MHz,
and that is negligable compared to other errors. You can
veryfy the error by measuring an empty PL259.
I can't think of any amateur systems using stubs that are
critical to one degree.
There is a much larger and more common source of error,
harmonics in the analyzer. Make sure the analyzer has at
least 20-30dB of even harmonic attenuation. Even harmonics
like the second harmonic can skew readings by quite a bit.
Myself, even in my most fussy moments of life, I don't worry
about a degree or two in a system with such very low Q.
73 Tom
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