Topband: Stub measurement error

[Tom Rauch]

> 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