Topband: Stub measurement error

George Cutsogeorge w2vjn at
Wed Sep 5 22:38:47 EDT 2007

Tom writes:
By transmission and reflection, are you referring to S21 and 
If so, how could it be different with a symmetrical T? The 
fact it is different, if you are using a symmetrical T, 
means something is wrong in the test setup."
A transmission measurement uses a 2 port set up like a spectrum
analyzer/tracking generator.  In this case you tune to the desired null
frequency and cut the stub.  A reflection measurement uses a single port and
to find resonance, you cut until the phase is zero degrees.
Regarding tee connectors, if you look at them carefully you will see they
are not symmetrical.  The two straight arms are the same length, but the
center arm is shorter.  This is true for F-M-F UHF and BNC tees.  This is
why you cannot just average the two readings mentioned before.
I have explained why the measurement comes out differently in my earlier
Tom writes:
"That 35 dB wouldn't be true in an antenna system ... etc"
All may be true, but this is not what I was addressing.  If you want the
accuracy that an instrument of the AIM caliber is capable of, you need to
understand what you are measuring.  
Hopefully, we will see a two port AIM in the future.
George, W2VJN


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