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I used to use a tee and dummy load on my RigExpert AA-170. it was an older one without the stub feature (b+w screen).  The reason for the tee and dummy load was so that when the coax was 1/2 wavelength, that side of the tee would see an infinite impedance and the RigExpert would only see the 50ohm dummy load and read 1:1 match.  Any other length will be a reactance in parallel with the dummy load and change the SWR.  It is a good method if you are careful and stop an inch early to account for the leg of the tee.  These impedance analyzers are not very accurate at high impedances, which is why I figured measuring the stub at 50Ω was ideal. It saves a lot of messing around trying to short the end between trimmings.


Edit:

Perhaps these new RigExperts have a built-in 50Ω load that is switched in when that impedance is selected. I don't know, but the idea of having to short it when it asks tells me they cheated and are doing it with math and sketchy high-Z/low-Z readings. With the poor accuracy at high impedances (like half-wave open-ended cables), I cannot imagine why they would design it to measure stubs any other way. I am curious if anyone knows how they did it...  Is there an internal load to parallel with the stub or did they cut that corner?