The AIM or any other reflection type device does not measure stubs
accurately. The error is introduced because in normal use, a stub is
connected to one port of a tee that is inserted in a transmission line.
When measuring with a generator and receiver or a spectrum analyzer, the
stub frequency is read correctly. This is a 2 port measurement. The
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.
It can also be measured with a tee in line and it will measure too low in
frequency. This error is due to the fact that in a transmission measure
only one of the tee arms is included as part of the stub. In the reflection
measurement two arms of the stub are included. This error can be calculated
out by a series of measurements on the tee, but a direct measure on the stub
will not be correct.
The error increases with frequency and the error is about 400 kHz on 28 MHz
with UHF connectors and RG 213.
When measuring impedance, the AIM 4170 is almost as accurate as my N2PK
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