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I have been trying to tune some ferrite rod antennas to a specific
frequency. The target frequency is 530 kHz, and for reasons that may be too
complicated to describe here, I want to use a fixed capacitor, not a
variable capacitor. I got some ferrite rods that are 8 inches long and
0.375 inches in diameter. I wound 50 turns of wire on one to give an
inductance of about 300 uH. I tried to measure the inductance of the coil
with my NanoVNA, and found the measured inductance varied depending on
whether the ferrite rod was vertical or horizontal, and changed with
direction that the ferrite rod pointed when horizontal. When I used the
calculated capacitance value to give a resonant frequency of 530 kHz, I
also found that the resonant frequency shown on the NanoVNA seemed to
change when the antenna was vertical vs. horizontal, and varied with
direction of the horizontal position.

This baffled me until I got a hunch and connected the ferrite rod assembly
to my TinySA. A local AM station on 580 kHz was extremely strong in one
direction when the antenna was horizontal, I could null it out at right
angles to that, and with the antenna vertical, a signal was observable but
not that strong. So it appears the signal from the local AM station is
affecting the readings I get on the NanoVNA.

I tried making the same measurements with my RigExpert AA-600, which also
measures resistance, reactance and impedance of circuits. The results
seemed more consistent. I believe the AA-600 generates a stronger signal
that is sent to the DUT. I have suspected this for some time, because I
could get an SWR reading on the AA-600 over 400 feet of coax cable, but
none on the NanoVNA.

I thought I'd send this e-mail as a curiosity of something I discovered. If
you're trying to measure something with the NanoVNA and there is a strong
transmitted signal source nearby close to the frequency you're trying to
measure at, it could affect your readings.

Zack W9SZ