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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

Yes Zack. Been there done that. Tried outdoor antenna measurements carting a
hp vector voltmeter and sig gen. Always saw reflected power greater than incident!
Gee an active antenna! Could not get realistic data until I moved to a new measurement
frequency or drove the input power up to over ride the interference.

I can second that as well !? I inherited some old antennas from a defunct CATV head end and have been trying to take what I thought was the easy way out and sweeping them for resonance with my Nano VHA-H but I can't trust the reading given regardless of calibrations I get a different value each scan LOL.


Wt

example measure a 40m dipole and have a strong 41m radio station close .. yes

even a signal outside of your normal measure range may cause this (think also on harmonics or unwanted mixing products)

dg9bfc sigi

Am 03.06.2023 um 23:49 schrieb Zack Widup:

One of the lesser known issues is transmitter sideband noise. No carrier is
perfect as it generates noise eithe side of the carrier and if the carrier
is very powerful so the sideband noise is considerable.

*Clyde K. Spencer*



On Sun, Jun 4, 2023 at 9:40?AM Siegfried Jackstien <
siegfried.jackstien@...> wrote:

example measure a 40m dipole and have a strong 41m radio station close
.. yes

even a signal outside of your normal measure range may cause this (think
also on harmonics or unwanted mixing products)

dg9bfc sigi

Am 03.06.2023 um 23:49 schrieb Zack Widup:

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

This is where the professional devices excel.? I have an Anritsu SiteMaster and a CellMaster that have the capability of ignoring these extraneous signals. The Anritsu devices send a coded signal up the coax and only use that coded reflected signal to make measurements.? This is almost a necessity at highly Rf populated sites that I occasionally do work at, especially if you are sweeping an antenna that is in the range of other services on the site.? The exception is when the interfering signal is so strong that it overpowers the coded signal.

Joe

Hi Zack,

You said “I believe the AA-600 generates a stronger signal that is sent to the DUT. I have suspected this for some time”

The AA-600 output level is specified as approximately-10dBm into a 50 ohm load and some models of the NanoVNA have a lower output level than that but the NanoVNA-F V3.1 as an example has an output level of 0 dBm into a 50 ohm load which is considerably greater than the AA-600 and that’s one reason I went with that particular model.

Bottom line is that the output level of the NanoVNA series varies between models.

Just FYI,
Don wd8dsb

Zack W9SZ wrote:

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

Here's a way to avoid the problem at least sometimes.

I've had something similar happen and since I was measuring the properties of small parts, I used a biscuit tin as a noise chamber. I drilled a hole in the lid to fit an SMA connector and attached my fixture to the inside of the lid. I sanded the lid and tin edge to improve connectivity. With the part to be measured attached to the fixture I closed the tin and made the measurements. It worked quite well. Extending the frequency range where measurements were possible.