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Here is a quick check - put the 50 ohm load that you used for calibration
and run the same scan as you did for the tri-band antenna. I have had
several dual band and tri-band antennas over the years and have generally
found the dual band will have acceptable VSWR on both bands, but the
tri-bands will generally be ok on 2m/440 and high on 220 or ok on 2m/220
but high on 440. I suspect that for VHF/UHF tri-band antennas to work well
on all three bands is not an easy thing to do.

--
David S

Hi Jack,

I'm very familiar with Ed Fong's Tri-band antenna and I'm pretty sure that
there is only one model which consists of a dual-band J-pole for 2m/440 and
a helical wound, center-fed dipole for 220 inside a PVC pipe. The J-pole
itself is very robust but some care needs to be taken when taking apart or
putting together the antenna in order not to bend the 220 dipole.

However Comet makes several different models of Tri-band antennas (and not
all of them for same 3 bands).
I own one each of the following 2m. 220, 440 Comet Tri-band antennas:
CX-333, SBB-224, SBB-224NMO.
The CX-333 is my base station antenna and installed on top of a 10ft
antenna mast (putting the top of the antenna 20ft above the roof). I use
the SBB-224 and SBB-224NMO for portable operation (the former with a mast
mount with radials and the latter with a magmount on vehicles).

Regardless of model, the Comet antennas are very different designs
(end-fed vertical dipole) from the Ed Fong antenna and therefore different
results are to be expected. That part of your question is therefore easily
answered with a very definite: Normal!

I'm getting a VSWR of less then 3:1 on all bands for all of those antennas
when measured under good conditions. This means that I'm doing the
measurement outdoors and that there is nothing in the proximity of the
antenna that would react and change the results (the antenna is on top of
the 18ft MFJ-1919EX tripod+mast or on a mag-mount in the center of the car
roof). The "somethings" that commonly effect the result of antenna
measurements are people, wires (including the feedline) and trees.

I'd consider a 5:1 result as something that warrants further investigation
to find out why it happened.

For my purposes a "close enough" result is sufficient and therefore don't
put too much effort into increasing precision in my VSWR measurements.
If you are concerned about accuracy, be sure to upgrade the firmware of
the NanoVNA so that it includes the enhancements made over time (such the
number of steps used during calibration and measurement). While the
software is very good in interpolating, the most accurate results are
obtained when using the same steps (number of steps and frequency range)
for calibration and measurement and to keep the frequency range as small as
the desired measurement allows.
If 2m/220/440 tri-band antennas are what you commonly test, consider
saving a wide-band calibration in C0 and then one band-specific calibration
for each band of interest saved to C1,C2 and C3. Don't forget to recall the
appropriate calibration before switching to measure a different band.

73,
Thomas

Hi, new user so thanks for your patience.

Calibrated and confirmed appropriate procedure per reference videos.
Did a wide sweep, and then resonant sweeps for two different triband
antennas and found: good VSWR for two bands (~1.2:1 , high VSWR (±5:1) for
other two bands, both antennas on different bands (one was high on 75cm,
the other 220(?)). So two different bands had one high SWR, and two low SWR
readings on _different bands_. (Comet vs. Fong)

Would the reason be calibration, normal, or user error? Thanks!