Jim
I calibrate a coax pig tail that connects to my antenna system (50ft + UnUn
+ Wire + Counterpoise). That way, I eliminate a known good out of my
measurement by “calibrating out” the known good. I have also worked my way
out of diagnosing the antenna system by measuring the window connection,
coax then with unun + antenna + wire. Not sure if this is best practice or
where I learned it. Just seems to make logical sense. How are others doing
it? From posts I have read, it seems people just wing it without any
logical step thru of the antenna system!
On Mon, Jan 30, 2023 at 9:14 AM Jim Lux <jimlux@...> wrote:
On 1/30/23 3:42 AM, PDXer wrote:
Does it need new calibration every time when different frequency range
is selected?
Or is it enough just one initial full freq. calibration?
That depends on the level of accuracy you want. If you do a full range
cal, and then set a narrower range for the measurement, it interpolates
from the full range cal. That may or may not be great (particularly at
the points where it changes which harmonics to use, at N*300 MHz points)
If you cal over, say, 0-900, and there's 100 points, that means there's
a calibrated point every 9 MHz. Leaving aside the "transition points
between harmonics" it's likely that the device's internal errors (what
you're calibrating out) smoothly vary in amplitude and phase between
points.
OTOH, if you are calibrating with a fixture or coax in the system (e.g.
you've got 50 ft of coax, and you're doing the cal at the end of the
coax, to measure your antenna at the feedpoint) then you potentially
have a problem -That 50 ft of coax has multiple wavelengths (and a
different number of wavelengths, depending on frequency) - and 9 MHz
between calibration points may have one or more phase wraps. Now,
linear interpolation might not work so well.
The other thing where you definitely want to recalibrate for the "range
of use" is where you want the best possible measurements. Why
interpolate if you can use real data?
For what it's worth, I cal 0-100 MHz (because it's easy) when working
with HF stuff.
And, if you're using an application (like NanoVNA-Saver, and others)
that does multiple sweeps to get more points in the range, then you can
calibrate with fine resolution over a wide range.
--
73, ko4aqf@...
