On Mon, 28 Oct 2019 at 15:37, Rich NE1EE <NE1EE@...> wrote:
New to VNAs, I picked up a NanoVNA after a fellow ham showed one around
at a hamfest. (I think I got "lucky" and got a good one.) I've been
following my nose, learning as I go, and studying various technical VNA web
sites. I came to wonder why all the NanoVNAs that I see advertised come
with at /most/ one short, one open, one load test fixture. It seems to me
that we /need/ 2 of each to do a decent cal...or is what we are doing
actually a valid, but "incomplete" cal...we have actually only calibrated
Port 1 (Ch0), and I am not sure about the implications of that. There are
no instructions with my device, so I have been reading docs and messages
here.
I am not following you 100%, but just because the VNA has two ports,
doesn¡¯t mean you need two calibration kits. However, you do in general want
male and female standards.
In ideal wor you would have the following.
* A set of a set of male standards (open, short, load), as well as *one
extra male load* for an isolation measurement.
* A set of a set of female standards (open, short, load), as well as *one
extra female load* for an isolation measurement.
* Female-female adapter, so you can do a thru calibration with cables
having two male ends where you connect the device. *This is a common
occurrence. *
* Male-male adapter so you can perform a thru measurement is the cables
connecting to the DUT have two female ends, which is *very rare*.
What is supplied with NanoVNAs appears to be a subset of those, to keep
costs down, on what is an *amazing value for money*. For example, you can
combine the male load and a female-thru to get a female load. Exactly what
can be done, depends on exactly what parts you have been supplied.
The extra loads I mentioned do not have to be good loads. In fact, in
professional VNAs an isolation measurement is rarely desirable, and no
extra loads are supplied. The NanoVNA does however benefit from an
isolation measurement.
One of the issues that I see as hams do various "tests" and "demos" is
that those activities seem to be not well understood by those executing
them or those reading|observing them. Because those activities are
presented with a modicum of authority, they are repeated around the ham
community, leading to confusion (I observe).
There¡¯s a *lot* of rubbish written about VNA calibration kits.
My own company
designs, builds and sells calibration kits - we have sold the to many
places, including the US military . I can assure you that there¡¯s a *lot*
of junk written about VNA calibration.
Someone mentioned on here the other day about a good YouTube video. I
looked at it and pointed out it was *seriously flawed*. The author is on
this list, and between me and someone else on the list, we managed to
convince him that the advice he was given was just plain wrong.
One of my questions is this: if I cal with only one set of short and load
fixtures, then is the through valid? And does it really matter at
frequencies below, say 600 MHz?
At 600 MHz you should be concerning yourself with these issues. There are
some limitations in the NanoVNA firmware which are stopping ideal
calibrations, but the I think these will be addressed.
Most of the stuff that I do as a ham does not seem to need super precision.
OTOH, I want to use this device in the best way possible, just as I do all
my gear, including test gear. I've ordered a 2nd NanoVNA from hugen (or so
I hope), and it will come with a 2nd set of cal fixtures.
All you really need is an extra load for the isolation measurement, which
doesn¡¯t need to be a good load.
Please understand that you don¡¯t need to have as many calibration standards
as ports on the VNA. I think you are under the impression that you need two
opens and two shorts because you have two ports. That is simply wrong. Lots
of professional VNAs have 4 ports, and several can be configured with more
- certainly 32 port VNAs have been produced. But only one calibration kit
is required.
--
On the banks of the Piscataqua
Rich NE1EE
Dave, G8WRB
(not too far from the banks of the river Crouch ????)
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...
Telephone 01621-680100./ +44 1621 680100
Registered in England & Wales, company number 08914892.
Registered office:
Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United
Kingdom
