开云体育

On Sun, 29 Sep 2019 at 07:28, Starsekr via Groups.Io <Starsekr=
[email protected]> wrote:

On Sat, Sep 28, 2019 at 03:49 PM, Dr. David Kirkby from Kirkby Microwave
Ltd wrote:

I don’t think your simple model is really suitable for the following
reasons

1) The variation of C with homemade standards is likely to exceed that of
commercial standards - this is from experience measuring them.

2) The inductance of shorts is likely to be more with homemade standards
than commercial ones - again this is based on experience measuring them.

3) People may well want to make measurements in a 75 ohm system.

4) it is possible to improve upon the accuracy of loads at low

frequencies

by using a DC resistance measurement.

5) In the case of a female N, a simple standard can be made by just

leaving

the connector open. This will create a higher impedance transmission line
than 50 ohms as the centre conductor sits in a cylindrical section with a
greater diameter than when its mated.

6) The loss of homemade standards is likely to be greater than commercial
ones from Keysight - again this is based on actual measurements I have
performed.

Dr. Kirkby, if I understand your post, and objections to Dave Anderson's
"Very Simple Characterization Model", You are against Mr. Anderson's idea
because it doesn't account for the possibility that inductance, capacitance
and loss terms are likely to be significant in home-made standards and a
desire to have an option to change the reference resistance to account for
actual load resistance, or to use the VNA at something other than 50 ohms,
but you agree with the idea that Offset Delay should be an input option.

Yes, essentially

Dr. Kirkby, I too like the idea of being able to reference the NanoVNA with

75 ohms or measureing a home-made load with a 4 terminal system and getting
a more accurate result. But I'm not sure if your other objections are
valid (1) (2) (6), because most home-made loads won't be characterized
anyway. The operator will take the answer he gets, publish it in QST or
RadCom, and move on.

I believe implementing the full model could be beneficial for homemade kits
in *some* circumstances, such as

* Have the ability to measure homemade standards at work. I would suspect
that a fair few NanoVNA users work in the RF field.

* Know someone with a VNA able to measure them

* *Possibly* compute the properties using a software package like openEMS



* *Possibly* compute the approximately properties, then tweak them to
produce the best calibration possible by using the T-checker.



Jeff has convinced me that for the HP kits, C0 is sufficient.

I think we can all accept the possibility of making slight tweaks of the
load based on 4-wire resistance measurements, are the possibility of
working in 75 ohms.

*ONE OTHER THING I HAD FORGOTTEN ABOUT IS THE NEED TO BE ABLE TO ENTER THE
DELAY OF A THRU FOR A 2-PORT CALIBRATION *

Of course this brings up the operations of data entry and storage; which
starts taking away from the original concept of turn it on, do a simple
1-2-3 cal, and use it, so I think that option should be 1st choice in the
software.

If the firmware could

a) Define a number of calibration kits
b) Default to the most used one

then once the VNA is configured once, the rest would be a simple 1-2-3.

There seems a good argument for the VNA defaulting to the parameters of the
supplied kit (50 fF on the open, some small negative delay on the short).
But I would like to override that, as I will never use the supplied kit, as
it’s impossible to avoid rotating the male pin in the female.

Jim McEwen, KA6TPR


Dave

--

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
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