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Re: [TowerTalk] To get a truly balanced antenna feed

To: "Richard (Rick) Karlquist" <richard@karlquist.com>, towertalk@contesting.com
Subject: Re: [TowerTalk] To get a truly balanced antenna feed
From: jimlux <jimlux@earthlink.net>
Date: Fri, 24 Jun 2016 13:07:11 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 6/24/16 12:48 PM, Richard (Rick) Karlquist wrote:


On 6/24/2016 8:08 AM, jimlux wrote:

Or they use multiport VNAs and get to the result by algebra.  If you
look at the measured test data for a minicircuits RF transformer, you'll
see full 4 or 5 port data.

http://www.minicircuits.com/MCLStore/Agreement.jsp?file=ADT1-6T_S5P.zip


I used to design network analyzers for Agilent (now Keysight).
A 4 port VNA is the gold standard for dealing with any sort of
balanced system.  I had the use of an E5071C 4 port at work
and was able to make very definitive measurements of CAT5
twisted pair with it.

A 4 port VNA actually consists of:

1.  A 2 port VNA with one source and one or two receivers.

2.  A switch matrix to connect two of the VNA ports
to various combinations of two of the four DUT ports

3.  A bunch of math to calculate common mode, differential,
and mixed S-parameters.  Also, a calibration routine.

It does NOT use any baluns whatsoever.  It never generates
pure balanced or pure common mode drive signals nor does
it have the corresponding receivers.

It is therefore possible to fake a 4 port VNA using only
a 2 port VNA if you do the various interconnections manually
and have a math routine to crunch the data.  However, the
2 port VNA needs to be a true 2 port VNA.  Many so-called
2 port VNA's should probably be called 1.5 port VNA's because
they don't have full 2 port calibration.

This is way beyond the capability of virtually all hams because
of the cost of the 4 port VNA (5 figures) and the complexity
of understanding how to use it.  Let alone faking it
with a 2 port VNA.

Rick N6RK


ya know, though..

for HF, a box full of suitable relays could make the test set for the underlying 2 channel VNA. I think that's within the scope of an ambitious amateur. I'm not so foolish to think that one can calibrate out everything from crummy relays, but there's probably some middle ground that's realizable.

The math is probably the sticky part. I started building a 4 port unit for phased arrays using the tentec VNA and some surplus coax relays, but the integration of the VNA software with the (relatively simple) software to switch the relays with the (not so simple) post processing software got to be overly complex. The VNA software for most ham VNAs didn't lend itself to being "part" of an overall measurement system, so there was a lot of trying to fool it with reloading cal files for each relay configuration.

I suspect that had I spend some substantial time on it, I could have done all the cal outside the VNA software, but that was also tricky.

OTOH, measuring mutual Z among phased array elements is probably less challenging for a VNA than characterizing a balanced transformer. There's a reason they charge what they do for N-port analyzers.

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