[TowerTalk] 4 port network analyzers for balun measurements (was: Re: Myantennas)

[Richard (Rick) Karlquist]

On 11/21/2015 4:53 AM, Jim Lux wrote:
> On 11/21/15 4:04 AM, Richard (Rick) Karlquist wrote:

>> Not with just "any network analyzer", like the 8711; you need a 4 port
>> network analyzer, such at the Agilent E5071C with 4 port option.

> Isn't there a process (perhaps tedious and manual) to do this with a 2
> port analyzer?  Where you connect each pair of two ports in turn (so
> there's 6 of them AB, AC, AD, BC,BD, CD) and then in each configuration
> connect a short, open, load (or any 2 or 3 known Z) to the other ports.
>   Then you can solve algebraically for the full 4 port matrix.

Yes, that is correct.  Once you calculate 
s11,s12,s13,s14,s21,s22,s23,s24,s31,s32,s33,s34,s41,s42,s43 and s44
as you described, you then need to convert them to common
mode and differential mode two port s parameters.

And the "secret" is that most "4 port"
analyzers are just 2 port analyzers with a switch matrix
attached to the 4 external ports to do exactly what you said.
The 4 ports, like the programmable port impedances, are just
a mathematical abstraction.  Some of Agilent's competitors
have made a big deal about having a true balanced source,
but I have not heard a convincing argument about why that
is necessary for reasons other than marketing.
>
> Given the number of inexpensive two port analyzers out there these days,
> it might be worth figuring out how to do this kind of measurement

Yes, I keep waiting for someone to market a cheap 4 port analyzer,
since "it's just software".


> Just out of curiosity, how do you connect the 4 ports to a Balun, which
> is sort of a 3 terminal device?  Where do the "ground/shield"
> connections go?

You should fixture the balun under test on a ground plane with
4 connectors grounded to that plane.  The 4 analyzer cables go
to these connectors.  You don't connect anything from the DUT
to the ground plane.  Alternately, you can do a 3 port measurement
in which case the unbalanced port goes directly to the analyzer.
Again, the Agilent software walks you through the setup possibilities.


Rick N6RK