[TowerTalk] 4 port network analyzers for balun measurements

[Jim Lux]

On 11/21/15 9:50 AM, Michael Tope wrote:
> On 11/21/2015 8:22 AM, Daniel Danny Horvat wrote:
>>>>> Placing an "open" wound 2.4" toroid BALUN on/near a ground plane will
>> affect the real performance of that BALUN which is not the case with
>> common
>> "enclosed" binocular BALUNS used in industry.
>>
>>   Just my $0.02
>>
>> Danny E73M
>>
>>
> This is a good point. If the cores are sitting close to a ground plane
> will the small asymmetries between the conductors in the bifilar turns
> and the ground plane be large enough to skew the results of a 4 port
> measurement toward a misleading conclusion about balun performance?
>



like with measuring antenna traps, I suspect fixturing is everything..

But thinking about measuring a Balun.. say you supported it in mid air. 
  Port 1 of the VNA goes to one of the balanced terminals, Port 2 goes 
to another.  Ports 3 and 4 (or just 3) go to the other end of the Balun.

All the port shields are connected together (just the plate is some 
distance away)

So when you're measuring Port 1 to Port 3, Port 2 is terminated into a 
load, so the current path for port 1 is a combination of Port 1 wiring 
to the surroundings, and port 1 wiring to the port 2 wiring.

I suppose that when you flip it around, and terminate port 1 and test 
with port 2, the situation is reversed.

the farther the DUT is from the "ground plane" the less effect there 
would be from asymmetry in construction or position.

Just how much of the measurement would be from "wire from connector to 
balun" and "balun to ground plane".

if it's, say, 1 foot from connector to balun, the wire has an inductance 
of about 0.3 uH.  that's about 50j ohms at 30 MHz, so it's big, but not 
ungainly huge.

I guess you could make a calibration "balun" with suitable resistive 
terminations inside a balun like package, so you could move the 
reference plane to the balun, also.  Do your usual SOLT cals at the 
balun fixture terminals.