[TowerTalk] 4 port network analyzers for balun measurements

[Michael Tope]

On 11/21/2015 10:36 AM, Jim Lux wrote:
> 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.

It would be interesting to change ground plane distance and lead length 
to see if the results were consistent over a broad range or super 
sensitive to ground plane spacing and lead length. The former would 
suggest a robust measurement.

BTW, it seems one could get a good feel for common-mode performance 
using E73M's back-to-back method by alternately shorting one of the two 
balanced nodes to ground. This is especially important when power 
testing as it provides a measure of how much power the balun can handle 
when subjected to maximum common mode stress.

73, Mike W4EF............