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............
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