On 9/3/2019 4:23 AM, Neal Sulmeyer wrote:
How do I make measurements on open wire feedline with my analyzer? I have a
RigExpert which has four discrete impedance settings (25,50,75, and 100 ohm).
Obviously, the connection is for unbalanced (coax) lines. A 4:1 balun would
work with window line (440 ohm), but what about open wire 600 ohm?Email sent
using Axigen Free Mail Server:
You need to have an analyzer that allows "calibration". The RigExpert
model I have allows this, however you didn't specify which
model you have, so you need to check on your particular analyzer.
Buy or build a balun that has an impedance that is in the
general range of what you want to measure, say 600 ohms.
If you want the analyzer effectively have a 600 ohm Zo,
then install the balun on the analyzer and do a calibration
using a 600 ohm resistor for the "load", along with usual
"short" and "open". The impedance setting on the RigExpert
should remain on 50 ohms. Any measurements you take with
this calibration in effect with display a reading that is
50/600 = 1/12 of the true impedance.
To measure Zo of the OWL, connect a non wirewound pot at
the far end of the line and measure the impedance at the
near end over a range of frequencies. Adjust the pot
to achieve minimum variation in impedance over frequency.
Then measure the resistance of the pot with an ohmmeter
This is the Zo of the OWL Let's say it turns out to be
550 ohms.
Knowing this, you can now re-calibrate your analyzer using
a 550 ohm resistor for "load". Now the readings on the
analyzer will read 1/11 of the true impedance. If you
display SWR, it will be the true SWR in a 550 ohm system.
It will not be scaled like impedance is.
Because of the calibration, it is not necessary to have a
"precision" balun. Any imperfections will be "calibrated
out". Except imperfections due to imbalance. A hand held
analyzer with tend to float at HF and alleviate any small
imbalances in the balun.
I don't have any details of your OWL, but generally the
usual formula (276 LOG 2D/d) will be very close to correct
for any reasonable construction. It is fairly hard
to screw it up. (Doesn't apply to window line).
Rick N6RK
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