On 11/8/2011 2:54 PM, Owen Duffy wrote:
John wrote in :

...

If you are not concerned with trying to calibrate out the directivity of
the coupler (and if that is greater than the expected / tolerable Return
Loss, you don't need to do so), and you have convinced yourself that Vf
is independent of load impedance (as it will be if Zs=50+j0 and you use
short low loss line, or a large attenuator at the coupler to control
Zs), then the simple approach is to do the following.

I have convinced myself of nothing. Hence, my questions here.

I measured the coupler several years ago:

Narda Dual Directional Coupler...

435 MHz

Forward direction:
Coupling = -33.1 dB
Directivity = -56.8 dB

reverse direction:
Coupling = -33.5 dB
Directivity = -74.7 dB

The manual for the Fluke generator says it is 50 ohms output Z. I used
your Line Loss Calculator to find that my 2.4m coax is 50-j0.1 and
0.671dB loss.

Nice numbers, but I don't know what to do with them.

And you understand that the Gamma found is at the reference plane (the
plane of the calibrating s/c), and you can adjust it, or the calculated
impedance to another point on a known feedline using the well known
Telegrapher's Equation (http://www.vk1od.net/calc/tl/tllc.php solves
this problem for a range of popular lines), albeit subject to error due
to uncertainty about the known line.

I understand very little.

The Fluke gen feeds the Narda dual directional coupler. The coupler
output has the 94 inches of RG-142B/U attached. The coax has all the
ferrite cores in my possession slipped onto it to moderate common mode
current. The vector voltmeter A input is attached to the forward coupler
sampling port and the voltmeter B input is attached to the reverse
coupler sampling port.

I put the best short circuit I can muster on the far end of the coax and
set the vector voltmeter to read 180 degrees. I record the A and B
voltage inputs.

(I did consider at one stage extending TLLC to allow specification of
mismatch in terms of Gamma, rectangular and polar, but no one ever asked
for it and I thought it not in demand. The complication is that finding
Z from Gamma needs to use the nominal Zo of the test equipment, not the
actual Zo of the lossy transmission line. I usually use a spreadsheet to
perform the calcs, Excel can handle complex numbers using the COMPLEX
and IM* functions either in the Analysis Tookpak in earlier versions, or
built in to the later versions.)

I put the antenna on the far end of the coax. I read the A and B
voltages and the angle between them. I use Excel to calculate the
results as indicated in the HP AN-77 app note.

An important thing to keep in mind is that while the measurements you
make are of the TL in differential mode, it may be carrying significiant
common mode components which will affect the differential currents. In
making your measurements, if you change the common mode current path
from the normal system configuration, you are measuring a different
system and the results might not apply. There seems an unwarranted
assumption in most discussion of such measurement projects that there is
inisignificant common mode current.

Has this made any sense?

Perhaps it is my turn to ask.

Owen

I am aware of the common mode current problems and I do everything I can
to minimize them. I don't know if I am successful, but I test the
effectiveness of my efforts by running my hand up and down the coax and
watching the voltmeter. I haven't been able to get all variation out,
but some of the variation is due to my hand proximity to the antenna itself.

Back to you.

John