John S wrote in :

....

The manual for the Fluke generator says it is 50 ohms output Z. I used

Ok, be aware that sometimes that impedance is gauranteed to less than
full output.

If it is 50+j0, then your Vf reading should not change in magnitude with
load variation. If it does change, you have to factor it into the calcs
as in the AN.

your Line Loss Calculator to find that my 2.4m coax is 50-j0.1 and
0.671dB loss.

Since it seems you are applying the cal short at the load end of that
line section, then its loss is not so important (so long as it is
stable).


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

....
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.

Ok, that sounds fine. If find it helps to measure something that is
known. Do you have a pair of 50 ohm terms and a T that you can measure
and convince yourself that it is working.


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.

Sounds fine.


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.

Ok, but it remains a potential problem.

Owen

On 11/8/2011 4:15 PM, Owen Duffy wrote:
John wrote in :

...

The manual for the Fluke generator says it is 50 ohms output Z. I used

Ok, be aware that sometimes that impedance is gauranteed to less than
full output.

If it is 50+j0, then your Vf reading should not change in magnitude with
load variation. If it does change, you have to factor it into the calcs
as in the AN.

your Line Loss Calculator to find that my 2.4m coax is 50-j0.1 and
0.671dB loss.

Since it seems you are applying the cal short at the load end of that
line section, then its loss is not so important (so long as it is
stable).

Okay! This is another answer for which I was hoping. This simplifies
everything.

Thanks again, Owen.

John

On 11/8/2011 4:15 PM, Owen Duffy wrote:
John wrote in :

...

The manual for the Fluke generator says it is 50 ohms output Z. I used

Ok, be aware that sometimes that impedance is gauranteed to less than
full output.

If it is 50+j0, then your Vf reading should not change in magnitude with
load variation. If it does change, you have to factor it into the calcs
as in the AN.

I just looked back at some of my data and I see that the Vf changes
slightly with some loads. However, this variation is used in my Excel
spreadsheet to calculate the results. All is according to the HP app note.

John

Dear John S: My comment of a few days ago was written under the assumption
that you wished to know the characteristics of the TL. It has developed
that you wish to measure the impedance of an antenna at the end of a piece
of coax.

The procedures for removing the effects of a fixture, which I expect to be
in the AN and require a short, an open, and a known load, may also be
applied to a piece of coax. The result is the ability to remove the
presence of the coax and be left with the antenna's impedance.

I do this in an automated way with the "calibration" feature of the AM4170.
One scans a span of frequencies three times storing information about what
the coax looks like with a short, an open, and (usually) 50 ohms at the far
end. This takes some time depending on the size of the frequency span and
the step size. The stored numbers (stored in a very inexpensive, tiny
computer) are then used automatically to calculate (over the same span) the
actual Z at the end of the coax.

Doing this procedure by hand, which I have done using the numbers measured
by a GR bridge, is labor intensive. Before the advent in 1972 of the HP35,
it was real pain. One did not use very many frequencies! It appears that
you are interested in only one frequency.

Comments about choking common mode are right on target. Owen Duffy is one
of the good guys.

73, Mac N8TT

P.S. Do you have a call sign?

J. C. Mc Laughlin
Michigan U.S.A.
Home:

On 11/9/2011 7:17 AM, J. C. Mc Laughlin wrote:
Dear John S: My comment of a few days ago was written under the
assumption that you wished to know the characteristics of the TL. It has
developed that you wish to measure the impedance of an antenna at the
end of a piece of coax.


Yes, I know. Sorry for the misleading subject line. I should have
presented my query differently.


The procedures for removing the effects of a fixture, which I expect to
be in the AN and require a short, an open, and a known load, may also be
applied to a piece of coax. The result is the ability to remove the
presence of the coax and be left with the antenna's impedance.

I do this in an automated way with the "calibration" feature of the
AM4170. One scans a span of frequencies three times storing information
about what the coax looks like with a short, an open, and (usually) 50
ohms at the far end. This takes some time depending on the size of the
frequency span and the step size. The stored numbers (stored in a very
inexpensive, tiny computer) are then used automatically to calculate
(over the same span) the actual Z at the end of the coax.

Doing this procedure by hand, which I have done using the numbers
measured by a GR bridge, is labor intensive. Before the advent in 1972
of the HP35, it was real pain. One did not use very many frequencies! It
appears that you are interested in only one frequency.

Comments about choking common mode are right on target. Owen Duffy is
one of the good guys.

Yes, he is. I've had some private correspondence with him and found him
to be very knowledgeable and helpful. I check his Web site often.

73, Mac N8TT

P.S. Do you have a call sign?

J. C. Mc Laughlin
Michigan U.S.A.
Home:

Thanks, J.C.

73,
John KD5YI