Measuring transmission line characteristics
 

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

Measuring transmission line characteristics
 

On 11/8/2011 2:37 PM, Jeffrey Angus wrote:
On 11/8/2011 2:13 PM, John S wrote:
I think I am the confused one. Do I even need to know the transmission
line characteristics if I am going to short the load end and set the
vector voltmeter for a phase reference of 180 degrees?

I am following the HP app note AN-77 and they do not mention a
transmission line. They say to short the load end of the coupler. I need
to get my antenna away from the test setup, so I add the transmission
line.

The problem is that these measurements are designed to be made
as close to the instrument as possible. Adding a piece of transmission
line adds loss and phase shift to the measurement.

So unless you know how to work backwards from the measure you get to
what you're really measuring at the far end, you won't really have a
valid answer.

Jeff

Hi, Jeff -

Yeah, that's what I'm trying to learn by my questions.

Obviously, the antenna can't be sitting in front of the test equipment
so I'm trying to find a way to minimize proximity effects.

I should have used a different subject for the post, I think.

John

Measuring transmission line characteristics
 

On 11/7/2011 8:51 PM, J. C. Mc Laughlin wrote:
Dear John S: Conventional wisdom and common sense suggests that
measuring Zin (with an open and then a short at the far end) at a
frequency where the transmission line looks like an odd multiple of 1/8
WL tends to provide the best quality of measurements to be used to
characterize a piece of coax. Such measurements tend to result in two
numbers that are similar. Extrapolation to 434 MHz should provide
reasonable estimates.

The UHF version of the AIM4170 and its software will provide the values
and do the indicated calculations. Of course, one needs to select the
reasonable value (from the infinite inherently provided) for rad/m - but
that is rarely an issue. Your equipment too should be able to provide
the two values of Zin and a good HP calculator will do the rest.

Measurements near, say, frequencies where the coax looks like multiples
of 1/4 WL produce numbers that are not favorable for calculation. Baron
provides other ways to think of the task.

No doubt you know this, but others might not. 73, Mac N8TT

"Baron" wrote in message ...

John S Inscribed thus:

I have about 94 inches of RG-142B/U. I am using a Fluke 6061A signal
generator, an HP 8405A Vector Voltmeter, and a Narda dual directional
coupler. I have tried to measure the line characteristics at 434 MHz
but I am not satisfied that the results are accurate. It is very
difficult to get good short and open circuits at this frequency and I
also wonder if the 8405A accuracy suffers since a short is well away
from the nominal system impedance of 50 ohms.

What if I simply calibrate the 8405 with a short on the end of the
line (the measurement plane) then attach my antenna and accept the
readings? Will they be very far from the real value?

Thanks,
John KD5YI

The easiest way to get the characteristics of the line is to look up the
manufacturers data. Somehow I don't think that this is really what you
are looking for !

Irrespective of line length if its terminated in its characteristic
impedance then you will only measure unity vswr. Open or short circuit
terminations are easy enough to obtain. Having a known input quantity
and measuring the return value will give you the line loss for that
particular line length.

I suspect that its actually the antenna characteristics that you are
seeking to measure ! In which case I would use a line, accurately cut,
to be number of half waves long, then the impedance presented at the
far end would be repeated at the near end. Of course you would need to
have an accurately cut quarter wave length in order to determine
whether the load was inductive or capacitive in nature.

I'm sure that if I'm mistaken some of the more knowledgeable will
correct my errors.

HTH

Yeah, well, sometimes I get turned around in my quests and lose my way.

It is the antenna characteristics I am after. What I want to know is, do
I need to know the transmission line characteristics which I use during
the test in order to modify my test results to show the true antenna
impedance?

What I want to do is build an antenna based on its radiation
characteristics (as shown with EZNEC) and then measure its impedance (at
the end of a few inches of parallel conductors) so that I can put in a
matching network to give my source what it wants.

John

Measuring transmission line characteristics
 

John S wrote in :

On 11/8/2011 2:37 PM, Jeffrey Angus wrote:
....
The problem is that these measurements are designed to be made
as close to the instrument as possible. Adding a piece of
transmission line adds loss and phase shift to the measurement.

The problem is that whilst putting the unknown right at the instrument
might solve some problems, it creates others, particularly when the
unknown is an antenna system.


So unless you know how to work backwards from the measure you get to
what you're really measuring at the far end, you won't really have a
valid answer.

Well, you can work backwards as I explained, though again at the expense
of some uncertainty (error).

....
Obviously, the antenna can't be sitting in front of the test equipment
so I'm trying to find a way to minimize proximity effects.

Don't overlook that the reference plane need not necessarily at the end
of the directional coupler, it could be anywhere that you can
conveniently or inconveniently apply the calibration s/c (eg at the
antenna connector for flange as appropriate).

Try this on the bench with a substantial length of coax to the reference
plane, and measure a 25 ohm load (pair of 50 ohm terms on a Tee). The
reference plane is determined by where you apply the calibration s/c.

It was common practice using slotted lines to make such measurements
with the slotted line a long way from the reference point, which might
be quite close to the antenna, possibly a s/c applied at the antenna
flange, or on a W/G switch near the antenna.

BTW, you have read the AN, and noted no doubt the issue with error due
to the probes loading the test circuit. I would not obsess too much over
perfection unless you are prepared to go to great lengths to try and
allocate the errors and obtain a better result.

Owen

Measuring transmission line characteristics
 

John S wrote in :

....
What I want to do is build an antenna based on its radiation
characteristics (as shown with EZNEC) and then measure its impedance (at
the end of a few inches of parallel conductors) so that I can put in a
matching network to give my source what it wants.

My mention of the common mode current path is very relevant.

You have test equipment that is 'not-balanced' and a load that is balanced
or more likely 'not-prefectly-balanced' in a different way. That is likely
(certain) to cause common mode current, which means the common mode current
path directly participates in radiation, making the antenna different to
your design.

Owen

Measuring transmission line characteristics
 

On 11/8/2011 3:58 PM, Owen Duffy wrote:
John wrote in :

On 11/8/2011 2:37 PM, Jeffrey Angus wrote:
...
The problem is that these measurements are designed to be made
as close to the instrument as possible. Adding a piece of
transmission line adds loss and phase shift to the measurement.

The problem is that whilst putting the unknown right at the instrument
might solve some problems, it creates others, particularly when the
unknown is an antenna system.


So unless you know how to work backwards from the measure you get to
what you're really measuring at the far end, you won't really have a
valid answer.

Well, you can work backwards as I explained, though again at the expense
of some uncertainty (error).

...
Obviously, the antenna can't be sitting in front of the test equipment
so I'm trying to find a way to minimize proximity effects.

Don't overlook that the reference plane need not necessarily at the end
of the directional coupler, it could be anywhere that you can
conveniently or inconveniently apply the calibration s/c (eg at the
antenna connector for flange as appropriate).

AHA! I think this is the answer I was needing. So, if I short the end of
the coax and calibrate my vector voltmeter I can then believe the
voltmeter when it says my load is a certain impedance?

I don't know were I got the idea that I had to have the coax
characteristics to be used to modify the readings for accuracy.

Try this on the bench with a substantial length of coax to the reference
plane, and measure a 25 ohm load (pair of 50 ohm terms on a Tee). The
reference plane is determined by where you apply the calibration s/c.

Okay. I'll do that as soon as I can find the instruments. They are
somewhere around here.

It was common practice using slotted lines to make such measurements
with the slotted line a long way from the reference point, which might
be quite close to the antenna, possibly a s/c applied at the antenna
flange, or on a W/G switch near the antenna.

BTW, you have read the AN, and noted no doubt the issue with error due
to the probes loading the test circuit. I would not obsess too much over
perfection unless you are prepared to go to great lengths to try and
allocate the errors and obtain a better result.

Owen

I don't obsess. I'm usually happy with errors of less than 1.5 to 1
(depending on circumstances, of course).

Thanks, Owen.

John

Measuring transmission line characteristics
 

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

Measuring transmission line characteristics
 

On 11/8/2011 4:04 PM, Owen Duffy wrote:
John wrote in :

...
What I want to do is build an antenna based on its radiation
characteristics (as shown with EZNEC) and then measure its impedance (at
the end of a few inches of parallel conductors) so that I can put in a
matching network to give my source what it wants.

My mention of the common mode current path is very relevant.

You have test equipment that is 'not-balanced' and a load that is balanced
or more likely 'not-prefectly-balanced' in a different way. That is likely
(certain) to cause common mode current, which means the common mode current
path directly participates in radiation, making the antenna different to
your design.

Owen


Yes, I am aware of that problem. Perhaps I will build a current pick-up
loop and run it up and down the coax. I can use the sensitive input of
the vector voltmeter or maybe my Boonton RF voltmeter. I can then try to
minimize the common mode current.

John

Measuring transmission line characteristics
 

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

Measuring transmission line characteristics
 

John S Inscribed thus:

On 11/7/2011 8:51 PM, J. C. Mc Laughlin wrote:
Dear John S: Conventional wisdom and common sense suggests that
measuring Zin (with an open and then a short at the far end) at a
frequency where the transmission line looks like an odd multiple of
1/8 WL tends to provide the best quality of measurements to be used
to characterize a piece of coax. Such measurements tend to result in
two numbers that are similar. Extrapolation to 434 MHz should provide
reasonable estimates.

The UHF version of the AIM4170 and its software will provide the
values and do the indicated calculations. Of course, one needs to
select the reasonable value (from the infinite inherently provided)
for rad/m - but that is rarely an issue. Your equipment too should be
able to provide the two values of Zin and a good HP calculator will
do the rest.

Measurements near, say, frequencies where the coax looks like
multiples of 1/4 WL produce numbers that are not favorable for
calculation. Baron provides other ways to think of the task.

No doubt you know this, but others might not. 73, Mac N8TT

"Baron" wrote in message ...

John S Inscribed thus:

I have about 94 inches of RG-142B/U. I am using a Fluke 6061A signal
generator, an HP 8405A Vector Voltmeter, and a Narda dual
directional coupler. I have tried to measure the line
characteristics at 434 MHz but I am not satisfied that the results
are accurate. It is very difficult to get good short and open
circuits at this frequency and I also wonder if the 8405A accuracy
suffers since a short is well away from the nominal system impedance
of 50 ohms.

What if I simply calibrate the 8405 with a short on the end of the
line (the measurement plane) then attach my antenna and accept the
readings? Will they be very far from the real value?

Thanks,
John KD5YI

The easiest way to get the characteristics of the line is to look up
the manufacturers data. Somehow I don't think that this is really
what you are looking for !

Irrespective of line length if its terminated in its characteristic
impedance then you will only measure unity vswr. Open or short
circuit terminations are easy enough to obtain. Having a known input
quantity and measuring the return value will give you the line loss
for that particular line length.

I suspect that its actually the antenna characteristics that you are
seeking to measure ! In which case I would use a line, accurately
cut, to be number of half waves long, then the impedance presented at
the far end would be repeated at the near end. Of course you would
need to have an accurately cut quarter wave length in order to
determine whether the load was inductive or capacitive in nature.

I'm sure that if I'm mistaken some of the more knowledgeable will
correct my errors.

HTH

Yeah, well, sometimes I get turned around in my quests and lose my
way.

It is the antenna characteristics I am after. What I want to know is,
do I need to know the transmission line characteristics which I use
during the test in order to modify my test results to show the true
antenna impedance?

What I want to do is build an antenna based on its radiation
characteristics (as shown with EZNEC) and then measure its impedance
(at the end of a few inches of parallel conductors) so that I can put
in a matching network to give my source what it wants.

John

Surely the antenna will have some means of adjustment ie gamma match,
and since you know that if the electrical length of cable equals a
number of half waves, adjusting the antenna to show minimum VSWR at
generator end, achieves your goal...

--
Best Regards:
Baron.