Measuring transmission line characteristics
 

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

Measuring transmission line characteristics
 

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
--
Best Regards:
Baron.

Measuring transmission line characteristics
 

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
--
Best Regards:
Baron.


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

Measuring transmission line characteristics
 

On 11/7/2011 2:59 PM, Baron wrote:
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


Good thoughts, Baron and J.C. Many thanks.

73,
John KD5YI

Measuring transmission line characteristics
 

John S wrote in :

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?

I am a little confused about your objective. The subject line seems
inconsistent with your discussion.

If you are trying to measure Z at the reference plane in the simplest
manner, then what you propose in your last par should give you the
magnitude and phase of the reflection relative to a s/c (where
Gamma=-1).

This simple paired measurement of the reflected wave from a s/c and
unknown load depends on the forward wave being constant. That is true if
the Thevenin source impedance of the source at the coupler is equal to
the nominal characteristic impedance of the coupler and the coupler
terminations, cables etc that you use. That would usually be met by a
standard signal generator etc, but some SSGs depart from ideal on their
highest output settings (check the specs). Measuring the forward wave
under significantly different loads will provide an indication as to
whether you can assume that it remains constant with different loads.

I give an explanation of why Vf is constant when Zs=Zo at
http://vk1od.net/transmissionline/VSWR/Zs50.htm .

You will recall that there is an ongoing argument that a ham transmitter
is well represented as a Thevenin source with Zs=50+j0 ohms, as some
accident of design. The article describes a simple test using an
accurate directional wattmeter to demonstrate that under different drive
level and different frequencies, that Vf is often not necessarily
independent of load impedance and that calculations that depend on
constant Vf (such a Mismatch Loss) are in error.

Owen

Measuring transmission line characteristics
 

Owen Duffy wrote in news:Xns9F9850D3A83Anonenowhere@
88.198.244.100:

If you are trying to measure Z at the reference plane in the simplest
manner, then what you propose in your last par should give you the
magnitude and phase of the reflection relative to a s/c (where
Gamma=-1).

I should have explained that this simple approach assumes the coupler etc
to be ideal, the measurement instrument accurate etc.

We make those assumptions in using something like a Bird 43 to find VSWR,
you could use your test setup with comparable accuracy to find more than
just the magnitude of Gamma (and hence VSWR), but also its phase and
therefore complex Z.

Owen

Measuring transmission line characteristics
 

On 11/8/2011 7:29 AM, Owen Duffy wrote:
John wrote in :

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?

I am a little confused about your objective. The subject line seems
inconsistent with your discussion.

If you are trying to measure Z at the reference plane in the simplest
manner, then what you propose in your last par should give you the
magnitude and phase of the reflection relative to a s/c (where
Gamma=-1).

This simple paired measurement of the reflected wave from a s/c and
unknown load depends on the forward wave being constant. That is true if
the Thevenin source impedance of the source at the coupler is equal to
the nominal characteristic impedance of the coupler and the coupler
terminations, cables etc that you use. That would usually be met by a
standard signal generator etc, but some SSGs depart from ideal on their
highest output settings (check the specs). Measuring the forward wave
under significantly different loads will provide an indication as to
whether you can assume that it remains constant with different loads.

I give an explanation of why Vf is constant when Zs=Zo at
http://vk1od.net/transmissionline/VSWR/Zs50.htm .

You will recall that there is an ongoing argument that a ham transmitter
is well represented as a Thevenin source with Zs=50+j0 ohms, as some
accident of design. The article describes a simple test using an
accurate directional wattmeter to demonstrate that under different drive
level and different frequencies, that Vf is often not necessarily
independent of load impedance and that calculations that depend on
constant Vf (such a Mismatch Loss) are in error.

Owen

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.

Has this made any sense?

John

Measuring transmission line characteristics
 

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


--
"Everything from Crackers to Coffins"

Measuring transmission line characteristics
 

John S 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 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.

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 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.)

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

Measuring transmission line characteristics
 

Owen Duffy wrote in news:Xns9F98507D98D82nonenowhere@
88.198.244.100:

....
(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.)

Agilen't AppCad can be a convenient tool for one-off calcs.

Nevertheless, there is value in see the results of measurement as you make
them, it helps to minimise the chance of leaving the task with errored
data.

Owen