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

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.

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:

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

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

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

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.

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

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

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