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