Antenna Input Impedance
 

Merry Christmas and a Happy New Year!

I am curious about the setup of measuring the input impedance of an
antenna. Is there any standard setup for this? What equipments are use?
Any pictures of a an example setup configuration?

I was talking to friend and I told him that you can probably use a
vector analyzer to measure the input impedance at a certain frequency
sweep. But I came to thinking that objects / persons around the antenna
during measurement would affect its impedance due to stray capacitance.
This is strictly referring to the vector analyzer fixed in a school lab
setting.

Any additional info would be greatly appreciated.

Thanks!

Antenna Input Impedance
 

wrote:
I am curious about the setup of measuring the input impedance of an
antenna. Is there any standard setup for this? What equipments are use?
Any pictures of a an example setup configuration?

It depends upon the required degree of accuracy. If you have
a perfectly characterized 1/2WL of feedline, you can measure
the antenna feedpoint impedance remotely within some degree
of accuracy. For what purpose is the measurement being made?
--
73, Cecil http://www.qsl.net/w5dxp

Antenna Input Impedance
 

Though what you say about measuring through a feedline is true, is
there some reason you limited it to 1/2 wavelength? I regularly make
measurements through an arbitrary length of transmission line--though
one key is that the line not have too much loss. And of course, in
making antenna measurements, it's important to realize that the
feedline you use for measurement (just as the one you use to feed the
antenna) may couple to the antenna and become part of the antenna.
There may be some advantage in using the same line for both measurement
and using the antenna as an antenna.

For the original poster: yes, an antenna system consists of what you
thought was the antenna, plus its environment. People around the
antenna, or big vector network analyzers around the antenna, or
feedlines, or masts--any of that may affect the measured impedance. On
the other hand, if you measure the antenna in the environment in which
it will be used, assuming you make accurate measurements, you should be
fine. You can even experiment to see just HOW MUCH effect a person
near the antenna will have. Modern analyzers make this very easy and
quick. Ah, for the "good old days" of hand-balanced bridges, measuring
one frequency at a time! But what people actually use to make the
measurement depends on the equipment they have available. Do you have
specific equipment in mind?

Cheers,
Tom

Antenna Input Impedance
 

HI Cecil,

For what purpose is the measurement being made?

I think he was trying to make an FM antenna for his transmitter. We
were talking about how he can design a matching network to match his
antenna to his RF output. I told him that he should pick the frequency
with the lowest reflection coefficient and design his matching network
base on the antenna input impedance at that frequency.

Does this sound valid?

Antenna Input Impedance
 

K7ITM wrote:
Though what you say about measuring through a feedline is true, is
there some reason you limited it to 1/2 wavelength?

I was thinking about my 80m dipole at the time.
--
73, Cecil http://www.qsl.net/w5dxp