how do you determine the impedance of an antenna?
is this something i can do with a multi-meter or is it very technical?

thanks
chuck

In article ,
"c.foster" wrote:

how do you determine the impedance of an antenna?
is this something i can do with a multi-meter or is it very technical?

You can't effectively measure it with a multi-meter.

An antenna is a 3rd to 4th dimensional problem involving electric and
magnetic fields depending on its construction.

The derivation thereof are technical but there are formulae and spread
sheets that can be followed to calculate the dimensions that make it
simple at the same time.

--
Telamon
Ventura, California

CF,

Consider one of these two Antenna Testers by "MFJ":

MFJ-204B = 1.8 - 30MHz Antenna Bridge
http://www.mfjenterprises.com/produc...rodid=MFJ-204B

MFJ-259 = HF/VHF Antenna/SWR/RF Analyzer w/ LCD, Counter & Meters
http://www.mfjenterprises.com/produc...rodid=MFJ-259B


iane ~ RHF
..
Some Say: On A Clear Day You Can See Forever.
http://groups.yahoo.com/group/SWL-AM...na/message/502
I BELIEVE: On A Clear Night... You Can Hear Forever and Beyond !
..
..
= = = "c.foster"
= = = wrote in message ...

how do you determine the impedance of an antenna?

is this something i can do with a multi-meter or is it very technical?

thanks
chuck

..

"c.foster" wrote:

how do you determine the impedance of an antenna?
is this something i can do with a multi-meter or is it very technical?

thanks
chuck

In electronics there are basically three kinds of signals, DC (direct
current), AC (alternating current) and more recently, digital. DC means
the current flows continuously in one direction but it can change in
value. AC current is constantly changing in value and periodically
reverses in direction. A digital signal can be seen as a kind of pulsing
DC where the signal is either on or off but not somewhere between those
two values or 'states'. In a DC circuit, the relationship between
voltage-V and current-I gives the resistance-R of the circuit. The
formula is V/I=R. However things are not so simple with an AC circuit,
particlarly one involving an RF (high frequency AC) signal. In that
case, the word 'impedance' is used to define a special kind of
resistance for AC signals. The impedance of an antenna is the
relationship between voltage and current at the feed point where the
lead wire is connected. This is not something which can be measured with
a typical multi-meter because it involves RF rather than pure DC or low
frequency AC. However the impedance of an antenna can be measured with
the proper equipment. In the days of vacuum tubes there was a device
called a 'grid dip meter' which could be used to evaluate an antennas
charecteristic impedance at a particular frequency. Today we have more
sophisticated solid state equipment for doing this. The first URL below
shows one of these devices. The other two addresses have more
information about antennas.

http://www.mfjenterprises.com/produc...rodid=MFJ-259B
http://www.ominous-valve.com/antennas.txt
http://digilander.libero.it/iw3sim/antennas.html


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STARMAN,

Nice Answer and Two Very Good Links:

http://www.ominous-valve.com/antennas.txt

http://digilander.libero.it/iw3sim/antennas.html

iane ~ RHF
..
..
= = = starman
= = = wrote in message ...

"c.foster" wrote:

how do you determine the impedance of an antenna?
is this something i can do with a multi-meter or is it very technical?

thanks
chuck


In electronics there are basically three kinds of signals, DC
(direct current), AC (alternating current) and more recently,
digital. DC means the current flows continuously in one direction
but it can change in value. AC current is constantly changing in
value and periodically reverses in direction. A digital signal
can be seen as a kind of pulsing DC where the signal is either
on or off but not somewhere between those two values or 'states'.
In a DC circuit, the relationship between voltage-V and current
-I gives the resistance-R of the circuit. The formula is V/I=R.
However things are not so simple with an AC circuit, particlarly
one involving an RF (high frequency AC) signal. In that case,
the word 'impedance' is used to define a special kind of
resistance for AC signals. The impedance of an antenna is the
relationship between voltage and current at the feed point
where the lead wire is connected. This is not something which
can be measured with a typical multi-meter because it involves
RF rather than pure DC or low frequency AC. However the impedance
of an antenna can be measured with the proper equipment. In the
days of vacuum tubes there was a device called a 'grid dip meter'
which could be used to evaluate an antennas charecteristic
impedance at a particular frequency. Today we have more
sophisticated solid state equipment for doing this. The first
URL below shows one of these devices. The other two addresses
have more information about antennas.

http://www.mfjenterprises.com/produc...rodid=MFJ-259B
http://www.ominous-valve.com/antennas.txt
http://digilander.libero.it/iw3sim/antennas.html


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-=jd=- wrote:

I had about 300' of #14 insulated wire strung in this "Z" shape down one
of the woodlines on my property. It really overloaded the receiver, but
with an external attenuator, it was a very useable signal snagger.

Last week, the new wore off and I decided to change it. I now have about
100' of that wire arranged in horizontal triangular loop.

The happy surprise is that the S/N ratio is much, much better.

The one thing that I would like to do is try to figure out the actual
characteristic impedance of the antenna. I've found information that shows
me how to do this with a noise-bridge, but I don't have one of those.

I was wondering if anyone knows of a link or a formula for finding a
relatively close "ballpark" figure for characteristic impedance. I'm
thinking there isn't (without the inclusion of one or more physical
measurements) due to all of the variables involved.

To add to the dificulty, the antenna is over sloping terrain and I recall
that height above terrain affects impedance.

The end goal is to match a coax feed to the antenna as closely as
*reasonably* possible. I understand that there's the distinct possibility
that after going through all the effort to determine what ratio is needed
for the transformer, a SWAG may well have produced the same or similar
results... But, it's a hobby and I have the time.

A general rule of thumb might say that for that single random wire you'd be
looking at 500 ohms, give or take.

Here's some good reading material:

http://members.aol.com/DXerCapeCod/z_transformers.pdf

http://www.dxing.info/equipment/impe...ing_bryant.pdf

Steve
Holland, MI
Drake R7, R8 and R8B

http://www.iserv.net/~n8kdv/dxpage.htm

Even if you do match it, it will be at only one frequency. At any other, the
mismatch returns.

"-=jd=-" wrote in message
...
I had about 300' of #14 insulated wire strung in this "Z" shape down one
of the woodlines on my property. It really overloaded the receiver, but
with an external attenuator, it was a very useable signal snagger.

Last week, the new wore off and I decided to change it. I now have about
100' of that wire arranged in horizontal triangular loop.

The happy surprise is that the S/N ratio is much, much better.

The one thing that I would like to do is try to figure out the actual
characteristic impedance of the antenna. I've found information that shows
me how to do this with a noise-bridge, but I don't have one of those.

I was wondering if anyone knows of a link or a formula for finding a
relatively close "ballpark" figure for characteristic impedance. I'm
thinking there isn't (without the inclusion of one or more physical
measurements) due to all of the variables involved.

To add to the dificulty, the antenna is over sloping terrain and I recall
that height above terrain affects impedance.

The end goal is to match a coax feed to the antenna as closely as
*reasonably* possible. I understand that there's the distinct possibility
that after going through all the effort to determine what ratio is needed
for the transformer, a SWAG may well have produced the same or similar
results... But, it's a hobby and I have the time.

-=jd=-
--
My Current Disposable Email:

(Remove YOUR HAT to reply directly)

CW wrote:

Even if you do match it, it will be at only one frequency. At any other, the
mismatch returns.

There are ways to get more of a broadband match, via the use of a matching
transformer.



"-=jd=-" wrote in message
...
I had about 300' of #14 insulated wire strung in this "Z" shape down one
of the woodlines on my property. It really overloaded the receiver, but
with an external attenuator, it was a very useable signal snagger.

Last week, the new wore off and I decided to change it. I now have about
100' of that wire arranged in horizontal triangular loop.

The happy surprise is that the S/N ratio is much, much better.

The one thing that I would like to do is try to figure out the actual
characteristic impedance of the antenna. I've found information that shows
me how to do this with a noise-bridge, but I don't have one of those.

I was wondering if anyone knows of a link or a formula for finding a
relatively close "ballpark" figure for characteristic impedance. I'm
thinking there isn't (without the inclusion of one or more physical
measurements) due to all of the variables involved.

To add to the dificulty, the antenna is over sloping terrain and I recall
that height above terrain affects impedance.

The end goal is to match a coax feed to the antenna as closely as
*reasonably* possible. I understand that there's the distinct possibility
that after going through all the effort to determine what ratio is needed
for the transformer, a SWAG may well have produced the same or similar
results... But, it's a hobby and I have the time.

-=jd=-
--
My Current Disposable Email:

(Remove YOUR HAT to reply directly)

Steve, if i were to mount something like that,
and bring it to the radio by coax; how would
this be grounded for static and lighting?
i would rather ground outside and not at the
radio...thanks...
Drifter...

Drifter wrote:

Steve, if i were to mount something like that,
and bring it to the radio by coax; how would
this be grounded for static and lighting?
i would rather ground outside and not at the
radio...thanks...

You can do it several different ways. Here, I use the type of
transformer that utilises a ground at the feed point. Hence, the antenna
itself is always at DC ground.

It's simply a matter of how one connects the windings. It's certainly
possible to short the one end of the winding to the coax connector, and
then just get away with a ground at the receiver.

I believe this is the method that the RF Systems MLB balun uses. Myself,
I prefer to have a ground at the feed point.

From the feed point here it's about 9' down to an 8' or so ground rod.

I also have a separate ground for the receiver(s), and I would still
recommend it's use.

I used to offer them for sale, and could build one any way the customer
wanted. It's time consuming to make them though so I jut drifted away
from it. Last time I wound one up was for a fellow in Ireland a couple
years ago, and he has good success with it. I think I sold about 275 of
them, but I just could not keep up.

If you have other questions I'll try and help you out. I'm much better
at explaining in person and if I have some exhibits to use!

Steve
Holland, MI
Drake R7, R8 and R8B

http://www.iserv.net/~n8kdv/dxpage.htm