I searched all over the Internet and many books but just could not find
a formula or rough number of a wire antenna's impedance (not dipole or
anything else, just a simple, plain wire). I need this number to match
my small transmitter's final stage output, about 10mW, at 450MHz. The
final stage's transistor has fT of 6GHz, and is not unconditionally
stable at 450MHz. So I need to match it using Smith Chart. I know those
portion of work. But I just don't know the wire's impedance's range,
say, is it in the 50-80 ohms or in the 500-600 ohms range or even
1000-2000 ohms? Right now I do not guess this number right, and my
transmitter seems always oscillating at a wrong frequency. So take an
example, if I use a wire antenna, say, 22 AWG, spools of solid, and the
length=wavelength/4, what is its approximate impedance? Thanks.

wrote in message
ups.com...
I searched all over the Internet and many books but just could not find
a formula or rough number of a wire antenna's impedance (not dipole or
anything else, just a simple, plain wire).

Have you tried "Antennas for All Applications" by Kraus
(http://www.amazon.com/gp/product/007...1652?ie=UTF8)?
It has plenty of plots of input impedance for various antennas. Any
university library will have some edition of Kraus' book; it's been around for
decades.

I'm about 99% certain the ARRL handbook has plenty of antenna impledance plots
as well.

But I just don't know the wire's impedance's range,
say, is it in the 50-80 ohms or in the 500-600 ohms range or even
1000-2000 ohms?

For most antennas at least in the ballpark of resonance, you'll be in the
30-300 ohm range. Real antennas often have tens to hundreds of ohms of
reactance as well, though.

So take an
example, if I use a wire antenna, say, 22 AWG, spools of solid, and the
length=wavelength/4, what is its approximate impedance?

In the ballpark of 72 ohms assuming it's a dipole (lambda/4 on either side);
if you're operating lambda/4 above a ground plane, it'll be roughly half that.

---Joel Kolstad

I believe the demo (free!) version of EZNEC will answer your
questions. Google for it. One of the most critical aspects of any
antenna is where it is with respect to ground.
John W8CCW

On 7 Jul 2006 12:29:54 -0700, wrote:

I searched all over the Internet and many books but just could not find
a formula or rough number of a wire antenna's impedance (not dipole or
anything else, just a simple, plain wire). I need this number to match
my small transmitter's final stage output, about 10mW, at 450MHz. The
final stage's transistor has fT of 6GHz, and is not unconditionally
stable at 450MHz. So I need to match it using Smith Chart. I know those
portion of work. But I just don't know the wire's impedance's range,
say, is it in the 50-80 ohms or in the 500-600 ohms range or even
1000-2000 ohms? Right now I do not guess this number right, and my
transmitter seems always oscillating at a wrong frequency. So take an
example, if I use a wire antenna, say, 22 AWG, spools of solid, and the
length=wavelength/4, what is its approximate impedance? Thanks.
John Ferrell W8CCW

Thank you both above. But as I mentioned, I only wanted to know the
impedance of a simple, plain wire. No dipole, no complex antenna
structure. Please help.

As the other posters pointed out, it depends on it's length, where it is
above what kind of ground, and how it is fed. It will be influenced by
stuff around it like aluminum siding, fences, trees, and all sorts of
things. Some of the influences aren't minor, unless they are all
wavelengths away.

If you have an end-fed quarter wave, which I suspect is what you are
describing, then it needs to be fed against something, most likely ground.
In that case, it will be about thirty ohms at resonance, plus the resistance
of your ground, which could be substantial. That assumes "free space", of
course, which isn't bloody likely. More likely you are relatively close to
a less than perfect ground, so the impedance could be almost anything. Even
in a perfect world, with pefect ground, the impedance varies with height
above ground.

It isn't like there is some simple answer that is being kept secret. It is
a complex question, with lots of variables. As one poster pointed out, the
best answer short of measuring it, is to model it with something like EZNEC.

You indicated your objective was to come up with some sort of impedance
transformation from a rig you are building to this antenna. Given the
opportunity for significant reactance, I would suggest you locate someone
locally with an antenna analyzer and measure it! An SWR bridge won't tell
you enough to design the matching circuit; there will be reactance, and a
2:1 SWR says IF it is all resistive, maybe it is 25 ohms, maybe 100. You
have no reason to suspect either and likely there is some reactance so
probably it is neither.

So go measure it!

...

wrote in message
oups.com...
Thank you both above. But as I mentioned, I only wanted to know the
impedance of a simple, plain wire. No dipole, no complex antenna
structure. Please help.

On 7 Jul 2006 13:46:38 -0700, wrote:

Thank you both above. But as I mentioned, I only wanted to know the
impedance of a simple, plain wire. No dipole, no complex antenna
structure. Please help.

Very dependent on length, distance from groundplane, diameter
and skin resistance of the material.

The best answer is knowing it's only a wire and no defined information
more than that is: 42!

Allison

Unfortunately, there's no such thing as a "simple, plain, wire". Any
current you put into the wire will force an equal current into the
"ground" connection. If, for example, you just plug a wire into the
center conductor of your transmitter's output connector, a current will
flow out of the connector along the outside transmitter's chassis. This
current will equal the current into your wire. So the radio, along with
the path to the Earth, becomes the other half of a very asymmetrical
dipole. You can't avoid this, except by making the return current flow
where you want, for example into the other half of a regular symmetrical
dipole.

So to simply answer your question, if the wire is very close to a
quarter wavelength and connected as I've said, the resistance can be
anything from 10 or a few tens of ohms (if it's close to the chassis) to
a thousand or more ohms (if the path to the Earth is particular
lengths). The reactance can easily be plus or minus a few hundred ohms.

That's why you don't find a simple formula. If you did find one, it
would be wrong and useless.

Roy Lewallen, W7EL

wrote:
I searched all over the Internet and many books but just could not find
a formula or rough number of a wire antenna's impedance (not dipole or
anything else, just a simple, plain wire). I need this number to match
my small transmitter's final stage output, about 10mW, at 450MHz. The
final stage's transistor has fT of 6GHz, and is not unconditionally
stable at 450MHz. So I need to match it using Smith Chart. I know those
portion of work. But I just don't know the wire's impedance's range,
say, is it in the 50-80 ohms or in the 500-600 ohms range or even
1000-2000 ohms? Right now I do not guess this number right, and my
transmitter seems always oscillating at a wrong frequency. So take an
example, if I use a wire antenna, say, 22 AWG, spools of solid, and the
length=wavelength/4, what is its approximate impedance? Thanks.

wrote in message
ups.com...
I searched all over the Internet and many books but just could not
find
a formula or rough number of a wire antenna's impedance (not dipole
or
anything else, just a simple, plain wire). I need this number to
match
my small transmitter's final stage output, about 10mW, at 450MHz.
The
final stage's transistor has fT of 6GHz, and is not unconditionally
stable at 450MHz. So I need to match it using Smith Chart. I know
those
portion of work. But I just don't know the wire's impedance's range,
say, is it in the 50-80 ohms or in the 500-600 ohms range or even
1000-2000 ohms? Right now I do not guess this number right, and my
transmitter seems always oscillating at a wrong frequency. So take
an
example, if I use a wire antenna, say, 22 AWG, spools of solid, and
the
length=wavelength/4, what is its approximate impedance? Thanks.
========================================

It is not clear what impedance you are talking about.

If you are asking what is the Zo impedance of an antenna conductor
considered as a transmission line, then it is -

Zo = 60 * ( Ln( 4 * L / D ) - 1 ) ohms,

where L is length of the conductir, D is its diameter, and Ln is
natural logarithms to base e. L and D are in the same measurement
units.

You have mentioned Smith Charts in your query. Zo is usually in the
region of 300 to 600 ohms for wire antennas. The above formula is
approximate and is good enough for ordinary purposes.
----
Reg.

Thanks, everyone. But you guys still made things complicated. I said it
is just wire antenna, only for a very small output power, 10mW. In
order to eliminate the confusion or misunderstanding, let me put some
pictures to illustrate:

(1) This transmitter, 10mW, at 450MHz, wire length(lambda/4 for
450MHz)=16cm. What is the wire antenna's impedance?
http://www.charto.info/antq1.jpg
(2) This transmitter, 10MW, at 450MHz, Rod Extendable Antenna is
extended exactly as 16cm(lambda/4 for 450MHz). What is its antenna's
impedance?
http://www.charto.info/antq2.jpg

Given that such specific conditions are as shown in the pictures, is it
still difficult to tell, even for a rough number within a range such as
50 to 80 ohms, or 500-800 ohms?

wrote:
Thanks, everyone. But you guys still made things complicated.

No, they didn't. Antennas themselves are complicated. Formulas exist only
for specific well-defined environments, none of which fits your
configuration close enough to be meaningful.

I said it
is just wire antenna, only for a very small output power, 10mW.

Output power has nothing to do with antenna impedance.

In
order to eliminate the confusion or misunderstanding, let me put some
pictures to illustrate:

(1) This transmitter, 10mW, at 450MHz, wire length(lambda/4 for
450MHz)=16cm. What is the wire antenna's impedance?
http://www.charto.info/antq1.jpg
(2) This transmitter, 10MW, at 450MHz, Rod Extendable Antenna is
extended exactly as 16cm(lambda/4 for 450MHz). What is its antenna's
impedance?
http://www.charto.info/antq2.jpg

Given that such specific conditions are as shown in the pictures, is it
still difficult to tell, even for a rough number within a range such as
50 to 80 ohms, or 500-800 ohms?

Yes, it is. And, whatever number you are given will not necessarily be
correct from one time to another. The impedance will change if you pick up
the radio or the camera. The impedance will change if you change position to
be near another object.

Suppose you are told that the impedance will be 20-j35? What will you do
about it? Since you feel that the expert advice you received simply
complicated the issue, one can only assume that you do not have the
knowledge to cope with an answer.

Go to your local ham club and find somebody with an antenna analyzer. Put a
quarter-wave wire into the connector and wander around while recording the
maximum resistance and reactance. You can see it change all over the place.
No, it doesn't go from zero to infinity. The impedance will be different
from that when the antenna is installed on your actual object.

Here is a number. Do with it what you will.... Z ~ 35-j35. If you don't like
this number, get EZNEC and model your own antenna. Or, get your own analyzer
and measure your wire. Be sure to include hand and body capacitance in your
model. Also be sure to model your tripod, soil conditions, nearby trees, the
object enclosure, etc.

Good luck.

John