Hi,

For a whip, much shorter than a quarter wave against a poor
ground - who knows? However, you want a number?

So here's a number; 2 - j500 and it won't be too far wrong.

The name of the game in such a situation is "Suck-it and see."

Make an intelligent guess at what the impedance is likely to
be, rig up a far-field meter and adjust the tap/link/network
until it peaks. Then go out for a curry and maybe a drink or two
or...

Alternatively, buy an antenna book and RTFM :-)


Cheers - Joe

Paul Burridge wrote in message . ..
....
I'm simply trying to establish the radiation resistance of a non-ideal
antenna so I can reasonably match it to the output impedance of the
transmitter PA stage. The figure is likely to be very low, given that
this antenna is highly non-ideal. I'm not interested in plotting polar
patterns of the radiation distribution!

Then you want to know the feedpoint impedance, not the radiation
resistance.

You can measure that with any of a variety of impedance-measuring
devices. You need to be careful to not disturb the antenna with your
measurement, and to properly couple the measurement device to the
feedpoint, taking into account the effect of the coupling on the
measurement. For example, like Wes said, if you measure it through a
length of feedline, you need to accout for the impedance
transformation performed by that line, or calibrate your measurements
at the end of that line.

You can generally get a fairly accurate number through a simulation of
the antenna, too.

Cheers,
Tom

Paul Burridge wrote in message . ..
....
I'm simply trying to establish the radiation resistance of a non-ideal
antenna so I can reasonably match it to the output impedance of the
transmitter PA stage. The figure is likely to be very low, given that
this antenna is highly non-ideal. I'm not interested in plotting polar
patterns of the radiation distribution!

Then you want to know the feedpoint impedance, not the radiation
resistance.

You can measure that with any of a variety of impedance-measuring
devices. You need to be careful to not disturb the antenna with your
measurement, and to properly couple the measurement device to the
feedpoint, taking into account the effect of the coupling on the
measurement. For example, like Wes said, if you measure it through a
length of feedline, you need to accout for the impedance
transformation performed by that line, or calibrate your measurements
at the end of that line.

You can generally get a fairly accurate number through a simulation of
the antenna, too.

Cheers,
Tom

I can see from the way you have made your enquiry you havn't the foggiest
idea about what you are trying to accomplish. Do you know the frequency?
What transmitter power output do you have in mind - 10 milliwatts or 1KW?
For starters forget all about folding anything - you've been reading the
wrong books. However you now mention a short whip above a groundplane of
unknown construction.


The input radiation resistance at the base of a very short vertical antenna,
say less than 1/10th of a wavelength, is given by -


Rrad = Squareroot( 20 * Height in metres / Wavelength in metres ) ohms.


Rrad will be in the order of a few tenths of an ohm at 2 MHz but increases
fast as the square of frequency.


In series with this radiation resistance is a high value of capacitative
reactance which has to be tuned out somewhere by a lot of micro-henries.
Best located at or near the antenna base.


For a very crude guess the input reactance will be in the order of -


Xin = -300 * Cotangent( Angle ) ohms.


where Angle = 360 * Height / Wavelength degrees.


You will then have the task of winding the correct number of turns on a coil
former, of your chosen length and diameter, to provide an inductance of
similar value of reactance as presented by the whip. Download program
SOLNOID3 for coil design.

In series with Rrad and Xin there will be a loss resistance due to the
connection to the ground plane. If the ground plane is a vehicle then you
can expect a loss resistance between 3 and 15 ohms. If the ground plane
consists of a cigarette-pack size metal plate buried in your back yard then
expect a ground loss resistance between 500 and 5,000 ohms.


Overall antenna input resistance is then Rin = Rrad + Rcoil + Rground.


If it is your intention to connect the antenna directly to the transmitter,
or via a very, very, short length of coaxial line, then Rin is the
resistance which has to be matched to your 150-ohm transmitter by using an L
and C impedance matching network.


Frankly, it may be easier to redesign the transmitter to match the antenna
;o)


But you won't get very far without an impedance measuring device such as a
borrowed, begged or stolen antenna analyser.


As I have no idea of the purpose of the transmitter + antenna I suggest you
ask around for sombody who has already solved the problem and copy his.


It may be that a very short miniature centre-loaded dipole would do the job.
It doesn't need a groundplane and can be driven via a 150-ohm balanced,
twisted-pair line and, if needed at the transmitter end, a simple 150-ohm,
1-to-1 balun.


Download program MIDLOAD and amuse yourself. It also designs the loading
coil. I KNOW it works. Been there - done that!
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........


"Paul Burridge" wrote - .
"Reg Edwards"


[snip]

Hi Reg,

It so happens the uniformly distributed radiation resistance is exactly
twice the radiation resistance of a 1/2-wave dipole when concentrated at
its
centre. So the uniformly distributed radiation resistance along a
1/2-wave
dipole is about 140 ohms. It cannot be measured. It can be calculated
from
aerial dimensions. But best just to remember the approximate number 140.
It
does depend to small extent on wire diameter and 'end-effect'.

It's annoying, because the tx output Z I'm trying to match is (by a
strange coincidence) 140 ohms! So a folded dipole would be ideal, I
guess. However - and it's a big *however* - I can't use one. I'm stuck
with a telescopic whip and a ground plane the size of a box of Swan
Vestas. I imagine the radiation resistance of such a non-ideal antenna
is pretty low, but until someone can gimme a ballpark figure for it, I
can't even begin to think about how to go about matching it. :-(


--

"I expect history will be kind to me, since I intend to write it."
-
Winston Churchill

I can see from the way you have made your enquiry you havn't the foggiest
idea about what you are trying to accomplish. Do you know the frequency?
What transmitter power output do you have in mind - 10 milliwatts or 1KW?
For starters forget all about folding anything - you've been reading the
wrong books. However you now mention a short whip above a groundplane of
unknown construction.


The input radiation resistance at the base of a very short vertical antenna,
say less than 1/10th of a wavelength, is given by -


Rrad = Squareroot( 20 * Height in metres / Wavelength in metres ) ohms.


Rrad will be in the order of a few tenths of an ohm at 2 MHz but increases
fast as the square of frequency.


In series with this radiation resistance is a high value of capacitative
reactance which has to be tuned out somewhere by a lot of micro-henries.
Best located at or near the antenna base.


For a very crude guess the input reactance will be in the order of -


Xin = -300 * Cotangent( Angle ) ohms.


where Angle = 360 * Height / Wavelength degrees.


You will then have the task of winding the correct number of turns on a coil
former, of your chosen length and diameter, to provide an inductance of
similar value of reactance as presented by the whip. Download program
SOLNOID3 for coil design.

In series with Rrad and Xin there will be a loss resistance due to the
connection to the ground plane. If the ground plane is a vehicle then you
can expect a loss resistance between 3 and 15 ohms. If the ground plane
consists of a cigarette-pack size metal plate buried in your back yard then
expect a ground loss resistance between 500 and 5,000 ohms.


Overall antenna input resistance is then Rin = Rrad + Rcoil + Rground.


If it is your intention to connect the antenna directly to the transmitter,
or via a very, very, short length of coaxial line, then Rin is the
resistance which has to be matched to your 150-ohm transmitter by using an L
and C impedance matching network.


Frankly, it may be easier to redesign the transmitter to match the antenna
;o)


But you won't get very far without an impedance measuring device such as a
borrowed, begged or stolen antenna analyser.


As I have no idea of the purpose of the transmitter + antenna I suggest you
ask around for sombody who has already solved the problem and copy his.


It may be that a very short miniature centre-loaded dipole would do the job.
It doesn't need a groundplane and can be driven via a 150-ohm balanced,
twisted-pair line and, if needed at the transmitter end, a simple 150-ohm,
1-to-1 balun.


Download program MIDLOAD and amuse yourself. It also designs the loading
coil. I KNOW it works. Been there - done that!
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........


"Paul Burridge" wrote - .
"Reg Edwards"


[snip]

Hi Reg,

It so happens the uniformly distributed radiation resistance is exactly
twice the radiation resistance of a 1/2-wave dipole when concentrated at
its
centre. So the uniformly distributed radiation resistance along a
1/2-wave
dipole is about 140 ohms. It cannot be measured. It can be calculated
from
aerial dimensions. But best just to remember the approximate number 140.
It
does depend to small extent on wire diameter and 'end-effect'.

It's annoying, because the tx output Z I'm trying to match is (by a
strange coincidence) 140 ohms! So a folded dipole would be ideal, I
guess. However - and it's a big *however* - I can't use one. I'm stuck
with a telescopic whip and a ground plane the size of a box of Swan
Vestas. I imagine the radiation resistance of such a non-ideal antenna
is pretty low, but until someone can gimme a ballpark figure for it, I
can't even begin to think about how to go about matching it. :-(


--

"I expect history will be kind to me, since I intend to write it."
-
Winston Churchill

Paul Burridge wrote in message . ..
....
It's annoying, because the tx output Z I'm trying to match is (by a
strange coincidence) 140 ohms! So a folded dipole would be ideal, I
guess. However - and it's a big *however* - I can't use one. I'm stuck
with a telescopic whip and a ground plane the size of a box of Swan
Vestas. I imagine the radiation resistance of such a non-ideal antenna
is pretty low, but until someone can gimme a ballpark figure for it, I
can't even begin to think about how to go about matching it. :-(

Paul, it would be reeeeally helpful if you'd include enough info so we
could give you a ballpark figure. What wavelength (or frequency)?
How long is the antenna? How big is that box? (Will someone be
holding it during operation?) Yeah, someone did offer a ballpark
figure, but that depends a whole lot on what fraction of a wavelength
you have for your antenna and your ground plane. (Or perhaps the
posting in which you explained all that hasn't made it to this corner
of the universe.)

Cheers,
Tom

Paul Burridge wrote in message . ..
....
It's annoying, because the tx output Z I'm trying to match is (by a
strange coincidence) 140 ohms! So a folded dipole would be ideal, I
guess. However - and it's a big *however* - I can't use one. I'm stuck
with a telescopic whip and a ground plane the size of a box of Swan
Vestas. I imagine the radiation resistance of such a non-ideal antenna
is pretty low, but until someone can gimme a ballpark figure for it, I
can't even begin to think about how to go about matching it. :-(

Paul, it would be reeeeally helpful if you'd include enough info so we
could give you a ballpark figure. What wavelength (or frequency)?
How long is the antenna? How big is that box? (Will someone be
holding it during operation?) Yeah, someone did offer a ballpark
figure, but that depends a whole lot on what fraction of a wavelength
you have for your antenna and your ground plane. (Or perhaps the
posting in which you explained all that hasn't made it to this corner
of the universe.)

Cheers,
Tom

On Mon, 8 Dec 2003 21:32:52 +0000 (UTC), "Reg Edwards"
wrote:

I can see from the way you have made your enquiry you havn't the foggiest
idea about what you are trying to accomplish. Do you know the frequency?
What transmitter power output do you have in mind - 10 milliwatts or 1KW?
For starters forget all about folding anything - you've been reading the
wrong books. However you now mention a short whip above a groundplane of
unknown construction.

Perhaps you didn't see the earlier posts on the subject. The frequency
is 40Mhz (radio control band) and the tx output stage as it stands
puts out maximum power of 475mW with a 140 ohm resistor as load. I
only *need* 50mW ERP., however, so can stand to see quite a bit of
loss from an inefficient antenna. The antenna I will be using is a
telescopic whip - exactly the same set-up as you see with model
vehicle radio control transmitters- which is what it is, in fact.
It ain't rocket science.
Someone out there must know the radiation resistance of such a
telescopic whip (which has a ground plane of just around 16 square
inches contained within the remote control handset) and the best way
to couple it to a PA with a 140 ohm output impedance?
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Mon, 8 Dec 2003 21:32:52 +0000 (UTC), "Reg Edwards"
wrote:

I can see from the way you have made your enquiry you havn't the foggiest
idea about what you are trying to accomplish. Do you know the frequency?
What transmitter power output do you have in mind - 10 milliwatts or 1KW?
For starters forget all about folding anything - you've been reading the
wrong books. However you now mention a short whip above a groundplane of
unknown construction.

Perhaps you didn't see the earlier posts on the subject. The frequency
is 40Mhz (radio control band) and the tx output stage as it stands
puts out maximum power of 475mW with a 140 ohm resistor as load. I
only *need* 50mW ERP., however, so can stand to see quite a bit of
loss from an inefficient antenna. The antenna I will be using is a
telescopic whip - exactly the same set-up as you see with model
vehicle radio control transmitters- which is what it is, in fact.
It ain't rocket science.
Someone out there must know the radiation resistance of such a
telescopic whip (which has a ground plane of just around 16 square
inches contained within the remote control handset) and the best way
to couple it to a PA with a 140 ohm output impedance?
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

Its not a simple matter of matching the transmitter to the radiation
resistance. First a short antenna is going to have a capacitive
reactance. You must add an equal amount of inductive reactance (a
loading coil) in order to cancel the capacitive reactance and make the
antenna resonant at the operating frequency. Then what the transmitter
needs to match is the total load impedance of the antenna "system".
The load impedance includes the radiation resistance plus the
resistance of the loading inductance plus the ground losses. The
hardest thing to get a handle on will be the ground losses. The
physical size of the transmitter housing is a small portion of a
wavelength and losses will change as you handle the unit.