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. :-(

Depending on frequency in use, could you do it experimentally?

The way I tune up antennas for ham bands is to hook up a mfj 249 and the
tuner to the antenna, get a best fit with the 249 and then replace the 249
with the radio and fine tune from there. There always seems to be some small
differance between the result from the mfj and the meter built into the
tuner, and at full power I would rather trust the meter built into the tuner
(a Millen transmatch jr).

You could build an L match with a tapped inductor and variable cap, then
experiment with values until you get somewhere in the ballpark of being
matched. From there it is is just small adjustments to get perfect matching.
thanks, John.
KC5DWD

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

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

I read in sci.electronics.design that Reg Edwards
wrote (in
et.com) about 'Measuring radiation resistance', on Mon, 8 Dec 2003:

If the wire end-to-end resistance of a 40m, 14-gauge dipole is, say,
2.76 ohms then -


Aerial efficiency = 100 * 140 / ( 140 + 2.76 ) = 98.0 percent.


Which is very good, isn't it? It's equivalent to 1/68th of an S-unit
which cannot be detected even by using a magnifying glass and the bloody
needle stands still for long enough.


Which also serves to illustrate how VERY uncrtical are aerial impedance
measurements.

Reg,
The OP is working with very non-ideal antennas, for which the radiation
resistance is probably only an ohm or two, and he wants to know if it's
0.5 ohms or 5 ohms, for obvious reasons. I don't suppose there is any
realistic way of measuring it, and modelling may be extremely difficult
if the antenna shape is not simple.

I've seen antenna evaluation does on 1:10 and 1:20 scale models, but we
don't know what frequencies the OP is using, so even that may not be
practicable, but if it is, one could work back from field strength
measurements to radiation resistance, with a big pad between the
transmitter and the antenna to 'soak up' the mismatch.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

On Mon, 8 Dec 2003 15:29:59 +0000 (UTC), "Reg Edwards"
wrote:

[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 read in sci.electronics.design that Reg Edwards
wrote (in
et.com) about 'Measuring radiation resistance', on Mon, 8 Dec 2003:

If the wire end-to-end resistance of a 40m, 14-gauge dipole is, say,
2.76 ohms then -


Aerial efficiency = 100 * 140 / ( 140 + 2.76 ) = 98.0 percent.


Which is very good, isn't it? It's equivalent to 1/68th of an S-unit
which cannot be detected even by using a magnifying glass and the bloody
needle stands still for long enough.


Which also serves to illustrate how VERY uncrtical are aerial impedance
measurements.

Reg,
The OP is working with very non-ideal antennas, for which the radiation
resistance is probably only an ohm or two, and he wants to know if it's
0.5 ohms or 5 ohms, for obvious reasons. I don't suppose there is any
realistic way of measuring it, and modelling may be extremely difficult
if the antenna shape is not simple.

I've seen antenna evaluation does on 1:10 and 1:20 scale models, but we
don't know what frequencies the OP is using, so even that may not be
practicable, but if it is, one could work back from field strength
measurements to radiation resistance, with a big pad between the
transmitter and the antenna to 'soak up' the mismatch.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

On Mon, 08 Dec 2003 09:48:02 +0200, Paul Keinanen
wrote:

On Mon, 08 Dec 2003 00:55:46 +0000, Paul Burridge
wrote:


Hi guys,

How does one go about measuring (with a reasonable degree of
accuracy) the radiation resistance of antennas? And when I say
"antennas" I mean any radiator from a balanced dipole through to a
random length of wet string with a damp matchbox for a ground plane.

Are you really interested in the radiation resistance or are you
actually trying to figure out the efficiency or total radiated power
of the antenna ?

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!
--

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

Radiation resistance should always be referred to a particular point in an
aerial.


It is not of much use unless used to calculate radiating efficiency in
conjunction with conductor and other loss resistances.


The problem is not how to measure it but how to distinguish it from the
aerial loss resistance in series with it.


It is that fictional resistance which, if inserted in the aerial at that
point, dissipates the same power as is radiated when the same aerial current
flows.


Radiation resistance can also be considered to be uniformly distributed
along an aerial wire. It can then be directly compared with wire loss
resistance.


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'.


If the wire end-to-end resistance of a 40m, 14-gauge dipole is, say, 2.76
ohms then -


Aerial efficiency = 100 * 140 / ( 140 + 2.76 ) = 98.0 percent.


Which is very good, isn't it? It's equivalent to 1/68th of an S-unit which
cannot be detected even by using a magnifying glass and the bloody needle
stands still for long enough.


Which also serves to illustrate how VERY uncrtical are aerial impedance
measurements.
----
Reg, G4FGQ

Paul Burridge wrote:

Hi guys,

How does one go about measuring (with a reasonable degree of
accuracy) the radiation resistance of antennas? And when I say
"antennas" I mean any radiator from a balanced dipole through to a
random length of wet string with a damp matchbox for a ground plane.
Must it be done with a noise bridge or is there another way that
requires no special test equipment (aside from a scope/sig.gen etc.).
I'm primarily interested in checking out highly *non*-ideal antennas
for use in non-ideal situations/locations.

You can't directly measure radiation resistance, because it's embedded
in loss resistances and usually reactance too. Also there's more than
one definition of radiation resistance: some would say that the
radiation resistance of a half-wave dipole depends on where and how it
is fed (centre, end, off-centre); while others would say that the
radiation resistance stays the same, and it's only the feedpoint
impedance that depends on the method of feed.

Since you can't measure it and there isn't even a universally agreed
definition, it's best to tiptoe quietly away from "radiation resistance"
before the Holy Wars begin...

What you can measure directly is impedance at the feedpoint. That is of
much more practical interest. For that job, an R-X noise bridge is
probably the minimum entry-level instrument.

In principle you could measure impedance using a sig gen and a
two-channel scope, measuring the amplitude and phase difference across a
series resistor feeding the unknown load. But in practice you wouldn't
get very accurate results above MF, and it's a heap of equipment to
carry outside where you'd want to use it.

'Antenna analysers' such as the MFJ-259B are deservedly popular because
they will do R-X measurements with acceptable accuracy, and the whole
thing can be battery-powered and held in one hand. I'm looking forward
to making some outdoor antenna measurements using the new N2PK vector
network analyser... but not today brrrr!


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek