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Measuring radiation resistance

P

Paul Burridge

Jan 1, 1970
0
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.
 
W

Wes Stewart

Jan 1, 1970
0
On Mon, 08 Dec 2003 00:55:46 +0000, Paul Burridge

|
|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.

This was best posted to rec.radio.amateur.antennas but...

For ordinary wire antennas operated independent of ground and without
loading coils, the radiation resistance is very close to the resistive
part of the feedpoint impedance. Any number of instruments can
measure this with reasonable accuracy. A noise bridge or some of the
popular impedance bridges (AEA CIA, MFJ, etc) can do it.

If the antenna has loading coils, or is operated against ground, then
the losses in those (sometimes greatly) affect the measurement.

In either case, "special" equipment in the sense that it's more than a
scope and signal generator *is* required. Very often transmission
lines are a part of the equation and their effects need to be
accounted for as well.

I suspect that the more non-ideal the antenna, the more difficult the
measurement.

Wes N7WS
 
P

Paul Keinanen

Jan 1, 1970
0
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 ?

One way would be to first measure the total radiated power. To do
this, you would have to measure the EiRP to all directions and
integrate it. If the measurement is done in the far field, a
calibrated magnetic probe would be enough, but at closer distances,
additionally the electric field would have to be measured with a
reference dipole.

At short wavelengths the antenna can be put on a rotating table but
for instance to measure the radiated power for an LF system, you would
have to fly around in a plane with DGPS navigation and make the
measurements from all directions.

If the antenna has a symmetrical or otherwise well known radiation
pattern, the number of measurements could be greatly reduced.

When you have the total radiated power and then measure the input
power, you can calculate the efficiency and radiating resistance.

Assuming that the radiation resistance is in series with the loss
resistance, it would be enough to measure just the antenna current,
which also flows through the radiation resistance, producing the
already measured total radiated field.

Paul OH3LWR
 
I

Ian White, G3SEK

Jan 1, 1970
0
Paul said:
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!>
 
P

Paul Burridge

Jan 1, 1970
0
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!
 
R

Reg Edwards

Jan 1, 1970
0
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.
 
R

Reg Edwards

Jan 1, 1970
0
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 input impedance of a 1/2-wave resonant dipole is about 70 ohms. But
this may be at a considerable distance from the transmitter output
terminals. What do you have in mind to put in between?

Then all you have to do is find the purely resistive load the transmitter
would be most happy with. Almost certainly it will not correspond to an
impedance match.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Reg Edwards
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.
 
P

Paul Burridge

Jan 1, 1970
0
On Mon, 8 Dec 2003 15:29:59 +0000 (UTC), "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. :-(
 
J

john graesser

Jan 1, 1970
0
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
 
J

Joe McElvenney

Jan 1, 1970
0
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
 
T

Tom Bruhns

Jan 1, 1970
0
Paul Burridge said:
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
 
R

Reg Edwards

Jan 1, 1970
0
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
...........................................................


"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. :-(
Winston Churchill
 
T

Tom Bruhns

Jan 1, 1970
0
Paul Burridge said:
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
 
P

Paul Burridge

Jan 1, 1970
0
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?
 
B

Bob Lewis \(AA4PB\)

Jan 1, 1970
0
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.
 
C

Cecil Moore

Jan 1, 1970
0
Paul said:
... the best way
to couple it to a PA with a 140 ohm output impedance?

Use a loading coil to resonate the antenna to 40MHz. Use a transformer
or tank circuit to transform the impedance.
 
M

Mac

Jan 1, 1970
0
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?

You have had quite a few answers which seem pretty good to me.

I have a few comments.

Stop saying "radiation resistance" and start saying "input impedance" or
"feed impedance." Once you know the input impedance, getting maximum power
transfer is a straightforward matching problem that can be solved in a
variety of ways. True, you won't know for sure how much power is "getting
out," but some of the posts in this thread have given you some estimates.
If that isn't good enough, then you will have to measure by putting a
receive antenna in the far field.

Anyway, if you want to measure the input impedance, you should be able to
do it with a two-channel oscilloscope and a 40 MHz function generator.
Drive the antenna with the function generator and simultaneously measure
the input voltage and current, including the phase relationship between
them. The input impedance is V/I. To measure the current, put a suitably
sized resistor in series with the antenna and measure the voltage across
it. The voltage across the resistor should be small compared to the
voltage into the antenna. If it is too small to see, use a bigger resistor
until you can see it. Make sure both oscilloscope probes are grounded in
the same place. Unless you have a differential probe (in which case, use
it to measure the voltage across the current sensing resistor).

From what others have already said, it sounds like the input will be
extremely capacitive with a small series resistance. Once you know the
details, however, it will be easy to tell you how to match this to your
amplifier.

That's my $0.02.

Mac
 
T

Tony Williams

Jan 1, 1970
0
Paul Burridge said:
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?

Paul, standard CB antennas are loaded whips, for operation
at around 30MHz. Available in the CPC catalogue for about
£10 to £20, Perhaps you could start off with one of those
and modify to suit.

Note also that there is a 433MHz licence-free band for
short distance (100m) remote control. Pre-built Tx and
Rx modules are cheap, and a 433MHz antenna would be a
breeze.

<www.cpc.co.uk> or 08701 202530.
 
J

John Moriarity

Jan 1, 1970
0
I'm looking forward
to making some outdoor antenna measurements using the new N2PK vector
network analyser... but not today <brrrr!>

Gee, Ian, you only will have to go out and do
the short-open-50R calibration at the end of
the coax. The software can move the reference
plane from the VNA out to the antenna feedpoint,
and you can do the rest from the comfort of
your shack!

Yes, I'm on Harold's list for a kit.

73, John - K6QQ, who thought that after retirement
he'd never have the use of a VNA again!
 
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