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.
--

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

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.
|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

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.
|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

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 ?

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

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 ?

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

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

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

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

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