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.

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 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

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.
----
Reg G4FGQ

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'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.
----
Reg G4FGQ

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 remember as WN0GJS tuning the pi-net output to a piece of
300-ohm twin-lead thru a PL-259!

So make a little amp. with a pi output, tune it up, and
measure the components.

Since I learned this kind of stuff only in the school
of hard-knox, I refer to it as the "empirical" method. ;-)

Cheers!
Rich

Paul Burridge wrote:

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 Sun, 14 Dec 2003 02:10:56 GMT, Rich Grise
wrote:

I remember as WN0GJS tuning the pi-net output to a piece of
300-ohm twin-lead thru a PL-259!

So make a little amp. with a pi output, tune it up, and
measure the components.

Can you exand a bit on this please, Rich? Sounds like it might be a
useful technique.

Since I learned this kind of stuff only in the school
of hard-knox, I refer to it as the "empirical" method. ;-)

The best school all right, but the fees are high. :-(
--

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