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

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

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