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

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

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

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!

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

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

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

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