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.
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Reg, G4FGQ