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