On Fri, 28 Oct 2005 20:37:07 GMT, Ron wrote:

Assume an incoming rf signal has exactly the same strength in all 3
dimensions i.e., completely omnidirectional. Question: would an
antenna having gain capture any more signal power than a completely
omnidirectional antenna with no gain?

Your scenario is a little confusing.

Here are my thoughts:

I we took the case of say, noise that was sourced from all around you
(that is not to mean an isotropic transmitter antenna), a directional
antenna would receive about the same power as an isotropic antenna,
and the difference would be due to antenna losses, ground reflection
losses (if relevant).

Galactic noise on HF might nearly fit into that scenario (or perhaps
more topically, neighbourhood BPL interference), and I would expect
that a 8dB yagi would receive similar power to a half wave dipole.
Galactic noise is a little lower at the galactic poles, so in sweeping
the yagi you may observe a very small directional effect. Further,
ground reflection and different antenna + feed losses will introduce
small differences.

If at the end of that, you are trying to rationalise why a beam is
better than a dipole, although the beam does not receive more or less
of the "directionless" noise, it does increase the receive power from
noise, interference and signal from the main beam direction and reduce
receive power from noise and interference from away from the main
beam.

Does that hang together?

Owen
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