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?

Ron, W4TQT

"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?
__________

Whatever the net field that arrives from the transmit antenna (whether or
not the transmit antenna is omnidirectional), a receiving antenna having
gain in the direction toward the transmit antenna will perform better than
if it is omnidirectional.

Part of the improvement is due to its added gain, and part due to
interference rejection from co- and adjacent-channel signals arriving from
directions where the receiving antenna has less gain than an omni antenna.

RF

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
--

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?

Ron, W4TQT

Yes.

The amount of signal "captured" from a given direction is exactly
proportional to the gain in that direction. "Capture area", "effective
aperture" and "gain" are simply different ways of expressing the same
thing, as long as perfect efficiency is assumed; if you know any one you
know the other two.

Roy Lewallen, W7EL

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?

Does a vertical Yagi receive more signal than a vertical monopole?
--
73, Cecil http://www.qsl.net/w5dxp

Maybe I should restate my question. Assume a receiving antenna is in
the center of a sphere and the received signal is coming in equal
amounts from all points on the surface of the sphere. Which receiving
antenna would capture more power, an omni or a high gain beam? There
are no noise and no losses.

Ron

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?

Ron, W4TQT

On Sat, 29 Oct 2005 00:21:02 GMT, Ron wrote:

Which receiving antenna would capture more power, an omni or a high gain beam?

An isotropic antenna would be the best, as the field described came
from the same emitter (such a field would be impossible otherwise).
Stepping out of this enigma (that the emitter and detector are
different, and the field which could only be generated by an isotropic
would then suddenly turn and come back) would answer the omni.

73's
Richard Clark, KB7QHC

Ron wrote:
Maybe I should restate my question. Assume a receiving antenna is in the
center of a sphere and the received signal is coming in equal amounts
from all points on the surface of the sphere.

Are you trying to receive the background radiation left over
from the big bang? That's the only source outside of the
sphere that I know of that can accomplish your boundary
condition.
--
73, Cecil http://www.qsl.net/w5dxp

Ron wrote:
Maybe I should restate my question. Assume a receiving antenna is in the
center of a sphere and the received signal is coming in equal amounts
from all points on the surface of the sphere. Which receiving antenna
would capture more power, an omni or a high gain beam? There are no
noise and no losses.

They'll intercept equal amounts, assuming both are lossless. The
directional antenna will intercept a larger fraction than the isotropic
antenna in the directions it favors, and less in others. The total will
be be the same.

In reverse, this is equivalent to calculating the average gain of the
antennas, which is the same for all lossless antennas.

Roy Lewallen, W7EL

No, I'm just trying to understand antenna gain.

Ron

Cecil Moore wrote:
Ron wrote:

Maybe I should restate my question. Assume a receiving antenna is in
the center of a sphere and the received signal is coming in equal
amounts from all points on the surface of the sphere.

Are you trying to receive the background radiation left over
from the big bang? That's the only source outside of the
sphere that I know of that can accomplish your boundary
condition.