So true. Any TX power gain in one direction is offset by an equal loss in some
other direction. 100 watts in gets you 100 watts less resistance losses out.
As Billy Whitehead found when he built an antenna and transmission line from a
big roll of nice shiny wire he picked up at the Army-Navy store. For about two
weeks it was the greatest antenna in the world. Until someone pointed to the
roll and explained the 2 Omega ' penciled on the end meant 2 ohms per foot.
He'd made his antenna of resistance wire - and 400 ohms of series resistance in
his home brew open wire line didn't work too well.
The effective capture area for relatively simple antennas such as a dipole is
still (spelled out since I can't get Greek letters out of this E-mail program)
"equal to a circle bounded by circle which is at a radial distance from the
dipole of Lambda/1.6 Pi. The physical significance of this is that such a
circle very roughly forms the boundary between the region in which the local
induction field of the antenna predominates and the region in which the
radiation field is the major component."
And while the capture area is always larger than the physical antenna, that
does not mean a bigger antenna will either TX or RX better. Some 70 years ago
W.N. Christiansen first published the discovery that a recieve antenna must be
matched to the transmission line if maximum energy transfer is to be obtained.
Under the best circumstances half the energy impinging on an antenna will be
re-radiated and the other half goes down the transmission line.
If there is a mismatch at the antenna/feedline junction, more energy will be
reradiated and less goes to the reciever. In the case of a really terrible
match, such as a center fed full wave dipole, only a very small percentage of
the available signal ever makes it to the reciever. In that case, it's really
not feasable to make an antenna large enough to overcome the mismatch losses.
73 Pete Allen AC5E
> Regarding "capture area", please note that
> it is NOT separate from directivity (loss ignored).
>
> Therefore, NO antenna can have a "capture area"
> larger than some other antenna in all directions
> - just like NO antenna can have gain
> higher that some other antenna in all directions.
>
> This holds regardless of antenna size.
>
>
> 73,
>
> Sinisa YT1NT, VA3TTN
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