> This would be true for ALL arrays.. the performance is
proportional to the
> aperture. Yagi-Udas just happen to be end fire arrays
where the aperture
> is in the same direction as the preferred propagation
direction. There are
> some subtleties in superdirective arrays, but, as you say,
directivity/gain
The electrical aperture or performance is NOT defined by the
physical area an antenna occupies.
The electrical aperture is defined only by gain and
frequency, that's why it is more correctly called the
*effective* aperture.
For example, a Rhombic occupying an area of several
wavelengths with 10dBd gain has the same effective aperture
as a 10dBd Yagi occupying less than a wavelength on the same
frequency.
A longwire 20 wavelengths long can have less effective
aperture than a dipole. When you throw a resistor in a
given antenna, the electrical aperture decreases.
I've noticed over the years many people think physical area
makes an antenna more effective at receiving. The verbiage
is something to the effect of "it's a darned good receiving
antenna because it has a large capture area". That would
only be true for size if you are talking about the antenna
catching the wind, not radio signals. The capture area or
effective aperture not only NOT determined by physical size,
gain or effective aperture does not determine receiving S/N
at HF. Directivity sets receiving ability, until efficiency
is so low the receiver is no longer limited by external
noise. It's the gain on transmitting and the directivity on
receiving that matters, not the size.
This is actually pretty important, because MANY people think
7dB of additional antenna gain improves the receiving and
transmitting 7dB. The actual receiving change can be
anything from zero to dozens of dB. If for example you have
a 15 meter Yagi with 3dB of feedline loss and improve system
gain 2dB by replacing the feedline, the receive S/N will
remain the same.
73 Tom
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