[Antennas] B&W question

2003年2月16日 14:05:50 -0500

	There is a great deal of difference between reception of lf/mf/hf
signals and vhf/uhf signals with respect to antenna performance. In the
latter case (vhf/uhf or higher) in order to achieve satisfactory signal
to noise ratios (hear the signal above the noise) the signal must be
stronger than the noise generated internally at the input device of the
receiver -- that is the limiting factor. So the higher-gain, more
perfectly matched antenna will, of course greatly out-perform one that
is not.
On the other hand, for reception of lf, mf and hf signals the limiting
factor is frequently ambient noise, that is to say the noise coming into
the receiver along with the signal -- from outside. Here the aim of
"good" antenna design should be discrimination in favor of the signal
and against the noise -- not just "gain" as such. Viz: the
signal-to-noise ratio using any given antenna will remain the same if we
insert 6 to 10 dB of attenuation, because the noise is attenuated by the
same amount as the signal. Since this external noise appears to come
from all directions, directive antennas will very frequently give a
better signal-to-noise ratio than non-directive antennas (because they
are not looking at as much noise source as the non-directive antenna).
This will account for the many different opinions held by different hams
about antennas -- in a sense "y'all" are right to some extent! 
Harvey/W4TG