Thank you Roy - excellent as usual.

I recall a RADAR range equation where aperture (capture area) was one of the
terms

--
Caveat Lecter (a RADAR tech)



"Roy Lewallen" wrote in message
...
It applies just as well. However, while the aperture of a parabolic
reflector is about the area of the reflector, this isn't at all true of
simple wire antennas like a dipole. For example, a half wave dipole's
aperture is just slightly larger than a dipole of infinitesimally short
length, and about equal to that of a loop. The aperture of a loop stays
almost constant as the loop size is increased, until it gets big enough
for the pattern to appreciably change.

Aperture is the same as directional gain (not numerically, but when one is
greater so is the other), which is the same as gain when loss is
neglected. Since aperture has no direct or obvious connection to physical
size or dimension of most wire antennas, gain is usually used at HF as a
descriptive measure rather than aperture. Note that the gain of all but an
isotropic antenna is different in different directions, and therefore so
is the aperture.

People with a weak understanding of the principles involved often fall
into the trap of thinking that a larger antenna must have a larger
"aperture" or, as amateurs like to call it, "capture area". That mistaken
notion leads to all sorts of false conclusions. But the general
misunderstanding of the terms are a real boon to antenna charlatans.

Roy Lewallen, W7EL

Also, the aperture is different in different directions.

Caveat Lector wrote:
Just a question

In microwave we talk about aperature as a determing factor of antennas.

To what extent does this apply to HF ??