Forum

This link has a good explanation about the differences between effective aperture and physical aperture:

http://www.w8ji.com/capture_area_ae_effective_aperture.htm

?What is Capture Area?
Capture area, or more correctly effective aperture (Ae), is a direct function of antenna gain and operating wavelength. Ae is determined by the voltage available across a load matching the antenna feed impedance for a given electromagnetic field strength density. In simple terms if the antenna is placed in a electromagnetic field of a certain intensity, a certain amount of power will appear in the load at the antenna terminals. The area of space around the antenna that provided this amount of power is the effective aperture.
Many people confuse physical area, or Ap, with effective aperture. They are not the same. Physical size only determines effective aperture as physical size might affect gain of an antenna. Gain and wavelength determines capture area, but capture area itself has nothing to do with actual physical size or physical area of the antenna.
For example a 1/2 wave long dipole in freespace has a capture area of 0.13??. This means a lossless freespace dipole has an Ae of approximately 1/8 square wavelengths. This effective aperture is about 100 times larger than the actual physical area of a thin wire dipole antenna. Energy is extracted from an elliptically shaped area slightly longer than the dipole and about 1/4 wave diameter at the center. This is why increasing conductor diameter or using a cage of wires will not increase electrical aperture or capture area. As a matter of fact if we built a lossless or very low loss small dipole, perhaps ?/20 (1/20th of a wavelength) long, capture area or Ae would be within a few percent of a full size dipole!
A change in antenna element diameter does not affect gain, except as it might very slightly reduce power losses in conductor resistance. Length itself has very little effect unless the change in length significantly affects antenna gain. We must have a change in gain to change Ae (effective electrical aperture). Physical aperture (Ap) changes do not affect Ae unless the gain changes.?

Dave