Effective aperture (Ae) is a direct function of antenna gain and operating wavelength.
Ae (sometimes referred to as capture area) 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 λ/2 wave long dipole in freespace has a capture area of about 0.13λ². This means a lossless freespace dipole has an Ae of approximately 0.13 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 λ/4 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 λ/20 long, capture area or Ae would be within a few percent of a full size dipole!
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.
(FromFrom W8JI and Jasik's book, at www.w8ji.com/capture_area_ae_effective_aperture.htm.)www.w8ji.com/capture_area_ae_effective_aperture.htm.
Ae (sometimes referred to as capture area) 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 λ/2 wave long dipole in freespace has a capture area of about 0.13λ². This means a lossless freespace dipole has an Ae of approximately 0.13 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 λ/4 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 λ/20 long, capture area or Ae would be within a few percent of a full size dipole!
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.
