antenna-theory.com

[mustafa85]

Hi all :

im a little bit confused about the relation between wavelength and antenna size ( antenna size should be part of the wave length 1/2 for example ) why this consideration ? what is the physical effect of wavelength on the length of the antenna ;

can any one describe the idea in simple way ?[/b]

[helix]

a half-wavelength dipole is self-resonant. it's input impedance is similar to a series RLC circuit. if you lower the frequency (or, equivalently, make the dipole physically shorter), the C dominates the L and the thing just looks capacitive. and the radiation resistance is reduced proportional to the square of the frequency (or physical length).

the capacitive reactance is a big nusiance, it prevents good matching to a transmitter (or receiver) with a real source impedance across frequency.

you could resonate out the large capacitve reactance with a lumped inductor (that is not really part of the antenna). BUT lumped inductors are very lossy, especially when they are physically small. in fact the loss of an inductor scales proportional to something like the 4th power of physical volume! in a way, by extending the length of the dipole to about a half wavelength, you're putting that matching inductor out in the (very large) antenna volume, so it therefore has very low loss.

now the other issue i mentioned is radiation resistance; as you make the dipole much shorter than a half wavelength, the radiation resistance drops (rapidly!). the series RLC resonator therefore takes on a high Q, the input impedance therefore changes rapidly with frequency, making it difficult to match broad-band, leaving you with a very narrow return loss bandwidth. (look up the classic papers by wheeler and chu on Q limits of electrically small antennas).


the same ideas hold for loop antennas, except the equivalent circuit is parallel RLC; therefore, at low frequencies (or small loop sizes) it looks inductive.