Height of Antenna

i have a question which is simple as well tricky i read alot of material but not satisfied me .

can any one explain me why we take antenna height always λ/2 and λ/4?

The input impedance at those heights have desirable qualities, such as being purely real.

Antenna height is not always λ/4 or λ/2.
Antenna height is dictated by two factors: Fresnel zone, and ground image of the antenna.
In principle height shall be greater than λ to get best performances.
Increasing the height extends the line-of-sight distance and reduces the effects of objects on the ground.

but most of teh time it's λ/2 and λ/4 why ?give a solid reason

λ/2 is used for a symmetrical dipole, this one is feed in the middle and has an impedance of about 240 to 300 Ohm typically. The dipole itself consists of two wires with λ/4 length each.

If you cut away one half of the dipole and replace the middle line with a conducting ground plane you end up with a monopole of λ/4 in length. Varying the monopole distance over the ground plane will give a point were the impedance is around 50 Ohm.

When you want to build antennas with more gain you will have to "compress" the power in one direction, that means that the power in other (possibly unused) directions will be reduced. This can be achived by a 5/8λ monopole, but that one will never have an impedance of 50 Ohm so it must be connected via a small coil that transforms the impedance. This monopole reduces the side lobes so the power that will be "lost" in these side lobes will be fed into a torroid type power diagram.

You may want to read the famous antenna book by Balanis if you desire.

CU

I think here you mix words ?height? with ?length? or ?dimension?. These ones are usually λ/4 or λ/2.
Height from the ground of the antenna should be greater than λ, to get the best performances.

I agree with vfone .

The length for small antennas is usually λ/4 or λ/2 due to the resonance structure.

But the antenna height is usually close to the ground because of the coupling.

Hi
I think that this file will help youo to understund some element thinks about antenna.

THank you all