For the same antenna why the radiation pattern is different at different resonating frequency.
eg ant antenna resonates at 2.4, 5.2 and 7.5 GHz. but the RP do not maintains its shape. why? plz give reason with some references.
The radiation pattern depends on the frequency as the radiation intensity in any point in the space is the resultant radiation from the the infinitesimal parts of the antenna. Since this sum is vectorial and the radiation from the different parts has different path lengths in terms of the wavelength then as the wave length decreases any given path difference will will give more phase difference causing rapid fall of the radiation intensity with deviation angle from the peak radiation intensity. Also the number of side lobes may increase.
Best wishes.
Read about radiation from a linear aperture and you will see that even for the same aperture illumination the pattern depends on the ratio of the wavelength to the width of the aperture.
In addition, when the antenna resonates at different frequencies, the aperture illumination is not the same at each frequency. For instance current will be in different directions at different frequencies and the peaks and nulls will be in different places.
See this fundamental equation *G(θ,φ)= (4πA(θ,φ))/λ^2*. Wavelength (λ) varies with the frequency . So, if you change the frequency than antenna's radiation pattern will also be changed.
In linear Antennas the length of the antenna is equal to
n ƛ /2
n= 1, 2, 3, …………..
for every n we have radiation pattern.
ƛ is a function of frequency.
Kraus book (Antennas) is very good on this.
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