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Dipole Antenna null at theta equal zero

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Hi,

Does anybody knows why the dipole antenna (half-wavelength, short, infinitesimal) has a null at theta = 0? I know that the pattern of dipoles have sin(theta) and therefore at theta = 0 is zero. But what is the background physics?

Thanks
 

Hi,

The radiated wave in far field has an eletrical field E that:
a) is perpendicular to the direction of propagation
b) parallel to E must be a component of charge acceleration; this needs a component of current parallel to E

You are asking for the direction theta=0, i.e. along the z axis.
By a), E must be in the xy plane But as the current in the dipole is along z, not having any component in x nor y, by b) Ex=Ey=0.
So, there is no electrical radiated field along theta=0.

Z
 
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