RF beam width and antenna diameter?

[ctech4285]

it seems like looking at beam patterns from directional RF antennas the beamwidith is something like inversely proportional to the parabolic reflector diameter. why can you make an almost perfect strait parallel beam of light but not with RF?

 

[Bob S]

The angular divergence of an optical lens and a directional radio antenna is given by

α = 1.22 λ/d radians

where λ is wavelength, and d is antenna diameter.

For visible light, λ ~ 500 nanometers, and for a 3 GHz antenna, λ = 10 cm. Wavelength ratio is 200,000:1.

Bob S

 

[ctech4285]

what cause this?
what would happen if you use two directional antennas would the beam width be half on one plane? would it make a difference if you place them far apart?

 

[Bob S]

Two parabolic antennas are better than one. 27 parabolic antennas separated by multiple wavelengths are better than two. See Very Large Array (VLA) antenna "farm" photo at

http://en.wikipedia.org/wiki/Very_Large_Array

Read about diffraction, interference, and resolving power of lenses (the equation α = 1.22 λ/d radians) in physical (conventional) optics books.

Bob S