Dish Type Antennae
 

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Dish type antenna

There are two primary types of dish antenna.  The most common is the
paraboic.  The the other type is ths spherical antenna.  The reason
the parabolic is the dish of choice is overall mass.  A deep dish of
spherial design with the focal point in line with the outer lip would
be 1/2 the diameter of the dish.  A deep dish parabolic where the focal
point is in line with the outer lip would be 1/4 the diameter.

On either style the more shallow the dish,  the larger the F/D, (focus
to diameter ratio),  the more shallow the dish and the further out from
the dish to the focal point. A F/D of .5 is the shortest possible F/D
on a spherical antenna. The reason is that .5 F/D is the center of the
sphere and a shorter F/D would start closing the opening.

Examples of focal lendth of 100 inch diameter dishes with a given
F/D:              
                                    
  F/D Parabolic  Depth Focal Length  Spherical Depth Focal Length  
   2.00 
  3.125 in 
   200 in 
   6.35 in 
   200 in 
  1.00
 6.25 in
  100 in
   13.4 in 
  100 in
   .75
 8.34 in
   75 in
  19.1 in
   75 in
   .50
 12.5 in
   50 in
    50 in
   50 in
   .40
  15.63 in 
   40 in
N/A
N/A
   .35
 17.86 in
   35 in
N/A
N/A
  .325
 19.23 in
 32.5 in
N/A
N/A
  .300
 20.84 in
   30 in
N/A
N/A
   .25
    25 in
    25 in 
N/A
N/A

    

   

   
 
 

 

 
 

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