Another consideration of antenna design is the focal length to diameter ratio (f/D). It provides a measure of the depth or shallowness of the dish cross section. A shallow dish has the feed a long way from the surface of the dish (f/D = 0.4 - 0.5) and will tend to be more efficient as the wavefronts are more likely to arrive in phase at the feed. Deep dishes (f/D in the range 0.25 to 0.35) occupy less space but tend to have poorer efficiency. They also require specially designed feeds.
If the effects of surface inaccuracies are taken into account (and if all else is equal) a deep dish will have lower gain than a shallow dish.
Figure 6: Comparison of shallow and deep dish antenna designs [Ref 4].
Next Section
Return to Index