Parabolic Dish with BiQuad feeder - Page 5
Saturday, 12 September 2009 22:20

Attach the Feeder

Parabolic antenna types

The feeder, either the biquad antenna or a waveguide can-tenna, needs to be attached at the focal point of the parabolic dish.

There are two options for where the feeder is attached:

  1. Centre (axial) focal point, the feeder can be mounted (using supports) in the centre and above the dish at the focal point calculated on Dish with USB Feed page.
  2. Offset focal point, by using an offset focal point, a lot less of the reflected beam is blocked by the feed antenna and its support structure. In smaller dishes, feed blockage can cause significant loss in efficiency.

Calculating an offset focal point of a parabolic dish is not as simple at finding the dish's central (axial) focal point. If your keen, these articles have more information:

Simple Option: Use your dish's existing LNBF (low-noise block/feedhorn) position. You can assume the offset focus is the entrance of the feedhorn. By measuring the distances between the entrance and at least 3 fixed points at the edges of the dish, you can keep this information so that the offset focal point can be precisely determined and restored when the feedhorn is replaced with a biquad driver or can-tenna.

Note: When using a can-tenna feeder, the entrance of the can should be positioned at the dish's focal point.

sky dish with can feedersky dish with can feederdish with can feed on roof

An offset fed dish will need to be angled down about 45 degrees to be aimed at targets on the ground (and not at satellites!). Another option is to turn the dish upside down, with the feeder arm on top.

biquad and dishsatenna