Introduction:From a lecture I held in Weinheim back in 1993. Describing the principle behind the different antenna designs. Rebuilding feed horns for amateur use (10368 MHz).
Center feed dishes - Fig. 1, 2 and 3.
Offset feed dish - Fig. 4.
Gain, beamwidth and efficiency - Fig. 5 and 6.
Measured radiation pattern of a 60 cm offset dish - Fig. 7.
Calculated radiation pattern of a 60 cm dish - Fig. 8.
Design of choked feed horn (Scalar feed) - Fig. 9.
Other horn designs - Fig. 9.
Rebuilding the feed horns:The following two designs have been build and tested on several different dishes. Feeds for satellite TV are designed for higher frequencies - older models from 10.7 GHz to 11.7 GHz (they are the best to modify) and newer models for the full band from 10.7 GHz to 12.75 GHz.
Even without modification their performances are acceptable, however if you tune the feed horn by two adjustment screws you can make a perfect match - VSWR 1.1 : 1 or better - on 10368 MHz.
Rebuilding is shown on the drawings. First remove the LNB (screwdriver or hacksaw!). Then close the rear end with a piece of aluminium sheet or other conducting material. At a specified distance from the closed end of the wavwguide place an SMA connector with a probe going into the waveguide. Place two small adjustment screws in front of thwe probe. The screws must be spaced by 1/8'th of the guided wavelength (the wavelength inside the feed is much longet than in free space - see calculation).
With a suitable directional coupler you can now tune the feed for best return loss - best VSWR. On a prime focus dish this should be done with the feed horn in place at the front of the dish. With an offset dish that is not nescessary, as the interaction between feed an reflector is neglible.
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