The satellite ground station at Andover, Maine, that was used to communicate with the Telstar 1 satellite was a horn antenna. Why was a horn antenna used instead of parabolic dish antenna?
I did not find a explicit comparison with a dish, but this quote does contrast the design with "other possible forms"
The communications antenna at Andover is a much enlarged version of similar antennas widely used on Bell System microwave relay routes. A horn reflector of this type with a 20 X 20 foot aperture was used at Holmdel in the Echo experiments. Fig. 12 shows a model of the Andover antenna.* For structural reasons, the horn at Andover is conical rather than pyramidal, as was the case in the smaller versions. The antenna rotates in azimuth on two concentric rails and in elevation about the axis of the conical feed horn on two large bearings. Two equipment rooms are carried on the structure. The maser is in the upper room near the apex of the horn. This configuration has several advantages over other possible forms. It is very broadband, presents an excellent impedance to the transmitter, and the parabolic surface is efficiently illuminated. Most important, however, for the present application, the antenna has very low side and back lobes and may be connected to the receiver with short, low-loss connections resulting in a low system noise temperature.
Source: The Telstar Satellite System, included in NASA SP-32 Volume I
Lots of detail on the antenna are available in the first two papers in Volume II including better pictures and schematics
- The Mechanical Design of the Horn-Reflector Antenna and Radome
- The Electrical Characteristics of the Conical Horn-Reflector Antenna