FEED SELECTION GUIDELINES

• Antenna feeds are chosen with certain performance requirements imposed by the system in which the antenna is to be used - for example, frequency band, polarization(s), power handling, tracking capabilities, etc. Beyond these factors, one would appear to have wide choices. However, there are other constraints which may govern feed selection.

• A typical feed application question is whether a prime focus or a Cassegrain feed system is best. Experience has shown that, for small aperture reflectors (up to perhaps 5M diameter in C-band and 3M in Ku-band), a prime focus feed gives better efficiency, and lower sidelobes at dramatically lower cost. For C-band parabolic antennas in the 5-7M category, the Cassegrain design offers an advantage in that the feed ports are in a more accessible location thereby allowing for easier LNA servicing and for installation of complex equipment such as redundancy switches, etc.

• Consider shaped Cassegrain designs for C-band Earth Station antennas greater than 7M in diameter. With shaping, the Cassegrain subreflector contour is modified so that the main reflector aperture is nearly uniformly illuminated. This can result in aperture efficiencies approaching 80%. This design requires the main reflector contour to be modified also; therefore the technique cannot be used easily on existing parabolic reflectors. This shaped Cassegrain feed technique is the standard for nearly all large Earth Station antenna systems.

• In choosing a prime focus feed, attention should be paid to the proper specification of the feed’s primary pattern so that it matches the reflector’s focal length/diameter ratio (F/D). Optimum results for Earth Station work are obtained when the feed’s primary pattern edge taper is about 10 - 12 dB. In standard Seavey Engineering feeds, this can be specified by a dash number following the part number. With this edge taper, aperture efficiencies of about 70% can be achieved for scalar-type feeds. Refer to the graph on page 125 which shows efficiency vs. F/D ratio. For extremely deep reflectors (F/D = 0.25 or so) a special feed design such as our Model ESR-40X type is necessary to achieve best performance. Offset, circular projected-aperture reflectors are also optimally fed with 10 - 12 dB tapers.

• Feed polarization selection is important. Page 116 describes devices for modifying or selecting feed polarization. Also see page 122 for a general discussion of antenna polarization control. Certainly, the antenna system specification will dictate the polarization. However, questions will arise about the methods. For example, is remote polarization necessary or is a manual change at the feed possible? If the requirement is to remotely switch from linear to circular polarization, is there then a requirement that linear polarization also be remotely adjustable in angle? Complete polarization adjustability by remote control is expensive, so costs should be weighed in these decisions. To explore these questions, please call our applications engineering staff.

If several frequency bands are to be used,one should consider whether simultaneous access to all bands is necessary or whether a turret-style motional feed (see pages 98-99) could be used. For some applications, a set of feeds in a cluster around the focus may be useful if the off-boresight beams can be compensated for by suitably repositioning the antenna and if some pattern degradation is tolerable. Turret and cluster feeds offer the possibility of great flexibility in choice of frequency bands, polarization and feed function. Co-boresighted feeds offer good performance at relatively low cost in restricted frequency bands (generally an octave or more apart). Accessing several satellites at once with a parabolic reflector can be done using multiple feeds. Please see page 87. Special purpose antennas can be designed using unique feeds with parabolic reflectors. Examples are: angle diversity systems with two (or more) squinted beams, monopulse or conical scanning tracking feeds, and constant beamwidth antennas. Feeds may also be designed for non-parabolic reflectors such as radars with cosecant-squared coverage. When the choice is made for a moderate-gain (15 - 25 dBi) antenna using a parabolic reflector and feed, special methods such as the use of vertex plates are necessary to properly match the feed to the reflector. This is required because the focal length is a small number of wavelengths. In these instances, engineering consultation with Seavey Engineering is recommended.