Design and analysis of low frequency strut-straddling feed arrays for EVLA reflector antennas

A new feed system is designed for operation below 100 MHz. The only existing system on the EVLA operating below 100 MHz is the "4 m" (74 MHz) system which uses crossed half-wave dipoles located in front of the Cassegrain subreflector as the feed. However, the dipole feeds of this system introduce blockage, and a reduction in system sensitivity (estimated to be similar to 6% at 1.4 GHz) is observed at higher frequency bands; hence the dipoles are removed most of the time. An alternative feed concept is therefore proposed in this paper. The proposed system appears to reduce sensitivity degradation at 1.4 GHz by 3% and thus might be permanently mounted. Moreover, the new system has sensitivity comparable to the existing system at frequencies below 100 MHz. The feed for this system consists of dipoles mounted between the adjacent struts of the reflector and is thus referred to as a strut-straddling feed array. This design and the analysis methodology used in this paper should be applicable in meeting the contiguous frequency coverage requirement (50-470 MHz) of the new low frequency system proposed for the EVLA. Also, it may be applied in the modification of other existing large reflector antennas for low frequency operation.