Dual band quadrifilar helix antennas for UHF/VHF band operation

This paper details antenna design work on quadrifilar helices that can be applied to antennas for small satcom terminals (on boats and vehicles), for GPS applications, and in the space segment for communications monitoring and data down links. During the development of the antennas for the METOP Programme for ESA it was necessary to produce a design of antenna to operate over three frequency bands concurrently. These were centred on the V/UHF 121.5, 243 and 403 MHz frequencies respectively. As these two lower frequencies are harmonically related, a fractional turn quadrifilar helix (QFH) was proposed for the design. Coaxially mounting this antenna with another, smaller diameter, n-turn resonant length antenna at the higher frequency allowed a combined receive antenna (CRA), QFH antenna to be realised. The coaxial mounting of two QFH antennas in this manner has the potential to reduce the overall size of the combined receive antenna. However, the electrical performance of each volute is simultaneously affected by the presence of the other antenna volutes when the antennas are mounted at the same phase centre. Thus mechanical and electrical compromise is involved to optimise performance and minimise gain reduction. The extent of coupling and the best practical implementation can only be determined experimentally at this time.