Helmet mounted UHF antenna for Mobile User Objective System (MUOS)

The Mobile User Objective System (MUOS) is the Navy´s next generation narrowband tactical communications system that will provide a significant increase in capacity and link availability to disadvantaged users, including handheld terminals. Future MUOS antennas will have a receive band of 243 MHz to 270 MHz and a transmit band of 292 MHz to 317 MHz with a voltage standing wave ratio (VSWR) of less than three across both bands. Additionally, the antenna should be circularly polarized with a nearly omnidirectional radiation pattern above 10 degrees in elevation for optimum satellite coverage. A helmet-mounted antenna that can meet the space and weight constraints imposed by the application is a two-turn spiral with a different pitch to each turn. Our design is a two-arm Archimedes spiral encapsulated in a nylon shell that fits over the standard-issue US Kevlar helmet. The antenna was modeled and simulated using Ansoft´s High Frequency Structure Simulator (HFSS) software. A half-power beamwidth of 120 degrees, a front-to-back (zenith-to-nadir) ratio of 3.6 dB, and a VSWR of less than three were achieved using a feed port impedance of 150 ohms. Recent work includes specific absorption rate (SAR) calculations and the addition of a reflector ring, which increases the front-to-back ratio and reduces the feed port impedance to the standard 50 ohms.