Dual-band quadrifilar helix antenna

Resonant quadrifilar helix antennas are widely used on hand-held receivers for GPS (Global Positioning System) and for some mobile communication systems. The paper investigates a quadrifilar helix antenna over a ground plane and with trap loading. A model of this antenna was built to operate at two frequency bands, 1227 MHz and 1575 MHz, with more than 2% bandwidth in each band. The four wires of the helix were created using narrow copper strips on a flexible mylar sheet that is then rolled into a cylinder to form the helix. The trap circuit is placed across the gap in each strip. To obtain anti-resonance at the higher frequency, the trap circuit values are 6.8 nH and 1.5 pF. The four arms of the helix are attached to a base plate whose diameter is chosen to reflect the energy from the helix while still maintaining coverage for angles just below the horizon, a cardioid pattern. Examples of the right-hand circular polarization elevation plane patterns measured at those two frequencies are shown. The input match for the antenna is illustrated by an overlay of the reflection coefficients at the input to each of the four arms of the helix.