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This paper shows the design process and measurement data for a modified bi-directional equiangular spiral antenna fed by a microstrip to parallelstrip to twin-line transition on two substrates (20-mil thickness) with three layers (aligned vertically). By etching holes out of the spiral arms at the locations where the tapered parallel-strip line passed underneath, the impedance mismatch caused by the proximity of the spiral's metal layer to the parallel-strip line was reduced. Between 4 and 7 GHz, this antenna exhibits a typical radiation pattern for equiangular spiral antennas. Measured and simulated radiation patterns and VSWR plots for this frequency range show good performance.

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