Abstract
The normal mode helical antenna (NMHA) is a common antenna used on transceivers for wireless communications due to its omnidirectional radiation pattern, large bandwidth and small physical size. Applications such as ground penetrating radar (GPR) and Croswell radar tomography (CRT) require ranges of frequency in the order of 4 MHz to 1 GHz and can benefit from the use of NMHA. A scale model of a NMHA operating from 0.2 GHz to 10 GHz has been developed for a CRT system at Northeastern University. The antenna must operate with an omnidirectional radiation pattern and be less than 0.01 m in diameter. A new methodology and procedure to enhance the bandwidth of the NMHA is presented. The log-periodic principle is applied to the separation (S) between turns of the NMHA; the bandwidth of this antenna is increased considerably. This technique is based on the three Kraus' criteria to guarantee the helical normal mode operation. Several LP-NMHA antennas designed using this technique were built and tested; the bandwidth is significantly improved for each of them. The frequency range of operation obtained covered from 2.7 GHz to 12 GHz. To verify the normal mode operation, several radiation pattern measurements were performed.
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