A Tunable, High-Gain, Very Low-Profile Composite Monopole Antenna for Low-Frequency Applications

This paper presents an extremely low-profile frequency-tunable composite antenna with enhanced gain and an omnidirectional radiation pattern. The antenna is based on a recently developed very short two-legged monopole antenna and consists of one driven element and eight identical parasitic coupled elements. By utilizing the mutual coupling between elements, the radiation resistance of the antenna is significantly increased, leading to considerable gain enhancement and better matching performance. The composite antenna has the same height as the single monopole (lambda(0)/150) but a larger lateral dimension (lambda(0)/16). The performance of the proposed antenna is compared with that of a single monopole antenna. It is shown that the peak gain of the composite antenna is about 10 dB higher than that of the monopole antenna. The design is then modified for frequency tunability to achieve frequency agility and higher bandwidth for certain modulations like frequency hopping spread spectrum. The fractional bandwidth of the tunable antenna is 3.32%, and the gain is further improved at higher frequencies. A new method of effectively measuring a monopole antenna with an electrically small ground plane using cascaded transformers is also presented and discussed.