Vertically-polarized antennas with omnidirectional radiation characteristics have been the subject of many studies in the past. These antennas are commonly used for many communications applications, particularly military communications at the HF, VHF, and UHF frequency bands. In such applications, physically large monopole (whip) antennas are widely used. However, such monopole antennas suffer from a number of practical challenges. First, to radiate efficiently, monopole antennas need to be at least a quarter of a wavelength long. Therefore, it is not uncommon to see several-meters long whip antennas used at HF and VHF frequency bands. Such physically large antenna dimensions present numerous practical and security challenges in many military communications systems. Second, whip antennas tend to be narrowband and seldom can they cover the entire HF or VHF band without the use of external antenna tuners. This limits the application of such antennas in radio systems that exploit waveforms with instantaneously wide bandwidths. Therefore, compact wideband antennas that radiate like vertically-polarized monopoles but can cover an extremely large instantaneous bandwidths are desired.
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