A Study On Low-profile Antennas With End-fire Radiation

Vertically polarized antennas with low profile and end-fire radiation are very promising in airborne,carrier-borne,and vehicle communication and radar system.This dissertation focuses on the theory of end-fire antennas and techniques to achieve low profile and wideband.The main work is summarized as follows:1.An equivalent circuit of the top-hat folded monopole and a new top-hat(folded)helical antenna are proposed:The input impedances of a top-hat unfolded and folded monopole are studied first.The effect of the top hat and the shorting probe on the monopole is analyzed to establish a new equivalent circuit of the top-hat folded monopole antenna,which well explains its resonant character.On the other hand,a new top-hat(folded)helical structure is proposed.The input impedance and radiation characteristic of the helical antenna are studied.The specialties of these two kinds of antennas are concluded through comparison.2.A low-profile end-fire quasi-Yagi antenna based on top-hat(folded)monopole:A new low-profile top-hat quasi-Yagi antenna structure consisting of a top-hat folded monopole as the driven element and a grounded top-hat monopole as the parasitic element is proposed.By optimizing the parameters of the antenna,a broad band and a good front-to-back ratio are achieved.A 5-element quasi-Yagi antenna is designed and measured.Measured results show that a impedance bandwidth of 20.5%,a front-to-back ratio of 10 dB,and a gain up to 6.6 dBi are achieved while the antenna height is only 0.033?L at the lowest operating frequency.3.A planar low-profile end-fire quasi-Yagi antenna based on top-hat(folded)helix:A new planar helical quasi-Yagi antenna with low profile based on a substrate is proposed.By combining the antenna and a microstrip Butler matrix,a multibeam end-fire array antenna is achieved.4.A wideband low-profile end-fire log-periodic array based on top-hat monopole:The top-hat monopole is employed as the element of a log-periodic array to solve the critical problem of the profile of a conventional log-periodic monopole array.All the monopoles of the log-periodic array are made with the same height through different top-hat loadings.A wideband transition from the coaxial line to the conductor-backed coplanar strip line is designed to feed the array.The feed line is meandered to connect with the vertical monopoles aligned along a straight line to guarantee a symmetric H-plane pattern.The width of the feed line is varied all the way to achieve a good reflection coefficient of the antenna.The antenna has a very low profile of 0.047?L.Measured results show that the antenna has an impedance bandwidth of 4.53:1 and a gain up to 9.0 dBi.