The characteristics of coplanar waveguide (CPW) fed ring antennas (such as gain, efficiency and polarization purity) are degraded by the dielectric losses and excitation of surface waves on the substrates. To counteract such effects, we propose to remove some portions of the substrate in the vicinity of the metallic patches, which effectively eliminates the conditions for the generation of surface waves. The proposed technique is applied to a CPW-fed ring antenna and a prototype model is fabricated. The measurement data and the computer simulation results agree well and indicate the efficacy of the technique for the improvement of antenna gain and efficiency.
It is desirable to improve the antenna radiation efficiency by using suitable methods to decrease the substrate dielectric losses and limit the spurious surface waves. Although the dissipation losses of available substrates are usually low, the excitation of surface waves degrades the printed antenna performance such as polarization purity and radiation efficiency [1-3]. The surface waves are always present, because the cut-off frequency of their dominant mode is zero. Surface waves move towards the antenna edges, where they radiate into space. Consequently, it is advisable to devise some techniques to suppress the surface waves and prevent them from deteriorating the antenna efficiency and generating cross-polarized fields which spoil the antenna characteristics and polarization purity.
In this paper, we propose to remove the substrate in the neighborhood of the antenna, which counteracts the conditions for the excitation of surface waves. The proposed technique is applied to a printed CPW-fed ring antenna. A prototype model of the proposed antenna is fabricated and its Characteristics are measured and compared with computer simulation results.
We study the printed CPW-fed ring antenna as shown in
This leads to a formation of two equiphase currents on the ring, which generates the radiation perpendicular to the plane of antenna. The width of the feed line is calculated to produce a characteristic impedance of 50 ohm [
return loss is less than –10 dB. The radiation efficiency of the CPW-fed ring antenna as obtained by the computer simulation is shown in
Since the cut off frequency of dominant mode of surface waves is zero, they are always present in microstrip antennas and produce spurious radiations in undesired directions, such as towards the edges of substrates as shown
Therefore, the radiation efficiency of the antenna gets
deteriorated and its polarization purity is degraded.
Accordingly, it is proposed to cut out portions of the substrate, which are not under the metallic patches, in order to eliminate the conditions of propagation of surface waves and take them into the evanescent modes, which are drastically attenuated.
Consequently, the antenna radiation is directed mainly towards the antenna broad side direction. Note that this method is not suitable for the microstrip antennas, which operate on the basic of surface wave couplings like quasi-yagi antennas [
Furthermore, the removal of some portions of the substrate, changes its effective permittivity and also its operating frequency. Of course, this frequency variation is negligible. Our proposed method is applied to the CPW-fed ring antenna as shown in
Note that the proposed structure of cutting out portions of substrates has little effect on the return loss, operating frequency and radiation pattern of antenna.
Furthermore, the radiation efficiency of this antenna is drawn in
In this section, the computer simulation results and measurement data of two fabricated prototype antennas described in Sections 2 and 3 are shown. The photographs of the ring antenna and modified ring antenna are shown in
The return losses versus frequency of the two antennas are also shown in
Observe that the computer simulation results and measurement data for the return loss of the ring antenna agree quite well (see
Furthermore, there are some ripples in the pattern, which may be due to the antenna wide beam width (which is almost omnidirectional) and the small anechoic chamber (for such frequencies) which may generate spurious radiations, which are captured be the antenna due to it wide beam width.
In order to show the efficiency of the proposed antenna structure for the improvement of antenna efficiency, the gain of two antennas may be compared. Although their gains are small, and their measurements have some errors, they may yet be compared as shown in
This feature is important in high power array antenna, where any losses can be translated to major power consumption. In arrays, the proposed method can be used to significantly increase total efficiency by decreasing mutual couplings via surface waves. The measured antenna gain versus frequency is shown in
The surface waves are always excited and present on the printed antennas which propagate towards their boundaries. Consequently, they lead to the deterioration of their desired radiation properties, such as gain, efficiency and polarization purity. In order to counteract such adverse effects, we propose to remove some portions of the substrate in the neighborhood of the metallic patches. The efficacy of this technique is demonstrated by computer simulation, fabrication and measurement on a CPW-fed ring antenna. This technique may be readily applied to other types of printed antennas.
Furthermore, the proposed technique of removing dielectric substrates on the antenna structure may also be used in antenna arrays. This will cause the improvement of radiation efficiency and also the elimination of surface waves and as a result the reduction of mutual couplings among the antenna elements.