Me as u r e d Theory Me as u r e d 7.785 GHz 8.025 GHz 4.8 4.4 6.3% 15.1 100% 90%
for both planes and the discrepancy observed at small elevation angles, particularly for EB, is attributed to the finite size of the ground plane. The difference between the copolarized and cross-polarized levels was greater than 25 dB.
The antenna gain was measured with the use of the same receiving antenna as for the radiation patterns, for several distances away from the DRA, and the value obtained was 4.4, against the predicted value of 4.8. The bandwidth of the DRA for a VSWR I 2.5 was measured to be 6.3%, indicating a Q factor of 15.1. The radiation efficiency of the DRA, measured with the use of the Wheeler cap method [15, 161 with a metallic box of 25 (height) X 50 (width) X 50 (length) mm3, was 90%. The theoretical and experimental results obtained are tabulated in Table 1.
4. Conclusions
A theoretical formulation together with experimental investigation of an unmagnetized ferrite DRA operating at the HEZls mode have been presented. Expressions for the electric field components in the far field have been derived. Radiation patterns and the dependence of directivity on the dimensions of the DRA have been illustrated. The results indicate that the radiation pattern is more directional for small aspect ratios and that the directivity decreases as the height decreases and as the radius of the cylinder increases.
Experimental results showed that the magnetic-wall model employed gives good prediction in resonant frequency, radiation pattern, and directivity. More accurate prediction in these parameters would require the use of numerical methods. The gain of the DRA is 30% greater than the gain of an ideal monopole and the bandwidth is about four times greater than the bandwidth of a patch antenna. The radiation performance of a cylindrical DRA operating at the HEzls mode is superior to the performance of a monopole or patch antenna and hence, makes it a better choice for many antenna requirements.