Figure 3 VSWR curve for the 40 mm element, with feed gaps of Ž . Ž . Ž . 0.9 dotted , 1.8 solid , and 2.7 mm dot᎐dashed , based on an MoM analysis of the equivalent grid independent of the feed gap h, but the upper frequency is heavily dependent on it.
RADIATION PATTERN
The antenna exhibits an omnidirectional pattern in the H-plane with a maximum variation of "1.5 dB, and a typical monopolar pattern in the E-plane. The pattern variation with frequency is negligible over the impedance bandwidth. The gain was found to be 1.5 dBi in the plane of the ground plane, but increased to a maximum of 4.5 dBi at an elevation of 40Њ with respect to the ground plane, yielding a conical beam pattern. This is typical of monopoles, with the beam tilt due to the finite ground-plane effect.
CONCLUSION
A broadband square monopole antenna has been investigated, showing a constant radiation pattern over the impedance bandwidth. The effect of feed-gap distance is investigated. The frequency corresponding to the lower edge of the impedance bandwidth may be found using a simple formula, based on the equivalent radius concept. To obtain the upper edge frequency, a method-of-moments analysis which employs wire gridding is required.