Improving an inverted trapezoidal antenna for mobile communications

Impulse radio is seen as one of the possible solutions to the continuously increasing need for higher transmission capacities in mobile communication. One of its complicated elements is the ultra wide band antenna, which has to provide several gigahertz of usable spectrum and preferably should be of small size. The suitability of an inverted trapezoidal antenna has been evaluated. Simulations indicate that, contrary to earlier results, the frequency response has a second false pass band below the nominal one, for example in a 5 GHz antenna at 1800 MHz. However, prototype measurements confirm that the problem is in the simulation environment, not in the design itself. The radiation pattern is optimally aligned in the vertical plane and the unwanted coupling towards the user´s head is minimal but the horizontal structure needs improvements, which can be achieved by folding the two halves of the trapezoid slightly towards each other. In a 5 GHz antenna, a practical main beam is obtained with a 20-30 degree bend. The thickness of a trapezoidal antenna can be reduced to less than one half of the original without sacrificing SWR.