Miniaturization of TEM Horn Using Spherical Modes Engineering

The spherical mode expansion is exploited to miniaturize a transverse electromagnetic (TEM) horn antenna and achieve good frequency and time domain performances. To adjust the radiated powers of fundamental transverse electric and transverse magnetic modes, the combined loop and TEM horn antenna is engineered. The constructive combination of the two modes below the horn's turn-on frequency miniaturizes the antenna and improves its gain. The magnitude and phase tuning of the desired modes is achieved by modifying the loop's geometry. The spherical modal spectrum at higher frequencies is also studied and the antenna's geometry is further modified to maintain constructive interference between the higher order spherical modes. A prototype with electrical size of 0.188 λ x 0.145 λ x 0.087 λ (length x width x height) at the turn-on frequency is fabricated over a ground plane. Over three times lower turn-on frequency is obtained compared to a conventional TEM horn with up to 9 dB improvement in the realized gain at the low end and better radiation characteristics over more than 5:1 bandwidth (from 0.87 to 4.5 GHz). Unidirectional operation with front to back ratio 9 dB and excellent time-domain performance with fidelity factor 95% over wide field of view are also demonstrated.