The Relationship Between Feed Arm Position and Input Impedance in Reflector Impulse

For many years reflector Impulse Radiating Antennas (IRAs) were built with four feed arms positioned at ±45° to the dominant polarization (usually vertical). However, recent experimental and numerical investigations have demonstrated improved gain and crosspol performance when the feed arms were positioned at ±30° to vertical. Despite these results, there has been a belief that one could obtain a better TDR and lower return loss with feed arms positioned at ±45°. Because a flat TDR is especially desirable in radar applications, designs with feed arms at ±30° have been slow to be accepted. We compile here TDR data on several pairs of IRAs that are similar except for their feed arm positions. In controlled comparisons, we find no appreciable advantage in the TDRs of IRAs with feed arms positioned at ±45°. This suggests that the ±30° configuration is preferred because of its other advantages.