Sensor and Simulation Notes

Two offset IRAs are numerically modeled and analyzed. The reflector of the offset IRA is an offset portion of a paraboloid that is carved out by the intersection of the paraboloid and a cone whose apex is at the focal point of the paraboloid. A pair of conical curved plates is placed on the surface of the cone as TEM feed arms. The TEM feed arm of the first IRA is conical throughout its length and terminated at the reflector through two electrically small resistors. The TEM feed arm of the second IRA is partly conical and tapered to one electrically small resistor. The second IRA performs better in terms of the reflected voltage in the transmission line. The second IRA is further analyzed and compared with a center-fed IRA. The radiated waveforms both in the farand near-zones, radiation patterns, impulse and prepulse amplitude distributions as functions of observation angle, spot sizes, and power budgets for the offset IRA and the center-fed IRA are presented and compared. The results show that the tail waveform of the offset IRA is significantly lower in amplitude and simpler in shape than that of the center-fed IRA. Both IRAs have a shadow behind the reflector; the shadow location of the offset IRA may be useful in bistatic radar applications.