To: File

From: Jon Hagen

Date: Feb 12, 2000

Subject: Circular polarization at 401.5 MHz

The attached drawings are a design for a waveguide adapter to inject circular polarization into the 20" diameter neck of the 430 MHz dual-mode horn. HFSS confirms that this horn, designed for a center frequency of 430 MHz, should work well at 401.5 MHz.

Starting at the top, a half-height WR2100 waveguide-to-coax transition bolts to the half-height input waveguide which is mounted at an angle of 45 degrees on the top end of a 20" square waveguide. Only the fundamental mode can propagate in the half-height input guide. Symmetry ensures that equal amounts of x and y polarization are injected into the square waveguide. A capacitive tuning post in the input waveguide provides a match at 401.5 MHz.

A quarter wave plate in the square waveguide is formed by four square tubes - inductive posts, that span the waveguide. This polarizer provides a differential phase of 90 degrees at 401.5 MHz and has a return loss of about 25 dB.

The bottom end of the square waveguide has a 20" diameter hole connects directly to the 20" diameter input of the horn. The return loss at this square-to-round butt joint is better than 20 dB.