To: File \hagen\wpfiles\430tx\430tstl.wp6

From: Jon Hagen

Date: 9-17-99

Subject: Half-height Turnstile - Electrical Design

Implementation of 430 MHz radar capability from the Gregorian dome requires a dual circular polarization "self-duplexing" OMT (orthomode transducer). A turnstile junction has been used on the 430 line feed since the observatory first began operations and also in the S-band radar system. A more compact turnstile will be built for use in the dome. The waveguide run from the traveling collector, through the collar truss, and down to the feed room will be made of half-height WR2100. If a half- height turnstile is used, no impedance transformer is needed. The design goals for the half-height turnstile are sufficient bandwidth, and compactness - both in height and in the lengths of the shorted side arms. The output waveguide diameter is 20" to match the dual-mode feed horn. The input waveguides are 21" x 5.25" (half-height WR2100).

In Volume 8 of the Rad Lab Series, Dicke discussed the turnstile as a 6-port waveguide junction. Four of the ports are the rectangular arms and the other two ports are taken to be the two linear polarizations in the circular guide which are parallel to the arms. A turnstile will automatically have the properties needed to make OMT if it is matched. Matching simply means that if all but one of the six ports are terminated with matched loads, there will be no reflection at the remaining port. When matching is accomplished by any internal and/or external structure that preserves the 4-fold symmetry, the matched turnstile will have the following properties: half the power incident on a rectangular port will exit the circular guide while the other half will divide equally between the adjacent rectangular ports. No power will appear at the opposite rectangular port. Given these properties, it is straightforward to see how to install shorts on two opposite rectangular ports to produce an OMT with any desired polarization.

For matching, Dicke suggested concentric posts coming out of the floor of the turnstile. He showed that there must be at least three adjustable parameters. Most turnstiles use two posts; two diameters and two heights provide four parameters. The bottom post is adjusted first, changing its height and diameter until there is no reflection at the circular guide when all four rectangular guides are properly terminated. The top post or pin, is then adjusted so that there is no reflection at a rectangular port when all the remaining ports are properly terminated. If the pin diameter is much smaller than the wavelength, the first adjustment remains unchanged.

Dicke's tuning procedure was carried out with the Ansoft finite-element 3-d electromagnetic simulator HFSS. The result was a post with a diameter of 6.5" and a height of 8" and a pin with a diameter of 1", extending 5.75" above the top of the post. Once the 6-port device was tuned, the value of the phase shift from a rectangular port to an adjacent rectangular port was noted and used to calculate the side short lengths needed to produce an OMT with circular polarization. These dimensions are noted on the attached drawing, "tstil0.ecw." A table on the drawing shows the performance vs. frequency. At 430 MHz, the return loss looking into a rectangular port is -27.1 dB. The isolation of the opposite port is - 30.5 dB. The left-hand circular output power is 48 dB greater than the right-hand circular output power. The maximum field strength occurs at the top of the pin and is no more than about four times higher than the maximum field in the input waveguide. Thus, the device should be good for some 100/42 = 6 megawatts. Note also that our S-band turnstile has a 1/2" diameter pin and withstands 1 MW. That alone suggests that, with its 1" diameter pin, this turnstile should handle several megawatts.

For comparison, the dimensions of our spare full-height 430 MHz turnstile were measured and it was simulated with HFSS. Those results (surprisingly good) are shown on a second attached drawing, "tstil00.ecw." The half-height turnstile has essentially the same bandwidth as the full-height turnstile.

The consistent results of the HFSS, especially its verification of the performance of the old turnstile, suggest that there is little risk in building the half-height turnstile with no provision for adjusting the side shorts. Some adjustability (shims) will be provided for the post structure. When the device is finally installed, the cross-port isolation, vital for receiver protection, can be maximized by installing a small obstacle in the circular output waveguide. This will compensate not only for imperfect isolation of the turnstile itself, but also for stray power reflected back into the receiver by diffraction at the mouth of the horn or the tertiary reflector.