To: File C:\hagen\430tx\430_WR2100_turnstile.wpd

From: Jon Hagen

Date: 12-16-03

Subject: Arecibo Turnstile Junctions - Theory, Tuning, Performance

1. Bare 6-Port Turnstile Junction

The turnstile junction was invented and patented by Robert H. Dicke, who fully described its operation in Principles of Microwave Circuits, R.H. Dicke and E. M. Purcell, Rad Lab Series, Volume 8. The circular port supports two orthogonal modes, these modes are indicated on figure below as "Port 5 and "Port 6". The arrows indicate the electric field direction for these modes. At the rectangular ports, there is only one mode possible, and its electric field is perpendicular to the broad wall of the guide. If one begins with the bare turnstile, i.e. the rectangular and circular waveguide structure, it will be found that power applied to a side port will appear at the other side ports and also at Port 5 or Port 6. From symmetry, power applied to Port 1 cannot exit Port 6 and power applied to Port 2 cannot exit Port 5).

One can "tune" the device by inserting a structure of posts, concentric with the circular guide, or, in general with any other structure that, like the side ports, has 4-fold rotational symmetry about the axis of the circular waveguide (which could also be built with a square waveguide - or any other waveguide that has four-fold rotational symmetry). With an appropriate tuning structure, the junction will have the following properties:

a. All ports will be matched, i.e. none of the power incident on any port will be reflected, providing the other ports are connected to matched loads.

b. Power incident on a side port will split three ways: 25% will flow to each adjacent

side port and 50% will flow to either Port 5, or Port 6, as explained above. The opposite side port is isolated: no power flows to it.

2. Turnstile to Orthomode

When two opposite side ports are blocked off (shorted), the turnstile becomes

an orthomode junction, i.e. a device to separate or produce signals of different polarization in the circular waveguide.

Suppose we have built and tuned a turnstile junction. We now install shorts on the two unused sidearms and we make the length of these shorts differs by 90 degrees. Now we apply power to the transmitter port. Half this power proceeds out Port 6, as discussed above. One quarter of the power goes to each shorted side port and is reflected back into the turnstile. The reflections, as they reenter the turnstile, differ in phase by 180 degrees, due to the round trip through the 90-degree path difference. Thus the reflected waves cancel at both the transmitter port and at the receiver port (maintaining the isolation of the receiver). But each reflection also sends half of its power into Port 5. These voltages are in phase, so the total power arriving at Port 5 is equal to half the transmitter power, just as at Port 6..

The installation of the shorts on the sidearms has transformed the 6-port turnstile into a 4-port diplexer.

The turnstile in the carriage house is the original, installed around 1962. Its dimensions are shown below. The four side ports are WR2100 waveguides, 21" wide by 10.5" high. The circular waveguide has an inside diameter of 18.5". The post structure consists of three concentric cylinders of diameters 12", 8.500", and 2.000" whose nominal heights (all measured from the floor) are 0.250", 3.812" and 13.812". The heights of the two inner cylinders are adjustable.

21.3 Length of the turnstile side shorts

So far, the only requirement placed on the side shorts is that their electrical lengths differ by 90 degrees. But, for the device to act as a "self diplexer", the output wave must have circular polarization, i.e. the phase of the wave leaving Port 5 must differ by 90 degrees from the wave leaving Port 6. This is the same as saying that the wave entering Port 3, after being reflected from the side short at that port, differ in phase by 90 (or 270) degrees from the phase incident on Port 2, the transmitter port. If we know the the phase path, in degrees, between the transmitter port and an adjacent side port (i.e. the phase path between any two adjacent side ports), we can calculate the absolute lengths for the shorts. This phase path can be measured directly with a network analyzer or calculated using a finite element modeling program. If we do not know this phase path, we can fit the turnstile with adjustable shorts. The adjustment procedure would be first to set one short at an arbitrary position and set the other short farther out by guide/4. We can verify that the separation is correct by noting that feed through to the receiver port is minimized. Next we vary the positions of the shorts, while maintaining the guide/4 difference, to circularize the polarization leaving the circular waveguide. One way to do this is to put a metal cap over the circular waveguide. The cap provides a totally reflecting short which will change the sense of the circular polarization, causing the reflection to leave via the receiver port. If we put a matched load on the receiver port, we simply adjust the shorts until there is no reflection seen at the transmitter port.

21.4 Sense of the polarization: right-hand circular or left-hand circular? The turnstile pictured below, produces left-circular polarization. After reflections from the tertiary, secondary, and primary mirrors, the radiated polarization is right-circular.

We consider the polarization of the wave coming out of the turnstile. We will take the phase of the y- component to be the phase at the input plane. In our S-band turnstile, the phase path from the input plane to the left side plane is almost exactly 270 degrees, as shown by lab measurements and by HFSS simulation. The wave making this left turn is then shifted another 180 degrees when it reflects from the short. The total phase of the x-component is therefore 270+180 = 90 degrees. Expressing the x and y components, we have

Ex = Re (ej t- /2) = cos( t- /2) = sin( t)

Ey = Re(ej t ) = cos( t)

The E vector, as seen when the wave is approaching us, thus rotates in the clockwise direction. (As t increases from zero, Ex goes from zero to a positive number while Ey begins to decrease). By definition (IRE, IEEE) this is left-circular polarization. The tertiary, secondary, and primary reflectors each reverse the polarization so the telescope finally transmits right-circular polarization.