Prototype Design of New Orthomode Transition
David Stewart
(NRAO summer student, 6/04-8/04)
Prototype Design for a New Orthomode Transition
My name is David Stewart and I will be starting my second year of graduate school in electrical
engineering at Virginia Tech later this month. I earned my Bachelor’s degree in Physics. My advisor this
summer was Dr. Shing-Kuo Pan and the subject of my ten week assistantship from June 7, ’04 to August 13,
’04 has been to work on a new design for an orthomode transducer. It is hoped that the new OMT will
eventually be used in the Green Bank Telescope, and quite possibly in other applications as well.
An orthomode transducer is the part of a radio frequency receiver that divides incoming electromagnetic
waves into their orthogonal components in such a way that the two components can be effectively converted
into separate electrical currents. All EM waves are composed of an electric and a magnetic field. Each of
these fields points in a particular direction at any given point in time and space. As a wave enters the OMT,
the x-component of the wave’s energy will follow one path, and the orthogonal y-component, another path.
At the end of the OMT these orthogonal components are transformed into currents by the placement of a
metal coupler inside the metallic waveguide (which is the OMT.)
Much of the current receiver system on the Green Bank Telescope (GBT) is in the process of being redesigned. The OMT that I have been working on this summer is a part of a new receiver design which
includes four low noise amplifiers, a 180 degree hybrid junction, and other components. A schematic of the
OMT’s place in this new receiver design is shown below.
180* hybrid
junction
Mixer
Previous and new OMT designs
All previous OMT’s employing a quad-ridged waveguide design use two orthogonal couplers, one located
some small distance behind the other (see figure 1b.) The existence of this offset of the couplers in the current
designs has the unwanted effect of producing secondary, trapped mode resonances in the structure. This
means that the asymmetrical geometry of the couplers in the current design causes a small but significant
amount of the incident field energy to become trapped in the OMT so that this energy is like a standing wave
in the structure and causes small distortions in the detected signal. Cross coupling between the orthogonal
probes also exists to some extent in existing OMT’s, likely as a result of their particular placement in the
waveguide. The bandwidth (defined approximately by S11< -18 dB) of OMT’s such as that just described is
about 1.6.
The design proposed in this work has four coaxial probes located symmetrically in a quad-ridged
waveguide. The probes are arranged in the same plane, their direction perpendicular to the length of the
OMT, and they are separated by a fin traveling down the middle of the waveguide (see figures 1c,d,f .) The
center fin is tapered, as are the outer fins and the walls of the waveguide. The two outputs for each
polarization are combined at a 180 degree hybrid after leaving the OMT. This new balanced configuration
will cause the field generated by each pair of parallel probes to be properly coupled to the TE10 and TE01
modes.
Figure 1a
(previous design, back)
Figure 1b
(previous design, side)
Figure 1c
(new design, back)
Figure 1d
(new design, side)
Note: .470 is the distance from
end of waveguide to center of
coupler.
.426
1.512
.470
.684
.342
1.512
Note: the extension of the outer fins past
the teflon piece should be as small as
possible; .002 is acceptable.
Figure 1e
(new design, couplers)
.160
Teflon
(epsilon=2.1, mu= 1)
.050
The symmetrical arrangement of the structure, and the
placement of the fin in the middle of the waveguide ensures
that there will be almost no cross coupling between
orthogonal probes, and that trapped mode resonances will
not be produced.
Figure 1f
(new design, side)
6.000
1.750
2.000
Note: same function defines taper for all pieces with taper.
Function for taper is that defined by “CST Microwave
Studio’s” “loft” command, with value chosen as .3.
Note: the extension of the fins to the outside of the
waveguide was done only out of necessity in order
to enter the structure into Microwave Studio.
Order of this summer’s work
The first task I undertook this summer was to read several technical papers and one or two
textbook chapters in order to gain some understanding of what my project was about. I had never
before done a search for scientific literature on any specific topic, so this was a learning experience in
itself. I found that there were a very limited number of papers discussing OMT’s, (about 6 in all IEEE
publications for the past 40 years) and these few were short in helpful information for the design of the
new OMT. My next task after gaining some background in the subject was to learn how to use the
program, “Computer Simulation Technology’s, ‘Microwave Studio.’” The purpose of this software is
to simulate the performance of an electromagnetic device such as an antenna, a transmission line, or an
OMT, the design of which is inputted by the user. I was very impressed with this software and it took
only about a week to learn how to use it relatively well.
In the user’s manual that is included with the software it is stated, “CST MICROWAVE
STUDIO is a general-purpose electromagnetic simulator based on the Finite Integration Technique
(FIT), first proposed by Weiland in 1976/1977 [1].” This numerical method is the technique used by
the program to approximate solutions to Maxwell’s equations for many small grid spaces into which
the inputted structure is divided. This is a reliable technique, shown by the fact that “Microwave
Studio” has been widely used by engineers for several years. The factor I had to be most aware of
using this software was not the intrinsic reliability of the software, but rather the accuracy I chose to
specify for each simulation run. In each modeling study, higher specified accuracy causes a longer
simulation time. Some simulations took up to two days to complete, and so this balance between
accuracy and computation time was a consideration in my work with the OMT.
In optimizing the design of the OMT, I began by entering the basic design into “Microwave
Studio” and then running the first simulation of its performance. The parameters of the device were
varied until a best performance was achieved. Initially the bandwidth of the OMT was very narrow.
But shortly after beginning simulations, a simple modification was employed allowing the four outer
fins to end before the end of the waveguide so that there was a gap at the end of the structure. This
modification greatly increased the bandwidth of the OMT. After the structure was further optimized, a
separate tapered structure was drawn into CST for the purpose of providing a transition between the
quad-ridged waveguide and the “horn” that will attach to its mouth. In radio frequency receivers this
horn acts like a funnel to feed incident EM waves into the OMT. This transition piece presented some
difficulty to draw in “Microwave Studio” and the simulation times with this piece increased to about 4
hours per simulation or longer. It is appropriate to note that much of the work with the OMT could
have been done faster had the simulation times been shorter. The performance of this tapered piece
by itself was optimized and then this piece and the OMT were put together in “Microwave Studio” at
which time further simulations were run to analyze the performance of the full structure. Current
design parameters and pictures of the full structure are shown below.
Results
Current results for the OMT are very encouraging. The simulations indicate that a
bandwidth of 2 to 1 will be possible with the new design, cross coupling levels will be
satisfactorily low, and that no trapped mode resonances will occur. On all previous quad-ridged
OMT’s, including that currently in use on the Green Bank Telescope, the maximum bandwidth is
approximately 1.6 to 1. The improvement of around 25% in bandwidth with the new OMT is the
most significant result. In short, according to the simulations in “Microwave Studio” the new
OMT will give better performance than any quad-ridged OMT currently in use. The S-parameter
graphs below show the simulated performance for the new OMT.
Figure 2a
Figure 2b
Figure 1 shows the total amount of power reflected
out of the OMT (S11.) Best results have this
amount as low as possible since the goal is to
convert all incident power into electric currents,
thereby producing a stronger signal.
Figure 2 shows the amount of cross coupling
present in the OMT.
Figure 2c
Figure 3 shows the amount of power reflected
at the coax ports. This quantity should be
nearly identical to figure 1. Best results have
this quantity at a minimum.
Conclusion
The performance of the new OMT could likely be improved with further variations in the
geometry of the structure. These variations are part of the work that will need to be completed
before the new design is built and put into use. Other work will include finding the best geometry
to match the OMT to the four low noise amplifiers following it, and also to the horn in front of it.
It seems likely that the new OMT will eventually be built and that its performance will be good
enough to earn its use in the GBT and other applications.
I would like to say that NRAO has been a great place to work and that I have learned a
good deal this summer. I am impressed with the way the summer students have been treated and I
was especially grateful that we were given a tour of the Green Bank Telescope and that we were
even given observing time on it.