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Point-to-point radios at 18 GHz link many different communications systems, including terrestrial and satellite-base systems. A key component for these systems is the diplexer, which helps to manage and sort multiple signals. Fortunately, a simple and efficient technique has been developed for designing a waveguide diplexer for 18-GHz point-to-point communications applications, and the design approach can also be extended to the design of general junction-type waveguide multiplexers.

Microwave diplexers and multiplexers filter and separate the multiple frequency channels or bands in a common transmission line to direct each channel to its proper output port. In reverse, these components can combine signals of different frequencies at separate input channels into a single output port. Diplexers and multiplexers are essential when transmitting multiple, different-frequency signals through a single transmission medium, such as an antenna or transmission line.1 Figure 1 shows a typical multiplexer. Signals for transmission (T1, T2, through Tm) are in the m channels, while the received signals (R1, R2, through Rn) are in the n channels.

Waveguide Diplexer Links Point-To-Point Systems, Fig. 1

In general, the channels in a multiplexer can be contiguous but not overlap. Typical microwave multiplexers are implemented in rectangular or circular waveguide,2 on printed-circuit boards (PCBs),3 and as ceramic structures.4,5 Bandpass filters with a narrow bandwidth and a sharp rolloff property are used in each channel to screen desired frequency components and reject unwanted out-out-band signals. Bandpass filters are combined at the multiplexing junction into the common port or vice versa.

The multiplexing junction is a crucial part of a microwave multiplexer. Major types of multiplexing junctions used in waveguide multiplexers include the circulator/filter chain, the directional coupler/filter scheme, the manifold structure, and the branching filter scheme.1 Mutiplexers employing the latter two structures are often called junction-type multiplexers. In junction-type multiplexers, impedance-matching elements are usually employed in the junction region to achieve low reflection at all bands. This article will focus on the design of junction-type diplexers.

Diplexers are multiplexers with two channels. They are used for separating or combining transmitted and received signals in single-antenna communications or radar systems. Using a simple approach, it is possible to design high-performance T-junction waveguide diplexers, and the basic idea behind the technique can be extended to the design of general junction-type multiplexers.

Figure 2 shows the structure of a diplexer designed with this simple approach. It consists of bandpass filters 2 and 3, operating at bands 2 and 3 respectively. The diplexer is implemented in rectangular waveguide with a broad wall width of a and a narrow wall height of b. In most standard rectangular waveguide structures, b is one-half the value of a. For this multiplexing junction, an H-plane T-junction with a septum matching element is employed. Other types of waveguide junctions and matching elements can be used as well.

Waveguide Diplexer Links Point-To-Point Systems, Fig. 2

Figure 3 shows the design parameters for the diplexer junction. A combination of a septum of suitable dimensions and distance L3 from the junction center line to the first element of filter 3 enables impedance matching at band 2. Similarly, for impedance matching at band 3, the distance L2 is optimized with proper septum dimensions. A common design approach is to represent the diplexer junction and channel filters with network parameters and to optimize the T-junction structure and distances from the T-junction to channel filters—and in some cases, the first few elements in the channel filters.6-10 The design of the diplexer junction can be greatly simplified with the aid of modern electromagnetic (EM) simulation software tools.

Waveguide Diplexer Links Point-To-Point Systems, Fig. 3

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