The Frequency Diplexer as a Combiner

Mar 31, 2026 | Engineering Resources, Filters, Power Splitters / Combiners

Frequency diplexers (henceforth simply diplexers) serve to either divide (demultiplex) the frequency spectrum into two major sub-bands or perform the complementary function of frequency multiplexing where the two sub-bands are combined to form a full frequency spectrum. The focus of this article is the latter, the combining of two signals or sub-bands into one contiguous spectrum. Diplexers are a 3-port component comprised of 2 ports for the individual sub-bands and a third port defined as the full-band input or output, depending upon the orientation in which it is applied.

Diplexers are important because they allow two or more devices that operate on disparate frequencies to share a single transmission medium at the same time without interfering with one another. Since they reduce system cabling and complexity, they are considered economical to use for commercial wireless base stations, aerospace and defense systems, satellite communications (SATCOM) terminals, and for consumer electronics devices that operate over Wi-Fi and Bluetooth.

Diplexers come in many shapes and sizes and there are a multitude of technologies available with which to build them. Cavity, waveguide, dielectric resonator, substrate integrated waveguide (SIW), and microstrip/stripline diplexers are the most common types. Out of all these technologies, the one that arguably stands out as having the best balance of overall performance characteristics (SWAP-C) is the stripline variant, more commonly known as the suspended substrate stripline (SSS) diplexer. Almost 50 years ago, Rooney and Underkoefler first proposed applying SSS technology to multiplexers1, but David Rhodes was most instrumental in developing and applying this technology throughout the decade that followed.2,3,4 Mini-Circuits’ SSS diplexers appear directly on the website and many more custom options are available. SSS technology is described in Mini-Circuits’ Understanding Suspended Substrate Stripline Filters, dated April 26, 2022.5 In the paragraphs that follow, the performance of two Mini-Circuits’ SSS diplexer components is examined in terms of combining losses, frequency selectivity, and how they are applied to solve real-world challenges in applications where combining separate frequency bands is required.

Combining Separate Frequencies – Diplexing vs. Power Combining

In traditional in-phase and quadrature power splitters and combiners, signals of different frequencies at separate ports experience a theoretical 3 dB loss plus the additional, practical insertion loss of the component itself. Diplexers split and recombine in a frequency-selective manner such that the sub-bands are subjected to only the practical loss of the component. Diplexers not only have a 3 dB advantage in loss over their power-combining counterparts, but many systems benefit from the inherent filtering that they provide.

Many diplexer configurations are possible by combining filters with low pass, high pass, and band pass filter characteristics. Perhaps the simplest diplexer is the low pass-high pass realization shown in Figure 1.

Figure 1: Block diagram of a simple low pass-high pass diplexer with individual ports on the left and full-band port on the right

To fully define even the simplest of diplexers, several more characteristics are required. The nature of the stopband is important, since it may be reflective or absorptive. Additionally, the full-band output can be contiguous, as shown in Figure 1, or non-contiguous, with a gap introduced between the low pass and high pass outputs. Finally, the high pass section is generally open-ended, extending beyond the upper operating frequency, but may be closed or include a clean-up low pass filter, technically making the upper section a band pass.

Mini-Circuits’ Diplexer Use Case – 2.4/5/6 GHz ISM Band Applications

Mini-Circuits has designed and produced dozens of different diplexer models over the years, many of them using suspended substrate stripline (SSS) technology. One such diplexer that is compact, cost-effective and high-performance is the ZDSS-2R5G5G-S+. Figure 2 shows the insertion loss vs. frequency for this diplexer out to 6 GHz as well as markers for the 2.5 and 5 GHz ISM bands that are combined to illustrate the legacy dual-band application. These are the original bands in which Bluetooth, Wi-Fi, IoT and other ISM band systems operate. Most noteworthy is the very low insertion loss for each of the bands, as well as the high suppression of the second harmonic of the 2.4 GHz band (> 50 dB) as seen by the high pass (5 GHz) side of the diplexer.

Figure 2: Plot of S21 and S31 vs. frequency for ZDSS-2R5G5G-S+ diplexer with the 2.4 GHz ISM band marked in the low pass section (Port 2) and the 5 GHz ISM band marked in the high pass band (Port 3)

While Figure 2 is only a plot of the diplexer insertion loss to 6 GHz to clearly illustrate the original dual-band ISM application, the ZDSS-2R5G5G-S+ is an extended-frequency model that is more than capable of supporting tri-band operation as shown in Figure 3, which has been plotted to 8 GHz. In Figure 3, it is understood that the diplexer supports the 2.4 and 5 GHz bands, so the 6 GHz ISM band (5.925-7.125 GHz) is now marked in the high pass section to illustrate a full tri-band ISM use case. A typical application is for tri-band boosting and routing of Wi-Fi signals, particularly Wi-Fi 6E, which can take advantage of any of the bands individually, including the 6 GHz band, and Wi-Fi 7, which can take advantage of the bands simultaneously.

Figure 3: Plot of S21 and S31 vs. frequency for ZDSS-3G4G-3+ diplexer with Port 2 as the low pass, Port 3 the high pass and with the 6 GHz ISM band marked in the high pass section

The Multi-Band Radar Application

Over the decades S-band and C-band radars have not only proliferated but have evolved from mechanically steered analog systems to very sophisticated software-defined active electronically scanned arrays with extensive digital signal processing backends. One use case for a diplexer in the radar domain is to combine or split S- and C-bands. While S-band radar frequencies in use conceivably extend slightly beyond 3.5 GHz, for the purposes of this application, the focus will remain the legacy 2.7-3.1 GHz band. For C-band, of course radar altimeters operate in the 4.2-4.4 GHz region, and everything from there to 8 GHz is still considered C-band. For all intents and purposes, we’ll also focus our application on the popular 5-6 GHz region of this band. A radar diplexing the 2.7-3.1 GHz sub-band of S with the 5-6 GHz sub-band of C would take advantage of the SWAP-C benefit achieved by combining them together, the lower spatial losses of the S-band radar system as well as the higher resolution of C-band system.

Figure 4 illustrates the insertion loss vs. frequency of the ZDSS-3G4G-3+ SSS diplexer, truncated to 8 GHz. This diplexer actually performs quite well to 14 GHz and the ZDSS-3G4G-S+ performs well to 20 GHz. Designed specifically for handling 25W RF input power per port, the ZDSS-3G4G-3+ is a low pass-high pass, reflective, contiguous broadband diplexer with a crossover frequency of approximately 3.45 GHz. 25W per port makes for a robust combiner, but this diplexer can also be utilized as a splitter. If the application were not radar for instance, the splitter would often be made much smaller and lower in power to meet even more stringent SWAP-C requirements.

Figure 4: Plot of S21 and S31 vs. frequency for ZDSS-3G4G-3+ diplexer with Port 2 as the low pass for 2.7-3.1 GHz S-band and Port 3 the high pass for 5-6 GHz C-band

The frequency response characteristics of each of the ports of the ZDSS-3G4G-3+ diplexer are shown in Figure 4. Notice how the diplexer full-band response is contiguous. The crossover region is just below 3.5 GHz and is also very well-behaved, dipping little more than 3 dB for both the low pass and high pass sections. Although the insertion loss (y-axis) scale is 20 dB per division, it is still apparent that the in-band insertion loss for both S- and C-band is very low, only several tenths of a dB, which is to be expected. Most importantly, the diplexer does quite well in preventing the second harmonic of the S-band radar (5.4 GHz to 6.2 GHz) from spilling over into the C-band radar spectrum by attenuating it more than 60 dB beginning at 5.4 GHz.

The Diplexer – A Natural Combiner

In this article we introduced the diplexer in general and described its benefits in combining signals of differing frequencies when compared to a power combiner. An application utilizing the ZDSS-2R5G5G-S+ diplexer was provided in which 2.4/5 dual-band and 2.4/5/6 GHz tri-band ISM band Wi-Fi/Bluetooth combiners were envisioned. The ZDSS-3G4G-3+ diplexer was discussed in detail, and its insertion loss characteristics were both truncated and plotted versus frequency. An S-band/C-band radar use case was described and the performance characteristics of the ZDSS-3G4G-3+ analyzed, particularly the ability of the diplexer to ensure that the C-band radar spectrum remains well-isolated from the second harmonic of the S-band radar system.

References

  1. Rooney, J. P. and L. M. Underkoefler, “Printed circuit integration. of microwave filters,” Microwave J., Vol. 21, 68–73, 1978.
  2. Rhodes, J. D. (1979, August). Suspended substrates provide alternative to coax. Microwave Systems News (MSN), 9, 134–143.
  3. C. I. Mobbs and J. D. Rhodes, “A generalized Chebyshev suspended substrate stripline band pass filter,” IEEE Trans. Microwave Theory Tech., vol. MTT-35, pp. 397-402, 1983.
  4. J. D. Rhodes, “Suspended Substrate Filters and Multiplexers,” 1986 16th European Microwave Conference, 1986, pp. 8-18.
  5. Understanding Suspended Substrate Stripline Filters – Mini-Circuits Blog