Location and radii of the dielectric rods
1. Introduction
Metallic waveguides have major advantages, such as low propagation loss and high power transmission in the microwave frequency range. However, one disadvantage is that the usable frequency range is restricted to
Power sources, such as watt class IMPATT diodes or Gunn diodes, are readily available, and for high frequency use, power sources are sometimes combined, due to their low power rating. However, power combiners consisting of cavity resonators usually have narrow bandwidths (For example, Matsumura et al., 1987). Power dividers and power combiners may be easily setup using mode converters. For example, a TE10–TE30 mode converter easily offers a three-port power divider, and a three-way power combiner can be composed by reversal. A power combiner is useful for application to Gunn diodes in a waveguide array (Bae et al., 2000), because it converts the TE30 mode to the TE10 mode.
2. Design method of the mode converters
We have reported that single-mode propagation is available for a metallic waveguide with dielectric rods arrayed at the center of the waveguide in the frequency under twice the cutoff frequency region using the TE10 mode, and in the frequency over twice the cutoff frequency region using the TE20 mode, because of restrictions of the TE10 mode (Kokubo, 2007; Kokubo & Kawai, 2009 ). However, a TE20-like mode, which is propagated in the second band, is an odd mode, and generation systems for odd modes have seldom been reported. In this investigation, a mode converter is proposed which passes through the TE10 mode for the low frequency range and efficiently converts TE10 to TE20 mode for the high frequency range.
2.1. Design method of the TE10-to-TE20 mode converter
The frequency eigenvalues of a conventional metallic waveguide in a given
The group velocity is given by
If the group velocity is normalized using light velocity in a vacuum,
The metallic waveguide is assumed to be a WR-90 (22.9×10.2 mm, cutoff frequency
Rod Number i | Distance di from the Sidewall [mm] | Radius ri of the dielectric rod [mm] |
1 | 11.45 | 0.36 |
2 | 11.45 | 0.515 |
3 | 11.45 | 0.515 |
4 | 10.45 | 0.52 |
5 | 9 | 0.57 |
6 | 7 | 0.64 |
7 | 5 | 0.69 |
8 | 3 | 0.76 |
9 | 1.3 | 1.0 |
2.2. Simple fabrication method
For the fabrication of a mode converter, such as the Type A illustrated in Fig. 5(a), it is necessary to locate the dielectric rods in the waveguide without a gap at top and bottom.
Such a structure may be difficult to fabricate. As a solution, holes with diameters 0.2 mm larger than the rods were fabricated at the top of the waveguide and the dielectric rods were inserted (Type B, Fig. 5(b)). The S parameters were calculated using the HFSS software and the results are shown as dotted lines in Figs. 4(a) and (b). The results for these different structural conditions (solid lines and dotted lines) are almost same.
2.3. Reduction of reflection for TE20 mode
As shown in Fig. 4(b), reflection as TE20 mode is not small enough. This reason is dielectric rods are asymmetric arrangement for electromagnetic wave. Fig. 6 shows an improved structure of the TE10 to TE20 mode converter.
Rod Number i | Distance di from the Sidewall [mm] | Radius ri of the dielectric rod [mm] |
1 | 11.45 | 0.38 |
2 | 11.45 | 0.55 |
3 | 11.45 | 0.55 |
4 | 11.45 | 0.55 |
5 | 10.45 | 0.515 |
6 | 9 | 0.57 |
7 | 7 | 0.64 |
8 | 5 | 0.69 |
9 | 3 | 0.76 |
10 | 1.25 | 0.92 |
11 | 21.65 | 0.96 |
2.4. Design method of the TE10-to-TE40 mode converter
A TE10 to TE40 mode converter can be considered by combination of TE10 to TE20 mode converters. Another structure of the TE10-to-TE20 mode converter is proposed and is shown in Fig. 8. The locations of the dielectric rods are indicated in Table 1. The structure of the proposed TE10-to-TE40 mode converter, which is composed of three TE10-to-TE20 mode converters, is shown in Fig. 9. The S parameters between the input port (port 1) and output port (port 2) calculated using HFSS are shown in Figs. 10(a), (b) and (c).
2.5. Design method of the TE30-to-TE10 mode converter
A structure that contains two arrays of dielectric rods can convert the TE30 mode into the TE10 mode ( Kokubo, 2009 ). The TE30 mode electromagnetic waves in this type of waveguide are converted to the TE10 mode for 7.1–8.9 GHz with over 95% efficiency. However, this structure cannot pass through electromagnetic waves around 2.5–3 GHz without high reflection even if the waveguide is straight. Therefore, a mode converter is proposed that passes the TE10 mode at low frequencies and efficiently converts the TE30 mode into the TE10 mode at high frequencies.
A metallic waveguide that contains two in-line dielectric rods can propagate single modes in two frequency regions (Shibano et al., 2006). The propagation modes in a waveguide with two in-line dielectric rods with period
The metallic waveguide width is assumed to be
Rod Number i | Distance w2i between a pair of the rods [mm] | Radius ri of the dielectric rod [mm] |
1 | 7.13 | 0.22 |
2 | 7.13 | 0.32 |
3 | 7.13 | 0.32 |
4 | 7.13 | 0.32 |
5 | 8.3 | 0.32 |
6 | 10.2 | 0.36 |
7 | 12.7 | 0.43 |
8 | 15.2 | 0.51 |
9 | 17.7 | 0.62 |
10 | 19.5 | 0.78 |
The first pair of rods has
3. Conclusion
We have previously reported that single-mode propagation is available for a metallic waveguide with dielectric rods arrayed at the center of a waveguide using the TE10 mode, and the TE20 mode. However, a TE20-like mode, which is propagated in the second band, is an odd mode, and generation is not easy. In this investigation, a mode converter is proposed which passes through the TE10 mode for the low frequency range and converts TE10 to the TE20 mode for the high frequency range by small variation of the group velocity. It was shown that the electromagnetic waves pass through as the TE10 mode for 7-11.2 GHz and are converted to the TE20 mode for 14.1-16.1 GHz under a condition of over 95% efficiency.
It was shown that electromagnetic waves propagate as the TE10 mode around 8 GHz and that the TE40 mode is converted into the TE10 mode around 16 GHz.
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