Figure 12. Frequency response of the E-T diplexer with capacitive iris at the common port; w dashed line. response of the diplexer when no capacitive iris is used at the common portx.
dure, the iris opening and the location can be adjusted to satisfy eq. 6. in any region of the operating band. Figure 12 shows the computed frequency response of a diplexer with closely spaced channels designed by the Y-scheme for operation in the upper part of the waveguide operating band. The dashed curve shows the computed response of a diplexer of identical initial specification and designed by the scheme described in the previous section. Figure 12 shows that the approach in the previous section results in narrower bandwidth and poor return loss. The common junction based on a capacitive iris results in acceptable fulfillment of eq. 6. over a wide frequency range and allows one to design diplexers with very large guard bands. Figure 13 shows the characteristics of a diplexer with a very large guard band and the channel filters centered at 31.10 and 39.00 GHz.
CAD of Wa¨eguide E-Plane Diplexers 113
VIII. DIPLEXERS BASED ON E-PLANE STEP TRANSITIONS
Most compact waveguide diplexers use common junction step transitions and waveguide bifurcation, as shown in the inset of Figure 14. We discuss such diplexers only briefly because they have already been discussed in w8, 16x. Reference w8x designs the diplexers using brute force numerical optimization that uses no circuit theory. However, w16x considers initial equireflection based synthesis of the common junction and circuit theory based design of the channel filters. The simplest one-step transition is used as the common junction and the junction is tuned by an extra septum in the junction cavity. We would like to mention at this point that the simplest one-step transition, without any extra tuning septum, can be used as a common junction for Y-scheme based design of E-plane bifurcated common junction diplexer. Such a junction behaves like a close to ideal Y-junction near the waveguide cut-off frequency provided the waveguides constituting the three-port are identical. Figure 14 shows the performance of such a diplexer designed for operation in the lower part of the waveguide band. Although it has more than 20 dB return loss over the major part of the filter passbands, its performance is worse than that of a E-T diplexer, shown in Figure 10. Also, the required properties of the simplest common junction rapidly degrade with increasing frequency. This renders the Y- scheme approach unusable for designs supposed to operate in the upper part of the waveguide band.
The above difficulty can be overcome by using a two-step transition shown in Figure 4b. However, there is a restriction on the amount of guard