Page 324 - Maxwell House

304 Chapter 6

as possible, it is a disadvantage because even small WC shape deformations or some dielectric
or metal inclusions can transform the polarization into elliptical thereby creating the crosspol
interfering signal. From another point of view, single WC is equivalent to two WRs since it can
carry at once two waves of different and independent polarization at the same frequency. This
advantage lets twofold the capacity of communication channel or double power rating. But
nothing comes for free, in both cases the costly high-precision WC production becomes
obligatory.

It is worthwhile to point out that a WC is not so broadband in term of single mode regime as a
rectangular one. The cut-off ratio = 1.3 in WC while in WR = 2. A flexible WC in Figure
6.4.1h) is produced for the same applications as a rectangular one. Note that it is much harder
to keep the desired polarization purity in flexible WC than in similar WR.

6.4.5 Waveguide of Ridge Double (WRD)

Figures 6.4.1k) and l) demonstrate a WRD and its flexible variant. It was developed to extend
a single mode regime of conventional WR thereby making it more broadband. The cut-off ratio
can be up to = 4 and even higher but not very much. The main constrains are increase in
attenuation and drop in power rating. For example, an aluminum WRD marked as WRD-270
U36 with = 4.35 covering the single mode frequency band 270 MHz - 970 MHz has inner
dimensions 353.47x152 mm and wall thickness 5 mm. So WRD is certainly more compact and
weights much less than standard WR for the same frequencies. As well, a single mode WRD
can replace two frequency contiguous WRs thereby saving even more of space and weight.
What is the punishment? The attenuation in the standard WRD replacing WR-2300 is about 1.8
dB per 100 m or a 15-fold increase while the power handling drops to 4.6 MW that is a 40-fold
descent. Figure 6.4.6 illustrates the distribution of EM fields in WRD (CST numerical
simulation) and can be compared with the dominant mode in WR of Figure 6.4.3d.

Figure 6.4.6 Illustration of a) electric and b) magnetic field configuration and intensity in
WRD
The main alteration is that the E-field and its energy are now primarily confines inside the

small area of ridge gap the same way as in ordinary capacitance. Not by chance, the E- and H-
field distributions look almost identical if we forget about the variance in their polarization:
predominantly vertical for E-fields and horizontal for H-field. Recall (see (3.52) in Chapter 3
and (6.7) above) that the freely propagating wave in a transmission line does not carry any
reactive energy. If so, = in any cross section spot, and that is why the distribution

patterns are so similar. The visible differences can be detected only in areas close to the WRD
metal walls where the longitudinal and tangential magnetic field component reaches its