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7.2.3 Waveguide slot array design A sketch of a waveguide slot antenna with the pertinent dimensions is shown in Figure 7-5. The first design consideration is that the slots be resonant so that they provide a resistive load to the (waveguide) transmission line. Normally, it is desirable for an omnidirectional antenna to radiate in a horizontal (azimuth) plane. This is achieved by feeding all the slots in phase. The radiation pattern may be tilted upward or downward (visualize a shallow cone) by changing the phasing of the slots, if desired. I’m told that vertical collinear arrays for ground-based aviation antennas are sometimes designed with a pattern tilted upward. The slots are fed in phase by spacing their centers at electrical half-wavelength intervals along the waveguide. The electrical wavelength in waveguide is longer than in free space, so we must calculate the guide wavelength: λλ = 1 λλ g  1 2  1 2   −    λλ   λλ   λλ 0   λλ C  where λλ , the cutoff wavelength, equals 2 times the wide dimension of the waveguide. λλC If the spacing is wrong, or if the frequency is changed significantly so the spacing is no longer λλ /2, then the slots will not be fed in phase and the beam will be tilted — we can λλ g make a beacon for aliens, or for earthworms. A half-wavelength of transmission line has the useful property of repeating impedance: the input and output impedance are the same. As a result, the impedances, or admittances, of all the slots appear in parallel. Figure 7-6 shows this schematically. Each parallel resistor represents one slot, so there must be N resistances in parallel. The center of the last slot is a guide quarter-wavelength from the closed end of the waveguide.