Cavity-Backed Slot Antennas
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The previous page introducing slot antennas was primarily theoretical (giving you an intuitive idea of how slot antennas work); however, since it was about an infinite conducting plane it is not entirely practical. A practical slot antenna is the cavity-backed slot antenna. Unfortunately, the equations related to the cavity backed slot antenna are somewhat complicated and in my opinion don't give a good idea of how they work. Hence, I'll present the basics, present some experimental results and try to give an idea of design parameters. The basic cavity-backed slot antenna is shown in Figure 1 (in a rectangular cube of size A*B*C). The walls are metallic (electrically conducting), and the inside is hollow. On one end, a slot is cut out. The cavity is typically excited by a probe antenna in the intererior of the cavity, which typically is modelled as a monopole antenna. The exciting monopole antenna is shown in green.
 Figure 1. Cavity-backed slot antenna.
I'll give some experimental results for this antenna. Let the height of the cavity C=36mm, the length be A=87mm and the height B=36mm. The height of the monopole antenna will be 29.5mm, so that the monopole is a quarter-wavelength long at 2.55 GHz. The monopole will be centered about the cavity in the y-direction, and 61.5mm behind the slot in the x-direction. The slot is 58mm long (in the y-direction) and 3 mm high (in the z-direction). S11 is measured for this antenna (relative to a 50 Ohm source), and is plotted versus frequency in Figure 2.
 Figure 2. S11 as a function of Frequency for the Cavity-backed Slot Antenna. This cavity-backed slot antenna has a first resonance at about 2.45 GHz. At this frequency, the cavity backed slot antenna is roughly 0.474 wavelengths long - which is roughly the length of a resonant dipole antenna. S11 drops to below -20 dB at this frequency, indicating that most of the power is radiated away. The bandwidth, measured (somewhat arbitrarily) as the frequency span that S11 is less than -6 dB is roughly from 2.35 GHz to 2.55 GHz, giving a fractional bandwidth of slightly over 8%. Note that two other dips ('resonances') in the S11 curve occur, at approximately 3 GHz and 4.18 GHz. At these frequencies, the slot length is 0.58 and 0.81 wavelengths, respectively.
The volume of the cavity typically influences the bandwidth - a larger volume often yields a higher bandwidth. The material within the cavity (so far I have assumed it was filled with air), can be replaced with a dielectric medium. This reduces the resonant length of slot, allowing for a smaller antenna. The tradeoff is that the bandwidth and efficiency typically decrease with a dielectric cavity medium. The radiation pattern at 2.45 GHz is now presented. The H-plane (xy plane) is shown on the in Figure 3, and the E-plane (xz plane) is shown in Figure 4.
Figure 3. H-plane (xy plane). Angle measured off x-axis towards y-axis.
Figure 4. E-plane (xz plane). Angle measured off z-axis (to x-axis). The radiation pattern of the cavity backed slot antenna somewhat resembles that of a dipole antena in the forward H-plane. The 3-dB beamwidth is roughly 60 degrees in this plane. The radiation pattern is diminished in the rear H-plane, with a significant back lobe about 6 dB down from the peak of the main beam. In the E-plane, the pattern is fairly broad, with a 3-dB beamwidht of about 120 degrees. The broad pattern of these antennas make them well suited for use in antenna arrays. The peak gain of a thin slot is usually around 2-3 dB. In the next section, we'll look at slotted waveguides.
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