Horn Antenna - Intro

Horn antennas are very popular at UHF (300 MHz-3 GHz) and higher frequencies (I've heard of horns operating as high as 140 GHz). They often have a directional radiation pattern with a high gain , which can range up to 25 dB in some cases, with 10-20 dB being typical. Horns have a wide impedance bandwidth, implying that the input impedance is slowly varying over a wide frequency range (which also implies low values for S11 or VSWR). The bandwidth for practical horn antennas can be on the order of 20:1 (for instance, operating from 1 GHz-20 GHz), with a 10:1 bandwidth not being uncommon. The gain often increases (and the beamwidth decreases) as the frequency of operation is increased. Horns have very little loss, so the directivity of a horn is roughly equal to its gain. Horn antennas are somewhat intuitive and not relatively simple to manufacture. In addition, acoustic horns also used in transmitting sound waves (for example, with a megaphone). Horn antennas are also often used to feed a dish antenna, or as a "standard gain" antenna in measurements. Popular versions of the horn antenna include the E-plane horn, shown in Figure 1. This horn is flared in the E-plane, giving the name. The horizontal dimension is constant at w. Figure 1. E-plane horn. Another example of a horn is the H-plane horn, shown in Figure 2. This horn is flared in the H-plane, with a constant height for the waveguide and horn of h. Figure 2. H-Plane horn. The most popular horn is flared in both planes as shown in Figure 3. This is a pyramidal horn, and has width B and height A at the end of the horn. Figure 3. Pyramidal horn. Horns are typically fed by a section of a waveguide, as shown in Figure 4. The waveguide itself is often fed with a short dipole, which is shown in red in Figure 4. A waveguide is simply a hollow, metal cavity. Waveguides are used to guide electromagnetic energy from one place to another. The waveguide in Figure 4 is a rectangular waveguide of width b and height a, with b>a. The E-field distribution for the dominant mode is shown in the lower part of Figure 1. Figure 4. Waveguide used as a feed to horn antennas.