Reflector Antennas
Reflector antennas are used as high-gain, narrow-beam antennas in fixed radio links, satellite communication, radars, and radio astronomy. A parabolic reflector antenna is the most common of reflector antennas. Figure 9.26 a shows a parabolic antenna fed from the primary focus. The equation of the surface is where F is the focal length. The rays coming from the focal point are converted parallel by the reflector or vice versa. A more physical interpretation is that the fields radiated by the...
Horn Antennas
An open waveguide end operates as a simple antenna. It has a broad, unsym-metrical beam and a rather large impedance mismatch. A much better antenna, a horn antenna, is obtained by widening the waveguide end, as shown in Figure 9.23. H-plane, E-plane, and pyramidal horns are fed from a rectangular waveguide. An H-plane horn is widened along the broad side of the waveguide, an E-plane horn along the narrow side. A pyramidal horn is broadened in both directions. The distribution of the aperture...
Bending Refraction of Radio Waves in Troposphere
The refraction index n yje of the troposphere fluctuates over time and location. In normal conditions the refraction index decreases monotonically versus altitude, because the air density decreases. Because a phenomenon of this kind is a weak function of altitude, it causes slow bending of the ray. Fast changes in the refraction index cause scattering and reflections. Turbulence, where temperature or humidity differs strongly from those of the surroundings, gives rise to scattering. Reflections...
Waveguide Directional Couplers
We may make a directional coupler by placing side by side two rectangular metal waveguides having coupling holes in the common wall 1 . Figure 6.7 shows a simple directional coupler having two holes d Ag 4 apart in the broad wall of the waveguides. Let us assume that the coupling factor of a single hole is Bf in the forward direction and By in the backward direction. Figure 6.7 Two-hole waveguide directional coupler. For example, if a wave is applied to port 1 and its field is equal to 1 in the...
Scattering Matrix of a Directional Coupler
Let us next derive the scattering matrix of an ideal directional coupler. Because ports 1 and 3 as well ports 2 and 4 are isolated from each other, S13 S24 0. Let us assume that ports 1 and 2 are matched, or S11 S22 0. Due to the reciprocity Sj Sj . Thus, the scattering matrix is The scattering matrix of a lossless circuit is unitary. From rows 1 and 4 we get S14 S44 0 and from rows 2 and 3 S23 S33 0. Because S14 and S23 are nonzero, parameters S33 and S44 must be zero, and ports 3 and 4 must...
Fields in Media
In the above equations, the permittivity e and permeability l represent the properties of the medium. A medium is homogeneous if its properties are constant, independent of location. An isotropic medium has the same properties in all directions. The properties of a linear medium are independent on field strength. In a vacuum, e e0 8.8542 x 10 F m and 0 10 H m. In other homogeneous media, e ere0 and xr 0, where the dielectric constant er , that is, the relative permittivity, and the relative...
Design of Microwave Filters
At microwave frequencies we have two major problems in realizing the synthesized filters. First, good lumped elements do not exist instead we must use distributed elements. The frequency behavior of the distributed elements is more complicated than that of the lumped ideal ones, which makes the filter synthesis difficult. However, design of a narrow-band filter is easy, because over a narrow bandwidth many distributed elements may be modeled by ideal inductors and capacitors. Second, the filter...
Environment and Propagation Mechanisms
The troposphere is the lowest part of the atmosphere, where all weather phenomena occur. It extends on the poles to about 9 km and on the equator to about 17 km. The troposphere is inhomogeneous and constantly changing. Temperature, pressure, humidity, and precipitation affect the propagation of radio waves. In the troposphere the radio waves attenuate, scatter, refract, and reflect the amplitude and phase of the received signal may fluctuate randomly due to multipath propagation the...
Tropospheric Attenuation
At frequencies above a few gigahertz, the attenuation due to atmospheric absorption and scattering must be taken into account. This attenuation can be divided into two parts attenuation due to clear air and attenuation due to precipitation raindrops, hail, and snow flakes and fog. Attenuation of the clear air is mainly due to resonance states of oxygen O2 and water vapor H2O molecules. An energy quantum corresponding to the resonance frequency may change the rotational energy state of a gas...
Microstrip Line
A microstrip line consists of a metal strip on one side and a ground plane on the other side of a substrate, as shown in Figure 3-13- The substrate is made of a low-loss dielectric material such as polytetrafluoroethylene Teflon , aluminum oxide alumina , or quartz. A pure TEM wave mode can propagate in a microstrip line only if all fields are in the same medium. Then the solution for the field can be derived from Laplace's equation. In a case where the nonstatic fields are in two different...