Basic Antennas notes.pdfThe Wire Antenna.pdfThe Half Wave Dipole.pdfThe Aperture Antenna.pdfThe Reflector Antenna.pdf11/27/2007 Basic Antennas notes 1/1 Jim Stiles The Univ. of Kansas Dept. of EECS F. Basic Antenna Designs There are many, many, many different antenna designs, each with different attributes (e.g., cost, size, gain, bandwidth, profile, etc.). We will investigate a handful of the most popular and useful of these antenna designs. A wire antenna provides a wide beamwidth in a small package with low cost. As a result, they are very popular! HO: The Wire Antenna The most popular of the wire antennas is the half-wave dipole. It has some very special properties! HO: The Half-Wave Dipole Another important type of antenna is the aperture antenna. HO: The Aperture Antenna Satellite communication requires a very high gain antenna. This is perhaps most easily accomplished by using a reflector antenna. HO: The Reflector Antenna11/27/2007 The Wire Antenna 1/5 Jim Stiles The Univ. of Kansas Dept. of EECS The Wire Antenna The simplest and perhaps most popular and prevalent antenna is the wire antenna. The wire antenna comes in two prevalent “flavors”, namely the dipole antenna and the monopole antenna. The dipole antenna is a “balanced” wire antenna, whereas the monopole is an “unbalanced” wire design. Q: Why are wire antennas so popular? A: Two reasons! Reason #1: A wire antenna is really cheap! Tx/Rx 0Z0Zmonopole dipole Tx/Rx11/27/2007 The Wire Antenna 2/5 Jim Stiles The Univ. of Kansas Dept. of EECS A wire antenna is simply a straight piece of—um—wire. As a result, it is inexpensive to manufacture, a fact that makes it very popular for a plethora of consumer products such as mobile phones and car radios. Reason #2: It is azimuthally symmetric! Q: Huh?? A: Say we orient our wire antenna along the z-axis. Now let’s consider the gain pattern of this antenna, “cut” by the x-y plane (i.e., ()90G,θφ=D). We call this an “azimuth cut” of the antenna pattern provides antenna gain as a function of “azimuth” angle φ. The form of this azimuth cut ()90G,θφ=D should be readily apparent! Consider carefully what happens if we were to rotate the wire around the z-axis: x y z θ φ11/27/2007 The Wire Antenna 3/5 Jim Stiles The Univ. of Kansas Dept. of EECS Æ Nothing happens! The wire is a circular cylinder, and as such possesses cylindrical symmetry. Rotating this cylinder in azimuth does not change the structure of the antenna one whit. The wire cylinder has no preferred or unique direction with respect to φ—every direction is the same! Q: Why is this important? A: Since the antenna is symmetric with respect to φ—since every azimuthal direction is the same—the gain pattern produced by this antenna will likewise be azimuthally symmetric; the gain in every direction φ will be the same! Thus, the gain pattern of an azimuthal cut (i.e., ()90G,θφ=D) will be a perfect circle! x y z11/27/2007 The Wire Antenna 4/5 Jim Stiles The Univ. of Kansas Dept. of EECS In other words, a wire antenna will radiate equally in all azimuth directions φ. Q: Wait a second! Radiates equally in all directions?? That sounds like an isotropic radiator, but you said an isotropic radiator is impossible! A: An isotropic radiator is impossible—meaning a wire antenna is of course not an isotropic radiator. Note that we found that a wire antenna radiates equally in all azimuth directions φ—it most definitely does not radiate uniformly in elevation direction θ. In other words, if we take an “elevation cut” of a wire antenna by plotting its gain on the x-z plane (i.e., ()0G,θφ= ), we will find that it is definitely not uniform! x y φ ()90G,θφ=D11/27/2007 The Wire Antenna 5/5 Jim Stiles The Univ. of Kansas Dept. of EECS Q: You say that this azimuthal symmetry is one reason while wire antennas are popular. Why is azimuthal symmetry a desirable trait? A: For broadcasting and mobile applications, we do not know where the receivers are located (if we are transmitting), and we do not know where the transmitter is (if we are receiving). As a result, we need to transmit (i.e., radiate) across all azimuthal directions, or receive equally well from any and all directions. Otherwise we might miss something! Elevation cut of dipole antenna (()0G,θφ=).11/27/2007 The Half Wave Dipole 1/9 Jim Stiles The Univ. of Kansas Dept. of EECS The Half-Wave Dipole With most microwave or electromagnetic devices (e.g., μ−wave circuits, antennas), the important structural characteristic is not its size—rather it is its size with respect to signal wavelength λ. Instead of measuring the size of an antenna (for example) with respect to one meter (e.g., 1.5 meters or 0.6 meters) we measure its size with respect to one wavelengthλ (e.g., 0.3 λ or 1.1 λ). Thus, our electromagnetic “ruler” is one which varies with signal frequency ω! For example, we find that for a signal with an oscillating frequency of 300 MHz, its wavelength in “free-space” is: 8831010310c. meterfλ×== =× Whereas: 600 0 512 0253016512 2 5fMHz . metersf. GHz . metersfGHz . metersfGHz cmfGHz . cmλλλλλ=⇒==⇒==⇒==⇒==⇒=11/27/2007 The Half Wave Dipole 2/9 Jim Stiles The Univ. of Kansas Dept. of EECS Note then, as frequency increases, the value of one wavelength decreases. So now, let’s consider again wire antennas. The size of a wire antenna is completely defined by its length A . We can express this length in terms of: 1. meters (e.g., 06.m=A ). We call this value the antenna’s physical length. 2. wavelengths (e.g., 02.λ=A ). We call this value the antenna’s electrical length. Of course the electrical length of the wire antenna depends on both its physical length and the frequency (wavelength) of the propagating wave it is transmitting/receiving. For example, say an antenna with a physical length of 05.m=A is transmitting a signal with wavelength 025.mλ= . It is obvious (is it obvious to you?) that the electrical length of this antenna is 20.λ=A . But, if the frequency of the signal is decreased, such that its wavelength becomes 07.mλ= , that same antenna with physical length 05.m=A will now have a shorter electrical length of 14.λ=A11/27/2007 The Half Wave Dipole 3/9 Jim Stiles The Univ. of Kansas Dept.