Radiation Resistance Of Dipole Antennas
As I discussed in Chapter 5, the amount of power that an antenna radiates is a function of the electrical length of the antenna. A 1 MHz AM radio electric dipole antenna that is 150 m long radiates the same amount of power as a 100 MHz FM radio electric dipole that is 1.5 m long. The equivalence stems from the fact that both antennas have the same electrical length, 1 2. The radiating efficiency is usually expressed in terms of a quantity called radiation resistance. The power emitted by an...
Measuring Characteristic Impedance Tdrs
Although formulas are readily available for calculating the characteristic impedance of most transmission line geometries, no formula can ever give the assurance of physical measurement. I encourage all designers, RF and digital, to measure the typical characteristic impedance of the transmission lines in their designs. There are several ways to go about measuring characteristic impedance. The most useful method for measuring the parameters of transmission lines is to use a time domain reflecto...
Polarization
When an electromagnetic field travels in space, its electric field and magnetic field components are transverse to the direction of propagation and are at 90 degree angles to one another. Even with these limitations, the field components have 360 degrees of freedom in terms of which direction the fields point. The antenna geometry determines the direction of the fields. For an electric dipole, the electric field will be polarized in the direction of the dipole wire. The magnetic field is...
Power Supply Decoupling Problems And Techniques
Power supply decoupling is the term given to the technique of making sure the DC power line variations do not affect the loads amplifier, ICs, logic gates, etc. and vice versa. Since most ICs have AC signals as inputs and outputs, the current drawn from the power supply will vary in an AC manner. For example, an RF circuit that amplifies a 900 MHz signal will draw a supply current that varies at 900 MHz. A digital CMOS circuit that buffers a 100 MHz digital signal will also draw a supply...
The Fraunhofer And Fresnel Zones
To be even more precise, each electromagnetic field can be divided into four zones the near zone, the intermediate zone, the far zone, and the plane-wave zone. The near zone is the portion of the field close to the source. It is defined as the region where stored energy is much greater than any radiating energy. The far zone is the region where 1 the stored field energy is much less than the radiating energy, 2 the wave impedance is approximately ho 120p, and 3 the electric and magnetic fields...
The Quantum Vacuum And Virtual Photons
Now what about the energy in nonradiating electromagnetic fields, that is, the static field and the near field Quantum physics states that any energy must consist of individual packets or quanta, but this implies that even the static field must consist of particles. In fact, the static field does consist of particles virtual photons. To explain virtual photons, let's step further into the strange world of quantum physics. If we think about the uncertainty principle from another point of view,...
Ac Circuits Impedance And Reactance
An interesting thing happens when you apply an AC source to an inductor. Power is transferred to and from the inductor during each cycle, and the energy provided by the circuit is equal to the energy returned from the inductor. Because energy is not lost, the ideal inductor does not have an AC resistance. However, sinusoidal current flows through the inductor and a sinusoidal voltage develops across the inductor. In other words, the ratio of voltage to current is not zero. Moreover, the current...
Near Field Shielding Of Magnetic Fields
Magnetic fields are not reflected very well by conductors. A magnetic field has, by definition, a larger magnetic component than electric component. Therefore, its field impedance is very small. The intrinsic impedance of conductors is also small at low frequencies. Because the impedances are similar, not much reflection occurs. Absorption shielding must be used for magnetic fields. Consequently, to shield magnetic fields effectively, the shield must be several skin depths thick. Figure 9.7...
Characteristic Impedance
The simple circuit of Figure 8.2 demonstrates the behavior of a transmission line. In this circuit, a 5 volt battery is connected to a resistor through a transmission line that is modeled with a series of four L-C sections. In reality, a real transmission line is an infinite series of infinitesimal inductors and capacitors. However, this simplified model serves as a good heuristic tool. Between the battery and the transmission line is a mechanical switch, which is initially open. In the initial...
Fraunhofer And Fresnel Diffraction
Diffraction is a radiation phenomenon. Fraunhofer diffraction occurs when the wave source and observer are far from the aperture or scattering object. Fresnel diffraction occurs when the source and or observer are close to the aperture or scattering object. Figure 14.5 shows Fresnel diffraction patterns for apertures of various cross-sections. Figure 14.6 shows Fresnel diffraction patterns for opaque perfectly absorbing objects with different size cross-sections. In other words, Figure 14.6...
Common Impedance Ground Rise And Ground Bounce
Consider the circuit shown in Figure 12.9. Two circuits are connected to the same ground wire. Since all grounds are made from non-ideal wire, they will have some resistance. The circuit on the right is a highpower, high-current circuit. Its return current flows back to the ground wire to the power supply. The return current is so large that it causes a considerable voltage drop across the resistance of the ground wire. The end result is that the circuit on the left is referenced to a voltage...
The Electric Dipole
To create a half-wavelength dipole, start with an open-circuit transmission line of length equal to 1 4, as shown in Figure 11.1. Applying a voltage to the line creates a standing wave, and the input impedance appears ideally as a short circuit. With this geometry, very little radiation occurs. Now imagine that you take the ends of the transmission line and expand the wires outward 90 degrees. A standing wave still occurs on the line, with current going to zero at the ends. However, now the...
The Field Of A Static Charge
For a single charged particle, such as an electron, the electric field forms a simple radial pattern as shown in Figure 5.4. By convention, the field Figure 5.4 The static field of a positive charge is shown as a vector plot top left , streamline plot middle left , and a contour plot bottom left . The static field of a dipole is shown as a vector plot top right , streamline plot middle right , and contour plot bottom right . Figure 5.4 The static field of a positive charge is shown as a vector...
EFFECTS OF THE MAGNETIC FIELD The Dipole
Now that you understand how magnetic fields are created, you need to understand how magnetic objects are affected by an external magnetic field. The situation is more complex than the electric field, where charges just follow the electric field lines. The effect of the magnetic field is rotational. To analyze how the magnetic field operates, you need some form of fundamental test particle. For the electric field, we use a point charge i.e., a charged, infinitesimally small particle . Since...
And Maxwell Said Let There Be Light
In addition to being a physicist, Maxwell was also an extraordinarily talented mathematician. When he added his new term to the existing equations for electricity and magnetism, he quickly noticed that the mathematics implied that propagating electromagnetic waves could be created. Such waves had never been observed. This was a monumental discovery. Furthermore, when he performed the mathematical derivations, the speed of these new waves was predicted to be that of the speed of light Before...
Wave Propagation on Earth and in the Atmosphere
Boithias, L., Radio Wave Propagation, Boston McGraw-Hill, 1987. Carr, J. J., Practical Antenna Handbook, Fourth Edition, New York McGraw-Hill, 2001. Cummins, K. L., E. P. Krider, M. D. Malone, The U.S. National Lightning Detection Network and Applications of Cloud-to-Ground Lightning Data by Electric Power Utilities, IEEE Transactions of Electromagnetic Compatibility, Vol. 40, No. 4, November 1998. Jackson, J. D., Classical Electrodynamics, 2nd Edition, New York John Wiley amp Sons, 1975....
The Skin Effect
All real conductors exhibit a phenomenon called the skin effect. The skin effect gets it name from the fact that any electromagnetic wave incident upon a conductor diminishes exponentially in amplitude as it penetrates the conductor. The skin depth defines the depth at which the amplitude of the wave has diminished to about e-1 36.9 of its initial value. At a depth of five skin depths, the signal diminishes to about 0.7 of its surface value. Skin depth is inversely proportional to both...
Feeding Impedance And Antenna Matching
Just as the input impedance of a transmission line depends on length, so does the input impedance of an antenna. The input impedance that an antenna presents to the amplifier or transmission line feed is not necessarily equal to the radiation resistance. An electric dipole behaves like an open-circuited transmission line, in that it has a capacitive input impedance when its length is electrically short l lt 1 2 . At resonance l 1 2 , its impedance is purely real, with the resistance relating to...
Coaxial Cables Below the Break Frequency
To provide excellent electric field shielding, a metal cylinder can be placed around the signal conductor and used for the return current. This geometry forms the common coaxial cable. The term coaxial refers to the fact that the wire inner conductor and the shield outer conductor share the same central axis. In other words, the wire is placed in the exact center of the shield. For the coaxial cable to work effectively, it must surround the inner conductor completely, without any gaps or slots....
Near And Far Field
As mentioned earlier, an AC circuit will have a reactive field and a radiating field. The reactive field of an AC source circuit or system is often referred to as the near field because it is concentrated near the source. Similarly, the radiating field is referred to as the far field because its effects extend far from the source. Let's examine why. The power density of an electromagnetic field at a distance, r, from the source can be represented by a series in 1 r, Field power density Pd Q r2...
Bibliography Static Electricity And Lightning
Adams, J. M., Electrical Safety a Guide to the Causes and Prevention of Electrical Hazards, London The Institution of Electrical Engineers, 1994. Anderson, K., Frequently Asked Questions FAQ About Lighting, www.nofc.foresty.ca Edmonton Canadian Forest Service. Carlson, S., Detecting the Earth's Electricity, Scientific American, July 1999. Carlson, S., Counting Atmospheric Ions, Scientific American, September 1999. Carpenter, R. B., Jr., Lightning Protection Requirements for Communications...
The Effect of Holes in the Shield
If a shield has a hole, some of the incident radiation can avoid the shield and propagate through the hole. The amount of energy that can transmit through the aperture hole is greatly dependent on the electrical size of the aperture. If the aperture has dimensions greater than a half wavelength, the aperture lets all power at the opening through. Consider a 1 inch hole in an opaque screen at optical frequencies. Light cast on the opening will pass through unattenuated. The region behind the...
SpaceTime MomentumEnergy and Other FourVectors
You already learned that space and time form a single four-dimensional structure. Events in space-time can be described by a four-dimensional quantity called a 4-vector 4 x,y,z,ct . The length of a 4-vector is calculated using the Minkowki metric, which you learned about earlier in the chapter. Part of the beauty of relativity is that all the fundamental quantities of physics are condensed using 4-vectors. For example, the proper time derivative of 4-position gives proper 4-velocity
Near Field Shielding Of Electric Fields
In the near field, the wave impedance is no longer solely dependent on the intrinsic impedance of the air. Instead, the wave impedance is mostly dependent on the type of source and how far away the source is. For electric fields, the near field has a large electric component and a much smaller magnetic component. The corresponding wave impedance ratio of electric to magnetic field is very high. Since the wave impedance inside conductors is conversely very low, conductors act as excellent...
Electrostatic Induction And Capacitance
To understand capacitance, you need to first understand the process of electrostatic induction. For example, consider that you have a metal ball that is positively charged, near which you bring a neutral metal ball. Even though the second ball has overall neutrality, it still contains many charges. Neutrality arises because the positive and negative charges exist in equal quantities. When placed next to the first ball, the second ball is affected by the electric field of the charged ball. The...
Routing Of Signals Ground Planes Image Planes And Pcb Stackup
The ideal design for high-speed signals is as follows The outer layers have a ground directly underneath forming microstrip traces for all outer layer signals Every internal signal layer is surrounded by a ground above and below, forming the ideal strip-line geometry Signals traces have a maximum of two vias one via at the source IC pin and one at the destination IC pin. In between these two pins, the signal never changes layers It is a good idea to strive for these ideals, especially on the...
Impedance Matching For Rf Systems
Impedance matching is even more important in RF systems, and the techniques are slightly different. For RF transmission lines, the main goal is the same as for digital transmission lines eliminate reflections. The first reason for eliminating reflections in RF systems is to assure that proper voltage is transferred between the source and load. As the earlier example showed, transmission line reflections can cause the load voltage to be different than the voltage on the source end. Without...
Rf Decoupling
In general, the techniques of decoupling high-speed digital systems can also be applied to RF systems. RF systems are often easier to decouple. Most RF circuits create supply variations in very narrow frequency ranges. In such applications, a decoupling capacitor can be chosen such that the resonance of the capacitor including the PCB connections occurs at the frequency of the circuit. Furthermore, most RF circuits only require a high-frequency decoupling impedance of several ohms. In other...
Mesh Shields and Faraday Cages
Due to the radiation limiting effects of small apertures, shields can be made from a mesh of wires or from a metallic cage, as an alternative to using a solid sheet of metal. From the results of apertures, you can deduce that the holes in the mesh or cage should be considerably less than half a wavelength for such shields to work properly. Moreover, the mesh will be mostly transparent to signals with wavelengths smaller than the mesh holes. When the shield takes the form of a wire cage, it is...
Microwave Engineering and Distributed Circuits
Bigelow, S., Telephone Repair Illustrated, Blue Ridge Summit, PA TAB Books, 1993. Bigelow, S., Understanding Telephone Electronics, 3rd Edition, Boston Newnes, 1997. Bingman, G., Transmission Lines of Antennas, RF Design, January 2000. Caswell, W. E., The Directional Coupler 1966, IEEE Transactions on Microwave Theory and Techniques, February 1967. Cohn, S. B., and R. Levy, History of Microwave Passive Components with Particular Attention to Directional Couplers, IEEE Transactions on Microwave...
Layout Of Transmission Line Terminations
When using terminations on transmission lines, the placement of the termination is very important. For a series termination, the terminating resistor is placed in series with the driving source. The component must be placed within a critical distance of the IC output pin. The critical distance is the distance that it takes that signal 1 3 trise to travel on the transmission line, where trise is the rise time of the driving IC. For good designs, you should strive to meet 1 6 trise How close is...
BIBLIOGRAPHY AND SUGGESTIONS FOR FURTHER READING Dco
Abrikosov, A. A., Fundamentals of the Theory of Metals, Amsterdam North-Holland, 1988. Agilent Technologies, Making Precompliance Conducted and Radiated Emissions Measurements with EMC Analyzers, Agilent Technologies, AN 1328. Boithias, L., Radio Wave Propagation, Boston McGraw-Hill, 1987. DeWitt, D. P., and G. D. Nutter, Theory and Practice of Radiation Thermometry, New York John Wiley, 1988. Encyclopedia Britannica Inc., Sunlight Glass Electromagnetic Radiation, Encyclopedia Britannica,...
Multiturn Loop Antennas
I have focused on electric antennas, because these antennas are used most often in practice. One reason is that loops tend to take up more space than electric dipoles. However, the loop antenna can actually save space in low-frequency radio applications, where wavelength is tremendous. For instance, the wavelength at the center of the AM radio band is about 300 m. Creating an electrically long antenna at this wavelength is not feasible, except for the broadcast companies. An alternative to the...
Antenna Pattern Versus Electrical Length
Antennas do not radiate equally to all directions. The directional dependence of antenna radiation is aptly called the antenna pattern. An electric dipole that is 1 2 1 in length or smaller exhibits a fairly simple antenna pattern. The most power is radiated in the direction broadside to the antenna that is, perpendicular to the antenna's length. The power decreases with decreasing angle such that at zero degrees, no power is radiated. In other words, no power is radiated in the direction of...
Conducted Versus Induced Versus Radiated Interference
In EMC literature and EMC regulations the mechanisms of electromagnetic interference are divided into two categories, conducted and radiated. Historically, this categorization has been used in the regulations and, unfortunately, it continues today. I use the word unfortunately because both near-field coupling and far-field radiation are lumped under the term radiated emissions. While experts in electromagnetics are quite familiar with the difference between near-field coupling and far-field...
SingleEnded versus Balanced Signaling
So far all the cable techniques I have discussed assume that one of the conductors is connected to the signal common. In the case of coaxial cables, the outer conductor is always connected to the signal common and possibly ground . Because twisted pair is a symmetric transmission line, it doesn't matter which conductor is connected to ground. This method of cabling is referred to as single-ended cabling. A better way to send signals is to use balanced signaling. In a balanced system, each wire...
Lumped Element Circuits Versus Distributed Circuits
At high frequencies parasitics start to dominate the behavior of lumped circuit elements such as resistors, transistors, capacitors, and inductors. As the wavelength of the signal becomes small enough to be comparable in size to the devices, lumped element design becomes impossible. On printed circuit boards, state of the art, surface mount device SMD technology allows for designs to reach frequencies of several GHz. As components become even smaller, this frequency limit will increase. Inside...
Scattering By Antennas Antenna Aperture And Radar Crosssection
A receiving antenna captures a certain amount of the radiation incident in its vicinity, but how much The most common method for defining the amount of radiation an antenna intercepts is the use of effective cross-section. In this method, you calculate the effective area of the antenna as if the incident radiation consists of rays, like the rays of geometric optics. For rays of light, the cross-sectional area is simply equal to its area. The concept is familiar to us. If a distant light is...
Differential Mode And Common Mode Radiation
Unintentional radiation can be described as either differential mode or common mode. The two terms relate to the currents that create the radiation. Differential mode currents are equal but travel in opposite directions. Common mode currents travel in the same direction. On any transmission line, the signal current and the return current travel in opposite directions. As long as the two currents are close together the radiation is very small, albeit non-zero. The two currents Figure 12.18 Here...
Conductivity And Permittivity Of Common Materials
To conclude this chapter on electric fields, I have listed the conductivity and dielectric constants of several common materials in Table 2.1. Note that salt water and body tissue are much more conductive than distilled water. The conductivity is caused by dissolved chemicals like salt, which dissolves into Na and Cl- when placed in water. These ions act to conduct electricity. Also notice that the typical ground is not a very good conductor. Keep in mind that these values are for DC and low...
Paramagnetism
In addition to orbiting the atom, each electron has a spin, which can be thought of in simple terms as similar to the Earth spinning on its axis. Because the electron has a net charge, the spin causes a circular current and a magnetic dipole. We learned earlier that although currents are governed by the same magnetic law as free electrons, they behave in opposite ways. Therefore, the inherent dipole of the electron will be rotated to line up with the external magnetic field, thereby increasing...
Analog Signals And The Effects Of Frequency
You can use a similar analysis for analog signals. Using the circuit of Figure 8.6, let the source be a IV sine wave with a frequency of lMHz period T l msec . This signal undergoes the same reflections as the step signal. The amplitude of the waves incident on the load is also the same, namely 1.33V, -0.44, 0.15, -0.05, etc. The sum of the series of waves is a 1V sine wave Figure 8.9B . In this example, no transmission line effects are noticeable because the delay of the transmission line is...
Antennas In Parallel And The Folded Dipole
Electric dipole antennas can be placed in parallel to produce more radiation. In fact, two electric dipoles placed in close proximity create a near field coupling that produces radiation power that is a factor N2 greater than a single dipole, where N is the number of dipoles. The folded dipole, shown is Figure 11.12, is another common and simple antenna. At first glance, the folded dipole looks like a transmission line, and therefore does not seems to be a good radiator. However, the key to the...
Inductors
At last it is time to learn about inductors. In contrast to the capacitor, which requires only one field namely the electric field to describe its operation, the inductor requires both fields to describe its operation even though it stores only magnetic energy. Here again is an inherent difference between the electric and magnetic fields. An inductor is a circuit element used to store magnetic energy. Typically, an inductor is created from several loops of wire stacked together to form a...
Antennas How To Make Circuits That Radiate 1
In Chapter 5, I discussed how and why circuits radiate, without any regard to practical implementation of antennas. This chapter covers how to make circuits that radiate efficiently. In Chapter 5, I based the results on an ideal line of current. In practice, circuits usually have a return current that flows in the opposite direction of the signal current. For example, in a two-wire transmission line, the two wires are parallel with signal current flowing in one direction and return current...
Bibliography And Suggestions For Further Reading
Adler, C. G., Does Mass Really Depend on Velocity, Dad , American Journal of Physics, vol. 55, no. 8, pp. 739-743. Albert, D. Z., Bohm's Alternative to Quantum Mechanics, Scientific American, May 1994. Baggot, J., The Meaning of Quantum Theory, Oxford Oxford University Press, 1992. Baylis, W. E., Electrodynamics, A Modern Geometric Approach, Boston Birkhaeuser, 1999. Blatt, F. J., Principles of Physics, 3rd Edition, Boston Allyn and Bacon, 1989. Boughn, S. P., The Case of the Identically...
Evanescent Waves and Tunneling
When the cross-section of a waveguide has dimensions smaller than half of a wavelength, waves cannot propagate. But the fields do penetrate a small distance into the guide, diminishing in an exponential manner. Such fields are called evanescent waves and are similar to the reactive storage fields that occur around circuits. In optics, such evanescent waves are referred to as frustrated waves. These waves have the interesting property that they can propagate energy if they are short and if they...
Ferromagnetism and Magnets
Diamagnetism and paramagnetism are both rather obscure phenomena to most engineers. The effects of ferromagnetism, however, are quite pronounced and quite well known. Ferromagnetism is responsible for the existence of magnets that is, permanent magnets like the ones you use to hang up notes on the refrigerator. A ferromagnetic material is like a paramagnetic material with the added feature of domains. Each domain is a microscopic patch of billions of atoms that have all lined up their dipole...
Voltage And Potential Energy
A quantity that goes hand in hand with the electric field is voltage. Voltage is also called potential, which is an accurate description since voltage quantifies potential energy. Voltage, like the electric field, is normalized per unit charge. Voltage Potential energy of a unit charge In other words, multiplying voltage by charge gives the potential energy of that charge, just as multiplying the electric field by charge gives the force felt by the charge. Mathematically we represent this by...
The Path Of Least Impedance
One of the most fundamental relations taught in basic circuit theory is that current follows the path of least resistance. At high frequencies this relation is modified to current follows the path of least impedance. The path of least resistance and the path of least impedance may or may not be the same, depending on the circuit geometry and the signal frequency. In any circuit, the signal consists of the flow of two currents, typically called the signal and return. Since the return current...