Antenna Fundamentals

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SURIANO / SURIANO /HOLMES /YU

ANTENNAS

Antenna fundamentalsA proper understanding of antennas requires familiarity with electromagnetics, circuit theory, electronics, and signal processing.CANDACE SURIANO, PH.D.Suriano Solutions

JOHN SURIANO, PH.D.Nidec Motors Auburn Hills, MI

TOM HOLMESAgilent Technologies Tipp City, OH

QIN YUAlcatel-Lucent Columbus, OH

H

OW DOES AN ANTENNA PICK UP A signal and convert it to something useful to a receiving circuit? What is the current path for the signals received or transmitted from an antenna? Why are there dierent types of antennas, and why do they have dierent shapes? What are the standard engineering terms associated with antenna technology? How are signals from antennas amplied? The subject of antennas may seem beyond reach for many engineers, but a working knowledge of the operation and functional characteristics of antennas is an essential component of the EMC knowledge base. It is the starting point for understanding many EMC requirements and test procedures and for resolving compliance issues. The basics of antennas can be deduced from fundamental principles of electromagnetics and electric circuits. Even a rudimentary understanding can prove to be invaluable in solving EMC problems. HOW DO ANTENNAS DETECT SIGNALS? Antennas have two complementary funcInterferenceTechnology.com

tions: converting electromagnetic waves into voltage and current used by a circuit, and converting voltage and current into electromagnetic waves which are transmitted into space. Signals are transmitted through space by electromagnetic waves consisting of electric elds measured in Volts per meter and magnetic elds measured in Amps per meter. Depending on the type of eld being detected, the antenna takes on a particular construction. Antennas designed to pick up electric elds, like the antenna of Figure 1(a), are made with rods and plates while antennas made to pick up magnetic elds, as in Figure 1(b), are made from loops of wire. Sometimes parts of electric circuits may have characteristics that unintentionally make them antennas. EMC is concerned with reducing the probability of these unintentional antennas injecting signals into their circuits or inuencing other circuits. Consider the antenna of a car radio. As the electric eld (V/m) hits the antenna it impresses a voltage along its length (m*V/m = V) relative to ground. The receiver detects the voltage between the antenna and ground. Another way to think of this type of antennaI

Electric Field

Magnetic Field

V

I

Figure 1. (a) Electric eld antenna and (b) magnetic eld antenna.INTERFERENCE TECHNOLOGY

1

ANTENNASis as one lead of a voltmeter measuring the potential in space. The other lead of the voltmeter is the ground of the circuit. WHAT IS THE SIGNIFICANCE OF AN ANTENNAS SHAPE? Some antennas are made of loops of wire. These antennas detect the magnetic eld rather than the electric eld. Just as a magnetic eld through a coil of wire is produced by the current in that coil, so too a current is induced in a coil of wire when a magnetic eld goes through that coil. The ends of the loop antenna are attached to a receiving circuit through which this induced current ows as the loop antenna detects the magnetic eld. Magnetic elds are generally directed perpendicular to the direction of their propagation so the plane of the loop should be aligned parallel to the direction of the wave propagation to detect the eld. Some types of electric eld antennas are biconical, horn, and microstrip. Generally, antennas that radiate electric elds have two components insulated from each other. The simplest electric eld antenna is the dipole antenna, whose very name implies its two-component nature. The two conductor elements act like the plates of a capacitor with the eld between them projecting out into space rather than being conned between the plates. On the other hand, magnetic eld antennas are made of coils which act as inductors. The inductor elds are projected out into space rather than being conned to a closed magnetic circuit. The categorization of antennas in this way is somewhat articial, however, since the actual mechanism of radiation involves both electric and magnetic elds no matter what the construction. HOW DO ANTENNAS FORM AND RADIATE ELECTROMAGNETIC FIELDS? As previously mentioned, electric eld antennas can be related to capacitors. Consider a simple parallel plate capacitor shown in Figure 2(a). The electric eld that occurs when a charge is placed on each of the plates is contained in between the plates. If the plates are spread apart so that they lie in the same plane, the electric eld between the plates extends out into space. The same process occurs with an electric eld dipole an2INTERFERENCE TECHNOLOGY

ANTENNA FUNDAMENTALS

tenna as shown in Figure 2(b). Charges on each part of the antenna produce a eld into space between the two halves of the antenna. There is an intrinsic capacitance between the two rods of the dipole antenna as shown in Figure 2(c). Current is required to charge the dipole rods. The current in each part of the antenna ows in the same direction. Such current is called antenna mode current. This condition is special because it results in radiation. As the signal applied to the two halves of the antenna oscillates, the eld keeps reversing and sends out waves into space. The charge and current on the dipole create elds that are perpendicular to each other. The electric eld, E, ows from the positive charge to the negative charge placed on the elements by voltage applied to the antenna as shown in Figure 3(a). Charging current applied to the antenna makes a magnetic eld, H, that circulates around the wire according to the right hand rule as shown in Figure 3(b). God made it so that when electrons move along the wire a magnetic wind is produced which circulates around the wire. Directing ones right thumb in the direction of the current ow, the ngers wrap around the wire in the direction of the magnetic eld. The circulation of this magnetic eld results in inductance of the antenna. The antenna is therefore a reactive device having both

capacitance from the charge distribution and inductance from the current distribution. As shown in Figure 3(c), the E and H elds are perpendicular to each other. They spread out into space from the antenna in a circular fashion. As the signal on the antenna oscillates, waves are formed. Transverse Electromagnetic (TEM) waves are produced in which E and H are perpendicular to each other. The antenna can also convert a TEM wave back into current and voltage by something called reciprocity. The antenna has complementary behavior when sending and receiving. The condition of antenna radiation is shown in Figure 4. The reactive components of the antenna store energy in the electric and magnetic elds surrounding the antenna. Reactive power is exchanged back and forth between the supply and the reactive components of the antenna. Just as in any L-C circuit where the voltage and current are always 90 out of phase, so too with an antenna the E eld (produced by voltage) and the H eld (produced by current) are 90 out of phase if the resistance of the antenna is neglected. In an electric circuit, real power is delivered only when the load has a real component to its impedance that causes a component of the current and voltage to be in-phase. This circumstance also

Figure 2. (a) Capacitor circuit, (b) dipole, (c) dipole showing intrinsic capacitance and charging current.

Figure 3. (a) Electric eld E and (b) magnetic eld H and TEM eld from dipole charge and current.EMC DIRECTORY & DESIGN GUIDE 2007

SURIANO / SURIANO /HOLMES /YU

ANTENNAS

Figure 4. Power ow resulting in radiation.

holds true with antennas. The antenna has some small resistance so there is a component of real power delivered that is dissipated in the antenna. For radiation to occur, E and H elds must be in-phase with each other as shown in Figure 3(c). With the antenna acting as both a capacitance and an inductance, how can this radiation take place? The in-phase components are the result of propagation delay. The waves from the antenna do not instantly form at all points in space simultaneously, but rather propagate at the speed of light. At distances far away from the antenna, this delay results in a component of the E and H elds that are in phase. Thus, there are dierent components of the E and H elds that comprise the energy storage (reactive) part of the eld or the radiated (real) part. The reactive portion is dictated by the capacitance and inductance of the antenna and exists predominately in the near eld. The real portion is dictated by something called radiation resistance, caused by the propagation delay, and exists at large distance from the antenna in the far eld. Sometimes receiving antennas, such as those used in EMC testing, may be placed so close to the source that they are inuenced more by the near eld eects than the far eld radiation. In this case, the receiving and transmitting antennas are coupled by capacitance and mutual inductance. The receiving antenna thus acts as a load on the transmitter. HOW DOES THE ANTENNA IMPEDANCE CHANGE WITH FREQUENCY? Antenna impedance is a function of frequency. The current and charge distribution on the antenna change with frequency. The current on a dipole is generally shaped as a sinusoidal function of position on the antenna as dictated by the frequency. Since the wavelength of a signal is dependent on the frequency, at certain frequenciesInterferenceTechnology.com

Figure 5. (a) Dipole current with wave excitation, (b) full wave excitation, (c) dipole impedance.

the antenna length is equal to key fractions of a wavelength. The current on a dipole for frequencies resulting in and 1 w