Associate Professor in the Department of Electrical and Computer Engineering at Rutgers University. Research interests include statistical and adaptive signal processing, audio signal processing, and electromagnetic waves and antennas.
How 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 different types of antennas, and why do they have different shapes What are the standard engineering terms associated with antenna technology How are signals from antennas amplified
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.
Antennas have two complementary functions: 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 fields measured in Volts per meter and magnetic fields measured in Amps per meter. Depending on the type of field being detected, the antenna takes on a particular construction. Antennas designed to pick up electric fields, like the antenna of Figure 1(a), are made with rods and plates while antennas made to pick up magnetic fields, 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 influencing other circuits.
Some antennas are made of loops of wire. These antennas detect the magnetic field rather than the electric field. Just as a magnetic field 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 field goes through that coil. The ends of the loop antenna are attached to a receiving circuit through which this induced current flows as the loop antenna detects the magnetic field. Magnetic fields 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 field.
Some types of electric field antennas are biconical, horn, and microstrip. Generally, antennas that radiate electric fields have two components insulated from each other. The simplest electric field 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 field between them projecting out into space rather than being confined between the plates. On the other hand, magnetic field antennas are made of coils which act as inductors. The inductor fields are projected out into space rather than being confined to a closed magnetic circuit. The categorization of antennas in this way is somewhat artificial, however, since the actual mechanism of radiation involves both electric and magnetic fields no matter what the construction.
As previously mentioned, electric field antennas can be related to capacitors. Consider a simple parallel plate capacitor shown in Figure 2(a). The electric field 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 field between the plates extends out into space. The same process occurs with an electric field dipole antenna as shown in Figure 2(b). Charges on each part of the antenna produce a field 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 flows 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 field keeps reversing and sends out waves into space.
As shown in Figure 3(c), the E and H fields 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 fields 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 field (produced by voltage) and the H field (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 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 fields 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 fields that are in phase.
Thus, there are different components of the E and H fields that comprise the energy storage (reactive) part of the field or the radiated (real) part. The reactive portion is dictated by the capacitance and inductance of the antenna and exists predominately in the near field. 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 field. Sometimes receiving antennas, such as those used in EMC testing, may be placed so close to the source that they are influenced more by the near field effects than the far field 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.
The three- or two-dimensional radiation pattern from an antenna is also called a power pattern, power plot, or power distribution. It visually illustrates how an antenna receives or transmits in a certain range of frequencies. It is normally plotted for the far field. An antenna radiation pattern is primarily affected by the geometry of the antenna. It is also affected by the surrounding landscape or by other antennas. Sometimes multiple antennas are used in an antenna array to affect directivity. As shown in Figure 6(a), two antennas fed by the same source can be used to cancel the fields in the plane of the antennas if they are spaced by wavelength. The top view of this arrangement is shown in Figure 6(b) with a sketch of the power pattern.
Another problem with connecting to antennas is signal unbalance caused by a ground plane. Figure 10(a) shows a dipole antenna connected to a source through a shielded cable. The shield is connected to the ground plane. Parasitic capacitance between the antenna and the ground plane causes some current to flow through the ground plane rather than through the shield. When this occurs, the current on the antenna is unbalanced, and the antenna loses efficiency. To correct this imbalance, a device called a balun (balanced to unbalanced) is used. A simple type of balun is shown in Figure 10(b). Here, the balun is comprised of a ferrite cylinder (bead) placed over the coaxial cable. The ferrite increases the impedance only for the common mode current and has no effect on the normal differential mode current in the cable. Consequently, the current that causes the unbalance is reduced, improving the operation of the antenna. For receiving antennas, the incoming signal may induce current on the shield that causes the unbalance. The ferrite bead reduces the current on the shield.
A proper understanding of antennas requires familiarity with electromagnetics, circuit theory, electronics, and signal processing. Such knowledge is indispensable to the EMC engineer who must interpret test results, improve accuracy and sensitivity of tests, and suggest ways to eliminate unintentional antennas from product designs.
In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment. 59ce067264