High-frequency electronic platforms, such as RF/microwave communications and radar systems, often rely on the propagation of electromagnetic (EM) energy from one location to another, as well as back and forth between two locations. James Clerk Maxwell is credited with a unified theory that explains the propagation behavior of EM energy. Maxwell noted the EM nature of light, and that light and EM energy will travel through a vacuum with the same speed, c, known as the speed of light (186,000 miles/s). EM energy will propagate through other media, but at speeds less than c.
RF/microwave systems depend on the propagation of EM energy or waves through certain materials to connect transmitters with communications receivers and the reflection from certain materials for radar systems to “illuminate” a target with EM energy. Understanding some of the basic propagation effects for EM waves can help when designing and planning for many high-frequency systems, even to the extent of preventing EM interference (EMI) from disrupting the proper operation of a nearby RF/microwave system. Reviewing the basics of EM propagation may help.
A Quick Primer on EM Propagation
EM propagation through space usually begins and ends with some form of antenna. Many different types of antennas are used at RF/microwave frequencies, from simple dipoles to more complex arrays with multiple antenna elements. However, the principles of operation are the same: to radiate EM energy during transmission and collect the energy from propagating EM waves and convert to voltage during reception.
Transmit antennas are often referred to as point sources of EM radiation, with the energy spreading out in all directions from the point source. When measured at a considerable distance from the point source, the radiated energy will appear to have the same power level or amplitude at all measurement points along an apparent plane that is perpendicular to the direction of travel for the radiated energy.
Radio waves travel from one ground-based antenna to another with the velocity of light, another form of EM radiation. A radio wave consists of magnetic (H) and electric (E) or electrostatic fields at right angles to each other and at right angles to the direction of travel. One-half of the energy of the propagating radio wave is in the form of electrostatic energy, while the other half of the wave’s energy is in magnetic energy form. The radio wave carries energy as voltage from one location to another, with antennas serving as the means of transmission and reception for that energy.
The direction of the electrostatic-flux lines is called the direction of polarization of a radio wave. When the electrostatic lines of flux are vertical, the waves have vertical polarization; when the lines of flux are horizontal, it is horizontal polarization. These are both forms of linear polarization. EM waves can also propagate in a kind of cork-screw-shaped motion known as circular polarization. If the E field is rotating in a clockwise motion relative to the direction of propagation, it is called right-handed circular polarization (RHCP). If the rotation of the E field is counterclockwise relative to the direction of propagation, it is referred to as left-handed circular polarization (LHCP).
As it propagates through space, EM energy can be visualized as a plane wave traveling in a single direction, such as from a transmit antenna to a receive antenna, rather than as energy spreading out in all directions. Ideally, such a plane wave would travel from transmit antenna to receive antenna with no loss in energy. But EM plane waves often come into contact with media other than the air through which they typically propagate. When that happens, the EM energy in the plane wave may propagate into (and through) the new medium, if it is a conductor, or bounce off another medium, if it is a reflector, as in a pulsed radar signal.
Communications signals, for example, typically propagate in many directions outward from a transmit antenna, whether it is an omnidirectional or directional antenna. Signals that follow a line-of-sight (LOS) path from the transmit to the receive antenna will also be joined at the receiver by EM energy from signals that have reflected from other media, such as buildings, with resultant slight delays or shifts in phase of these reflected signals.
Such reflections in EM propagation lead to what is known as multipath propagation, which can cause distortion at the receiver when the delays or phase shifts are significant. However, multipath propagation can also be put to good use, in the form of multiple-input, multiple-output (MIMO) antennas designed to take advantage of the multiple propagation paths to increase the effective data rate of a communications link.
Depending on frequency and such factors as antenna types and positions, radio waves are affected by the surface of the Earth and the atmosphere. Lower-frequency EM radiation, such as VHF and UHF radio waves, with longer wavelengths, tend to propagate along the Earth’s surface in a mode known as ground-wave propagation. Higher-frequency signals, with shorter wavelengths, will propagate as LOS signals or as skywaves that have traveled through the troposphere (which contains the air and clouds around the Earth) and have been reflected by the atmospheric layer above that, the ionosphere.
Propagation through the troposphere and in turn reflecting from the ionosphere as skywaves results in some EM energy loss. However, it also enables longer propagating distances for radio waves at some frequencies.
Efficient EM propagation relies heavily on the performance of the point source antenna. Many different types of antennas are used with radio waves at different frequencies. The simplest antenna is the dipole antenna, which is essentially a section of straight wire. It’s a configuration often used as a building block for other antenna types,. When a voltage is applied to the wire, current flows through it and electrical charges collect at either end of the wire. It is referred to as a dipole because a balanced set of positive and negative charges collect at either end.
When a voltage at some resonating or alternating frequency is applied to the dipole, its electric moment oscillates, resulting in oscillation of its positive and negative charges and oscillation of its electric current. The oscillating current creates the E and H fields which give rise to the outwardly propagating EM wave. The E field is oriented along the axis of the antenna and the H field is perpendicular to both the E field and the direction of propagation. This orientation of the fields is also the polarization of the antenna.
1. One of the more popular antenna formats for satcom and terrestrial LOS communications links is the parabolic or dish antenna. (Courtesy of RadioWaves)
The physical size of a dipole antenna determines its operating frequency. A standard dipole has a total length equal to one-half wavelength of the operating frequency. Each side of the dipole structure is equal to one-quarter wavelength of the operating frequency, with each side of the antenna fed 180 deg. out of phase from the other side of the antenna.
The omnidirectional behavior of dipole antennas makes them well-suited for EM waves with dominant LOS and ground-wave characteristics. But many applications, e.g., satcom systems, operate with EM propagation that is more directional in nature, through the use of space waves. This requires antennas that are more directional in nature, such as the apparently ever-present parabolic reflector or “dish” antenna (Fig. 1).
The antennas, which are also commonly used in terrestrial point-to-point communications systems that rely on LOS EM propagation, can be designed with single or multiple polarization modes, with sizes that vary according to wavelength and frequency. Similarly, horn antennas (Fig. 2) are quite directional in nature, operating with fairly narrow beam widths, depending on frequency, to transmit and receive in LOS mode.
2. Horn antennas are very directional in nature, with physical dimensions a function of the wavelength/frequency of the signals to be handled. The standard gain horn on the left measures 384 × 284 × 360 mm for use from 0.96 to 1.45 GHz, while the horn on the right is a mere 21.4 × 16.6 × 51.0 mm for frequencies from 90 to 140 GHz. (Courtesy of EC Microwave)
Sending Millimeter Waves
As the frequencies of EM waves increase, the distances traveled by those radio waves decrease as they lose EM energy due to propagation and attenuation of the Earth and the atmosphere. Very low frequencies, such as the bandwidths used for amplitude-modulated (AM) broadcast radios, achieve long distances due to ground-wave propagation and minimal atmospheric losses. Satellite communications (satcom) systems (see “Satellites Provide Distant Connections”) take advantage of orbiting satellites and directional antennas to bypass the losses of ground-wave propagation and achieve relatively low-loss EM propagation through the atmosphere.
With the coming of fifth-generation (5G) wireless communications networks and their multiple-frequency-band configurations that incorporate microwave and millimeter-wave frequencies, various EM propagation modes will be employed by these systems, along with the different types of antennas that will be needed for those propagation modes. As the frequencies used in 5G systems extend to 28 GHz and beyond, EM propagation at these higher frequencies will be more directional and LOS in nature over shorter distances to conserve as much EM energy (and the data it carries) as possible.
3. This miniature PCB-based phased-array antenna makes use of beamforming techniques for handling EM waves at 28 GHz. (Courtesy of Gapwaves)
In keeping with the relationship of wavelength and antenna size, some of the newer companies working on antennas for the millimeter-wave portions of 5G systems are at the chip and PCB level for their designs, including Gapwaves with its phased-array antennas and beamforming techniques for millimeter-wave frequencies in 5G systems (Fig. 3). The company’s PCB antennas include high-gain models with an effective isotropic radiated power (EIRP) level of +65 dBm at 28 GHz.
Anokiwave has shown tremendous innovation in its lines of active antenna integrated circuits (ICs) for millimeter-wave frequencies through 80 GHz. For example, the AWMF-0129 is a 28-GHz 5G active antenna design kit that includes a 64-element phased-array antenna assembled on a PCB with the company’s active antenna ICs. It operates from 27.5 to 30.0 GHz with linear polarization and has programmable beam widths, with independent phase and gain control in both transmit and receive operating modes. Its low-power, compact design is very much a sign of things to come for ubiquitous EM propagation at millimeter-wave frequencies as part of shorter-distance data links in 5G wireless networks.