Antenna Fundamentals & Radio Waves

Antenna Fundamentals

• An antenna acts as an interface between a transmitter and free space, and between free space and a receiver.
• Antennas convert transmitter RF power into electromagnetic signals at the transmitting end.
• At the receiving end, antennas pick up electromagnetic signals and convert them into signals for the receiver.
• A radio signal is an electromagnetic wave made up of electric and magnetic fields.
• Applying voltage to an antenna creates an electric field, causing current to flow and producing a magnetic field.
• These fields are emitted from the antenna and propagate through space at the speed of light.

Radio Waves: Magnetic Fields

• A magnetic field is an invisible force field created by a magnet.
• An antenna is a type of electromagnet.
• A magnetic field is generated around a conductor when current flows through it.
• The strength and direction of the magnetic field depend on the magnitude and direction of the current flow.
• The SI unit for magnetic field strength is ampere-turns per meter.

Radio Waves: Electric Field

• An electric field is an invisible force field produced by the presence of a potential difference between two conductors.
• An example of an electric field is that between the plates of a charged capacitor.
• An electric field exists between any two points across which a potential difference exists.
• The SI unit for electric field strength is volts per meter.
• Permittivity is the dielectric constant of the material between two conductors.

Radio Waves: Fields in a Transmission Line

• In a two-wire transmission line, the wires have opposite polarities at any given time.
• During one half-cycle of the AC input, one wire is positive, and the other is negative.
• The polarity reverses during the negative half-cycle.
• The direction of the electric field between the wires reverses once per cycle.
• The direction of current flow in one wire is always opposite to that in the other wire, causing the magnetic fields to combine.
• Transmission lines do not radiate signals efficiently.
• The closeness of the conductors keeps the electric field concentrated in the transmission line's dielectric.
• The magnetic fields tend to mostly cancel each other out in a transmission line.
• While electric and magnetic fields extend outward from the transmission line, the amount of radiation that occurs is extremely inefficient.

Antenna Operation

• If a parallel-wire transmission line is left open, the electric and magnetic fields escape from the end and radiate into space.
• The escaping radiation is inefficient for reliable transmission/reception.
• Radiation improves by bending transmission line conductors at right angles to the line.
• The magnetic fields no longer cancel each other out when the conductors are bent.
• Optimum radiation occurs when the transmission wire segment converted into an antenna is one-quarter wavelength long at the operating frequency, which creates an antenna that is one-half wavelength long.
• The ratio of electric field strength to magnetic field strength of a radiated wave is a constant called the impedance of space, or wave impedance.
• The electric and magnetic fields produced by the antenna are at right angles, and both are perpendicular to the direction of propagation of the wave.
• Antennas produce two sets of fields: near field and far field.
• The near field is the region directly around the antenna, where electric and magnetic fields are distinct.
• The far field is approximately 10 wavelengths from the antenna and contains the radio wave with composite electric and magnetic fields.
• Polarization refers to the orientation of magnetic and electric fields with respect to the earth.
• Antenna reciprocity means that an antenna's performance is the same whether radiating or intercepting a signal.
• A transmitting antenna converts voltage from a transmitter into an electromagnetic signal.
• A receiving antenna has a voltage induced by the electromagnetic signal that passes across it.
• A basic antenna can be a length of wire, a metal rod, or a piece of tubing.
• Antennas radiate most effectively when their length relates directly to the wavelength of the transmitted signal.
• Most antennas are some fraction of a wavelength in length, with one-half and one-quarter wavelengths being most common.

Common Antenna Types: The Dipole Antenna

• One of the most widely used antenna types is the half-wave dipole.
• A half-wave dipole is also called a doublet or Hertz antenna.
• A dipole antenna is two pieces of wire, rod, or tubing; each is one-quarter wavelength long at the operating resonant frequency.
• Wire dipoles are supported with glass, ceramic, or plastic insulators at the ends and middle.
• The dipole has an impedance of 73 Ω at its center, which is the radiation resistance.
• To make the dipole resonate at the operating frequency, its physical length must be shorter than the half-wavelength computed by λ = 492/f.
• Actual length relates to the ratio of length to diameter, conductor shape, Q, the dielectric, and end effect.
• End effect is due to support insulators adding capacitance at the wire antenna ends.
• The actual antenna length is about 95 percent of computed length.
• Using a dipole at a frequency different from its design frequency increases the SWR, and power is lost.
• Antenna bandwidth is determined by the frequency of operation and the Q of the antenna as BW = f/Q.
• Higher Q results in a narrower bandwidth.
• A low Q and wider bandwidth are desirable, allowing the antenna to operate over a wider frequency range with reasonable SWR.
• An SWR below 2:1 is generally considered good in practical antenna work.
• Antenna Q and bandwidth are determined by the conductor length to diameter ratio.
• Bandwidth is sometimes expressed as a percentage of the antenna's resonant frequency.
• A small percentage means a higher Q, indicating a narrower bandwidth.
• Using a conical antenna is a common way to increase bandwidth for a dipole antenna.
• The center radiation resistance of a conical antenna is much higher vs straight-wire conductors
• Conical antennas offer a tremendous bandwidth.
• They can maintain a constant impedance and gain across a 4:1 frequency range.
• Most half-wave dipole antennas are mounted horizontally to the earth and horizontally polarized.
• Horizontal mounting is preferred for lower signal frequencies because of the ease of construction.
• The antenna's radiation pattern is the shape of the radiated electromagnetic energy.
• Antennas have directional characteristics, radiating or receiving energy in a specific direction.
• Radiation is concentrated in a recognizable geometric pattern.
• The measure of an antenna's directivity is beam width, the angle of the radiation pattern over which energy is directed or received.
• A directional antenna can radiate more power in a given direction vs a nondirectional antenna.
• Antenna gain is the ratio of the effective radiated output power Pout to input power Pin.
• Effective radiated power is the power a reference antenna would need to produce the same signal strength as the actual antenna.
• The effective radiated power (ERP) is the power radiated by an antenna with directivity and gain, where ERP = ApPt.
• Folded Dipole is a popular variation of the half-wave dipole.
• The folded dipole is also one-half wavelength long.
• It has two parallel conductors connected at the ends with one side open at the center for connection to the transmission line.
• The impedance of this antenna is 300 Ω.
• Folded dipoles usually offer greater bandwidth than standard dipoles.
• The folded dipole is an effective, low-cost antenna for transmitting and receiving.

Common Antenna Types: Marconi or Ground-Plane Vertical Antenna

• The one-quarter wavelength vertical antenna, also called a Marconi antenna, is widely used.
• Its operation is similar to a vertically mounted dipole antenna.
• The Marconi antenna has the advantage of being have the length of a dipole antenna.
• Vertical polarization and omnidirectional characteristics can be achieved with a one-quarter wavelength vertical radiator: the Marconi or ground-plane antenna.
• It is usually fed with coaxial cable, where center connects to radiator and the shield connects to earth ground.
• The acts as an electrical mirror, providing the other quarter wavelength to equate to a vertical dipole.
• A good electrical connection to the earth creates a ground plane.
• If a ground plane cannot be made, an artificial ground can be constructed with quarter-wavelength wires on the ground or buried.
• Horizontal wires at the base of the antenna are called radials, and the collection is called a counterpoise.
• Antennas can be shortened by using lumped electrical components to compensate.
• This design decreases inductance, causing a higher resonant frequency.
• A series inductor, called a loading coil, connects in series with the antenna coil to compensate.
• The loading coil brings the antenna back into resonance at the desired frequency.

Common Antenna Types: Directivity

• Directivity is an antenna's ability to send/receive signals over a narrow horizontal directional range.
• The physical orientation of the antenna give it directivity.
• The directional antenna eliminates interference.
• A highly directional antenna acts as a filter.
• Directivity improves power transmission efficiency.
• Directivity allows the antenna to amplify the signal.
• Two or more antenna elements are combined to form an array.
• Two basic types of antenna arrays are parasitic and driven.
• A parasitic array is a basic antenna connected to a transmission line with one or more parasitic conductors NOT connected to the transmission line.
• Extra conductors are the parasitic elements, and the antenna is the driven element.
• A Yagi antenna is a driven element and one or more parasitic element.
• A driven array is an antenna with two or more driven elements.
• Each element receives RF energy from the transmission line.
• Elements arrangements produce different degrees of directivity and gain.
• Driven arrays come in collinear, broadside, and end-fire arrangement.
• A collinear antenna consists of two or more half-wave dipoles end to end.
• Collinear antennas use half-wave sections separated by shorted quarter-wave matching stubs, ensuring the signals radiated by each half-wave section are in phase.
• Collinear antennas are mainly VHF and UHF, as length becomes prohibitive at lower frequencies.
• A broadside array is a stacked collinear antenna.
• A broadside array consisting of half wave, spaced by half wavelengths.
• Has a highly directional radiation pattern perpendicular to the array
• The broadside is bidirectional with narrow beam and high gain.
• The end-fire array uses two half-wave dipoles spaced one-half wavelength apart.
• The end-fire array is bidirectional with narrower beamwidth and lower gain.
• Radiation is in the plane of the driven elements.
• A highly unidirectional antenna is created by careful element spacing.
• Wide-bandwidth log-periodic antennas are a special driven array.
• The lengths of the driven elements vary logarithmically shortening in length with spacing also variable.
• The great advantage of the log-periodic antenna over a Yagi or other array is its very wide bandwidth.
• Driving impedance is constant.
• Most TV antennas in use today are of the log-periodic variety.

Common Antenna Types: Impedance Matching

• Ensuring maximum power transfer is one of the most critical aspects of any antenna system.
• When the characteristic impedance of the transmission line matches the output impedance of the transmitter and the antenna, the SWR will be 1:1.
• When SWR is 1:1, maximum power transfer is achieved.
• A Q section, or matching stub, is a one-quarter wavelength of coaxial or balanced transmission line of a specific impedance.
• It connects between a load and source and is used to match impedances.
• A balun is a transformer.
• An antenna tuner contains a variable inductor, one or more variable capacitors, or a combination of these, in various configurations.

Radio-Wave Propagation

• A radio signal propagates through space to a receiving antenna.
• The signal's energy level decreases rapidly with distance.
• An electromagnetic wave is affected by trees, buildings, and other structures.
• A signal's path depends on signal frequency, atmospheric conditions, and time of day.
• Radio waves act like light waves.
• Light waves can be reflected, refracted, diffracted, and focused.
• Focusing waves concentrate a signal in a narrower direction, like a lens focusing light.
• Any conducting surface is a mirror for radio waves, reflecting them.
• Radio reflection follows light wave reflection principles.
• The angle of reflection equals the angle of incidence.
• The approaching electric field direction reverses, equivalent to a 180° phase shift.
• Refraction is the bending of a wave due to the physical makeup of the medium.
• The index of refraction is the speed of light or radio waves in a vacuum divided by the speed of light or radio waves in the medium.
• Snell's law expresses the relationship between the angles and the indices of refraction: n₁ sin Θ₁ = n2 sin Θ2.
• Diffraction is the bending of waves around an object.
• diffraction is explained by Huygen's principle.
• Huygen's principle assumes all electromagnetic waves radiate as spherical waveforms creating additional spherical waves.
• Waves pass around, above, and aside the obstacle's edge
• At the edge of the obstacle, point sources, create waves that fill shadow zone.

Radio-Wave Propagation Through Space

• A radio signal can take three basic paths through space: Ground wave, Sky wave, Space wave.
• Ground or surface waves leave an antenna and remain close to the earth.
• Ground waves follow the curvature of the earth.
• Ground waves need vertical polarization to be propagated.
• Ground-wave propagation is strongest at the low- and medium-frequency ranges.
• AM broadcast signals are propagated primarily by ground waves during the day and by sky waves at night.
• Sky-wave signals are radiated into the upper atmosphere, where they bend back to earth.
• The different levels of ionization bend the radio waves.
• The smaller the angle with respect to the earth, the more the waves bend and return.
• The higher the frequency, the smaller the radiation angle required.
• A space wave, or direct wave, travels in a straight line from transmitter to receiver.
• Direct-wave radio is line-of-sight communication.
• Direct or space waves are not refracted.
• Line-of-sight communication is above 30 MHz, VHF, UHF, and microwave.
• Repeater stations extend the communication distance at VHF, UHF, and microwave frequencies.
• A repeater is a combo receiver and receiver separate frequencies.
• When received, the signal is amplified and retransmitted to a remote receiver.
• Repeaters increase mobile and handheld radio range.
• Trunked repeater systems put multiple repeaters under computer control to balance communications load.
• Communication satellites act as repeaters.
• Satellites contain transponders: receiver-transmitter combos.
• Fading is defined as the variation in signal amplitude at receiver and smaller signals
• Fading is a function of distance, environmental changes, the presence of the multipath signals, and relative motion.
• A diversity system uses multiple transmitters, receivers, or antennas mitigate the multipath problems.
• Frequency diversity uses two sets of transmitters/receivers simultaneously.
• Space or Spatial Diversity uses two physically-distant receive antennas.