Antenna Fundamentals & Radio Waves

Antenna Fundamentals

• The antenna serves as the interface between a transmitter and free space, and between free space and a receiver.
• At the transmitting end, an antenna converts radio frequency (RF) power into electromagnetic signals.
• At the receiving end, an antenna picks up electromagnetic signals and converts them into signals the receiver can use.

Radio Waves: Magnetic Fields

• Radio signals are electromagnetic waves composed of both electric and magnetic fields.
• When voltage is applied to an antenna, an electric field is created.
• This voltage also causes current to flow in the antenna, generating a magnetic field.
• These fields are emitted from the antenna and propagate through space at the speed of light.
• A magnetic field is an invisible force field produced by a magnet.
• An antenna is a type of electromagnet.
• Whenever current flows through a conductor, a magnetic field is generated around it.
• The magnitude and direction of the magnetic field are dependent on the current's magnitude and direction.
• The SI unit for measuring magnetic field strength is ampere-turns per meter.

Radio Waves: Electric Fields

• An electric field is an invisible force field produced by a potential difference, also known as voltage, between two conductors.
• For example, an electric field exists between the plates of a charged capacitor.
• Electric fields exist between any two points with a potential difference.
• The SI unit for electric field strength is volts per meter.
• Permittivity is the dielectric constant of the material between two conductors.

Radio Waves: Magnetic and Electric 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 an AC input, one wire is positive, and the other is negative.
• During the negative half-cycle, the polarity reverses.
• The electric field direction between the wires also reverses with each cycle.
• Current flow in one wire is always opposite the current flow in the other. As a result, magnetic fields combine .
• Transmission lines are made of a conductor or conductors.
• Transmission lines aren't efficient radiators.
• The electric field is concentrated in the dialectric due to the closeness of the conductors.
• The magnetic fields tend to cancel each other.
• Any radiation from the electric and magnetic fields is very inefficient.

Antenna Operation: The Nature of an Antenna

• Leaving a parallel-wire transmission line open allows some electric and magnetic fields to escape, creating some radiation.
• This radiation is inefficient.
• Bending the transmission-line conductors at a right angle to the line will greatly improve radiation.
• The magnetic fields won't cancel each other out anymore, but instead aid one another.
• The electric field spreads between the conductors.
• Optimal radiation occurs when the segment of wire converted to an antenna is a quarter-wavelength long at the operating frequency.
• This makes the antenna one-half wavelength long.

Antenna Operation

• Wave impedance, or impedance of space, is the ratio of the electric field strength of a radiated wave to its magnetic field strength.
• In electromagnetic waves, the electric and magnetic fields are perpendicular to each other and to the direction of propogation.
• Antennas produce near and far fields.
• Near field is the region right around the antenna, where the electromagnetic fields are distinct.
• The far field is about 10 wavelengths from the antenna, and consists of the radio wave itself with the composite electric and magnetic fields.
• Polarization refers to the orientation of magnetic and electric fields with respect to the earth.

Antenna Reciprocity

• Antenna reciprocity means an antenna's characteristics and performance are identical regardless of whether the antenna is radiating or recieving a signal.
• Transmitting antennas take voltage from a transmitter and convert it to an electromagnetic wave.
• Receiving antennas have voltage induced into them as electromagnetic waves pass across.

The Basic Antenna

• Antennas can be made from a length of wire, a metal rod, or a piece of tubing.
• Antennas are most effective when their length is related to the wavelength of the transmitted signal.
• Most antennas have a length that is some fraction of a wavelength
• Common lengths are one-half and one quarter wavelengths.

The Dipole Antenna

• One of the most widely used types of antennas is a half-wave dipole.
• The half-wave dipole is also known as a doublet or Hertz antenna.
• Antennas consist of two pieces of wire, rod, or tubing, each being one-quarter wavelength long at the operating frequency.
• Wire dipoles are supported at their ends and middle using glass, ceramic, or plastic insulators.
• The dipole antenna has an impedance of 73 ohms at its center, which is the radiation resistance.
• Antennas are frequency sensitive.
• The physical length must be shorter than one-half wavelength to get the dipole to resonate at the frequency of operation.
• End effect, conductor shape, Q, the dielectric (when the material is other than air), and the ratio of length to diameter all relate to the actual length.
• End effect is a phenomenon caused by the support insulators at the ends of the wire antenna, which adds capacitance to the ends of each wire.
• The actual antenna length is only about 95% the computed length.
• If a dipole is used at a frequency different from its design frequency, the SWR rises and power is lost

Dipole Antenna: Antenna Q and Bandwidth

• Bandwidth of an antenna is determined by the frequency of operation and the Q of the antenna; equation is BW = f/Q.
• Higher Q means a narrower bandwidth.
• It it better for an antenna to have a low Q and a wider bandwidth because the antenna can operate over a wider range of frequencies with a reasonable SWR.
• A SWR below 2:1 is "good" in practical applications.
• The Q and bandwidth are determined by the ratio of the length of the conductor to the diameter of the conductor.
• Bandwidth is expressed as a percentage of the antenna's resonant frequency.
• Small percentages mean a higher Q, while a narrower bandwidth means a lower percentage.

The Dipole Antenna: Conical Antennas

• One way to increase bandwidth is to use a conical antenna, a version of the dipole antenna.
• The conical antenna's radiation resistance is typically much higher than 73 ohms, which is usually found with straight-wire conductors.
• Conical antennas posses a tremendous bandwidth.
• They can maintain a consitent impedance and gain over an approximate 4:1 frequency range.

The Dipole Antenna: Dipole Polarization

• Most half-wave dipole antennas are mounted horizontally to the earth.
• Mounting the antenna this way makes the electric field horizontal to the Earth, and the antenna is horizontally polarized.
• Horizontal mounting is preferred at lower frequencies because construction, mounting, and support are easier.
• This mounting also makes it easy to attach the transmission line and route it to the transmitter/receiver.

The Dipole Antenna: Radiation Pattern and Directivity

• The radiation pattern is the shape of the electromagnetic energy radiated or received by that antenna.
• Most antennas have directional characteristics, which cause them to radiate or receive energy in a specific direction.
• Radiation is concentrated in a pattern with a geometric shape.
• Beam width measures an antenna's directivity, representing the angle of radiation.

The Dipole Antenna: Antenna Gain

• Directional antennas are able to radiate more power in a given direction, so it acts like it has gain.
• Of this antenna type, antenna gain is expressed as the ratio of the effective radiated out power (Pout) to the input power (Pin).
• Effective radiated power is the actual power that would have to be radiated by a reference antenna to match the test antenna's signal strength.
• Effective radiated power (ERP) is the radiated power by an antenna with directivity and therefore gain.

The Dipole Antenna: Folded Dipole

• A popular variation of the half-wave dipole is the folded dipole
• Folded dipoles are also one-half wavelength
• Folded dipoles consist of two parallel conductors that are connected at the ends with one side open at the center for connection to the transmission line.
• It has an impedance of 300
• Folded dipoles typically have greater bandwidth than standard dipoles.
• It's an effective and low cost antenna which provides recieving and transmitting capabilities.

Marconi or Ground-Plane Vertical Antenna

• The one-quarter wavelength vertical antenna, also called a Marconi antenna, is widely used.
• It is similar in operation to a vertically mounted dipole antenna.
• Marconi antennas offer major advantages because they are half the length of a dipole antenna.
• Vertical polarization and omnidirectional characteristics can be achieved using a one-quarter wavelength vertical radiator, also called a Marconi or ground-plane antenna.
• It is usually fed with coaxial cable, the center conductor is connected to the vertical radiator and the shield with earth ground.
• The Earth acts as an electrical mirror providing the other quarter wavelength making it equivalent to a vertical dipole.

Marconi or Ground-Plane Vertical Antenna: Ground Plane, Radials, and Counterpoise

• Good electrical connections to the earth cause the earth to become a ground plane.
• If a ground plane cannot be made to earth, an artificial one can be constructed using one-quarter wavelength wires laid horizontally on the ground or underground.
• These horizontal wires at the base of the antenna are called radials.
• Counterpoise is the collection of radials.

Marconi or Ground-Plane Vertical Antenna: Antenna Length

• With equipment like portable or mobile equipment, it is not possible to make the antenna a one-quarter wavelength long.
• To overcome this, shorter antennas are used, with lumped electrical components added to compensate for the shortening.
• The practical effect is a decreased inductance.
• This means that the antenna no longer resonates at its desired operating frequency, but a higher frequency.
• To compensate for the increased inductance, a series inductor, called a loading coil, is connected in series with the antenna.
• The loading coil then makes the antenna resonate again at the desired frequency.

Directivity

• Refers to an antenna's ability to send or receive signals over a narrow horizontal directional range.
• The physical orientation of the antenna provides the directivity curve, which is a highly directional response.
• A directional antenna eliminates interference from all other signals except the desired one.
• Highly directional antennas act as a type of filter to provide selectivity.
• This type of antenna also provides greater efficiency of power transmission
• Directivity exhibits gain in antennas as well, which is one form of amplification.
• When creating an antenna with directivity and gain, two or more antenna elements are combined to form an array.
• The 2 types are parasitic and driven arrays.

Parasitic Arrays

• A parasitic array consists of a basic antenna connected to a transmission line, plus additional conductors that are not connected to the transmission line.
• These extra conductors are parasitic elements, and the basic antenna is called a driven element.
• Yagi antennas are made with 1+ parasitic elements and a driven element.

Driven Arrays

• A driven array has two or more driven elements.
• The elements receive RF energy from the transmission line.
• Different element arrangements can affect directivity and gain.
• The 3 kinds of driven arrays are collinear, broadside, and end-fire.
• A fourth type is a wide-bandwidth log-periodic antenna.

Driven Arrays: Collinear Antenna

• Collinear antennas are made using 2 or more half-wave dipoles mounted end-to-end.
• Half-wave sections separated by shorted quarter-wave matching stubs are used by collinear antennas.
• These stubs make the signals radiate by the half-wave sections be in phase.
• Collinear antennas are used on VHF and UHF bands because the length would be prohibited at lower frequencies.

Driven Arrays: Broadside Antenna

• A broadside array is a stacked collinear antenna made of half-wave dipoles spaced about one-half wavelength apart.
• This antenna has a radiation pattern that's highly directional, broadside, and perpendicular to the array's plane
• Although it's bidirectional, the broadside antenna has very narrow beam widths and high gain.

Driven Arrays: End-Fire Antenna

• The end-fire array uses two half-wave dipoles that are spaced about one-half wavelength apart.
• The end-fire array has a bidirectional radiation pattern, but with narrower beam widths and lower gain.
• Its radiation is in the plane of the driven elements.
• Unidirectional antennas can be created by selecting the optimal number of elements with appropriate spacing.

Driven Arrays: Log-Periodic Antennas

• The wide-bandwidth log-periodic antenna is a special type of driven array.
• The lengths of the driven elements in this antenna vary from long to short, and the length/spacing differences need to be related logarithmically.
• Its advantage over other arrays is their very wide bandwidth.
• In this range, the driving impedance is constant.
• TV antennas today tend to use log periodic variety with higher gains to pick up VHF and UHF TV channels.

Impedance Matching

• One of the most crucial aspects in any system is power transfer from the transmitter to the antenna.
• SWR(Standing Wave Ratio) = 1 : 1 - when the cha