Inverted-F antenna

Last updated

An inverted-F antenna in a DECT (a technology used for cordless phones and similar devices) base station Planar Inverted F-Shaped DECT Antenna.jpg
An inverted-F antenna in a DECT (a technology used for cordless phones and similar devices) base station

An inverted-F antenna is a type of antenna used in wireless communication. It consists of a monopole antenna running parallel to a ground plane and grounded at one end. The antenna is fed from an intermediate point a distance from the grounded end. The design has two advantages over a simple monopole: the antenna is shorter and more compact, and the impedance matching can be controlled by the designer without the need for extraneous matching components.

Antenna (radio) electrical device which converts electric power into radio waves, and vice versa

In radio engineering, an antenna 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. 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.

Monopole antenna type of radio antenna

A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane, which is often the Earth. This contrasts with a dipole antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna.

Impedance matching practice in electronics

In electronics, impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source to maximize the power transfer or minimize signal reflection from the load.

Contents

The inverted-F antenna was first conceived in the 1950s as a bent-wire antenna. However, its most widespread use is as a planar inverted-F antenna (PIFA) in mobile wireless devices for its space saving properties. PIFAs can be printed using the microstrip format, a widely used technology that allows printed RF components to be manufactured as part of the same printed circuit board used to mount other components.

Microstrip electrical transmission line for microwave-frequency signals on printed circuit board

Microstrip is a type of electrical transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave components such as antennas, couplers, filters, power dividers etc. can be formed from microstrip, with the entire device existing as the pattern of metallization on the substrate. Microstrip is thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact. Microstrip was developed by ITT laboratories as a competitor to stripline.

Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around twenty thousand times per second to around three hundred billion times per second. This is roughly between the upper limit of audio frequencies and the lower limit of infrared frequencies; these are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves. Different sources specify different upper and lower bounds for the frequency range.

Printed circuit board Board to support and connect electronic components

A printed circuit board (PCB) mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.

PIFAs are a variant of the patch antenna. Many variants of this, and other forms of the inverted-F, exist that implement wideband or multi-band antennae. Techniques include coupled resonators and the addition of slots.

Patch antenna

A patch antenna is a type of radio antenna with a low profile, which can be mounted on a flat surface. It consists of a flat rectangular sheet or "patch" of metal, mounted over a larger sheet of metal called a ground plane. They are the original type of microstrip antenna described by Howell in 1972; the two metal sheets together form a resonant piece of microstrip transmission line with a length of approximately one-half wavelength of the radio waves. The radiation mechanism arises from discontinuities at each truncated edge of the microstrip transmission line. The radiation at the edges causes the antenna to act slightly larger electrically than its physical dimensions, so in order for the antenna to be resonant, a length of microstrip transmission line slightly shorter than one-half the wavelength at the frequency is used. The patch antenna is mainly practical at microwave frequencies, at which wavelengths are short enough that the patches are conveniently small. It is widely used in portable wireless devices because of the ease of fabricating it on printed circuit boards. Multiple patch antennas on the same substrate (see image) called microstrip antennas, can be used to make high gain array antennas, and phased arrays in which the beam can be electronically steered.

Evolution and history

A: quarter-wave monopole, B: intermediate-fed quarter-wave monopole, C: inverted-L antenna, D: inverted-F antenna Inverted-F antenna evolution.svg
A: quarter-wave monopole, B: intermediate-fed quarter-wave monopole, C: inverted-L antenna, D: inverted-F antenna

The inverted-F antenna is an evolution of the basic quarter-wave monopole antenna. The wire F-type antenna was invented in the 1940s. [1] In this antenna the feed is connected to an intermediate point along the length of the antenna instead of to the base. The base is connected to ground. The advantage of doing this is that the input impedance of the antenna is dependent on the distance of the feed point from the grounded end. The portion of the antenna between the feedpoint and the ground plane is essentially behaving as a short-circuit stub. Thus, the designer can match the antenna to the system impedance by setting the position of the feed point (RF systems commonly have a system impedance of 50 Ω whereas a λ/4 monopole has an impedance of 36.5 Ω). [2]

Stub (electronics) short electrical transmission line

In microwave and radio-frequency engineering, a stub or resonant stub is a length of transmission line or waveguide that is connected at one end only. The free end of the stub is either left open-circuit or short-circuited. Neglecting transmission line losses, the input impedance of the stub is purely reactive; either capacitive or inductive, depending on the electrical length of the stub, and on whether it is open or short circuit. Stubs may thus function as capacitors, inductors and resonant circuits at radio frequencies.

The inverted-L antenna is a monopole antenna bent over to run parallel to the ground plane. It has the advantage of compactness and a shorter length than the λ/4 monopole, but the disadvantage of a very low impedance, typically just a few ohms. The inverted-F antenna combines the advantages of both these antennae; it has the compactness of the inverted-L and the impedance matching capability of the F-type. [3]

The inverted-F antenna was first proposed in 1958 by the group at Harvard led by Ronold W. P. King. [4] King's antenna was in wire form and was intended for use in missiles for telemetry. [5]

Ronold Wyeth Percival King was an American applied physicist, known for his contributions to the theory and application of microwave antennas. He published twelve books and over three hundred articles in his area, as well as mentored one hundred doctoral dissertations.

Telemetry data and measurements transferred from a remote location to receiving equipment for monitoring

Telemetry is the collection of measurements or other data at remote or inaccessible points and their automatic transmission to receiving equipment for monitoring. The word is derived from Greek the roots tele, "remote", and metron, "measure". Systems that need external instructions and data to operate require the counterpart of telemetry, telecommand.

Planar implementation

A: printed inverted-F antenna, B: meandered printed inverted-F antenna: C: patch antenna: D: Planar inverted-F antenna (PIFA)
Board without ground plane
Board with ground plane
Antenna feed pin
Ground pin PIFA antennae.png
A: printed inverted-F antenna, B: meandered printed inverted-F antenna: C: patch antenna: D: Planar inverted-F antenna (PIFA)
KeyBoardWithoutGroundPlane.png Board without ground plane KeyBoardWithGroundPlane.png Board with ground plane
KeyFeedPin.png Antenna feed pin KeyGroundPin.png Ground pin

A planar inverted-F antenna (PIFA) is used for wireless circuitry implemented in microstrip. The microstrip format is the format of choice for modern RF electronics. It can be used to implement required distributed-element RF components such as filters, while at the same time being economical because the same mass production methods are used as for printed circuit boards.

A printed inverted-F antenna can be implemented in the classic inverted-F shape, usually to one side of the circuit board where the ground plane has been removed from underneath the antenna. However, another approach is a modified patch antenna, the shorted patch antenna. In this approach, one edge of the patch, or some intermediate point, is grounded with grounding pins or vias through to the ground plane. This works on the same principle as an inverted-F; viewed sideways, the F shape can be seen, it is just that the antenna element is very wide in the horizontal plane. [6] The shorted patch antenna has a wider bandwidth than the thin line type due to the greater radiation area. [7] Like the thin line type, the shorted patch antenna can be printed on the same printed circuit board as the rest of the circuitry. However, they are commonly printed on to their own board, or on to a dielectric fixed to the main board. This is done so that the antenna, can be suspended and effectively be in air dielectric, is a greater distance from the ground plane than it would otherwise be, or the dielectric used is a more suitable material for RF performance. [8]

The term PIFA is reserved by many authors (e.g. Sánchez-Hernández [9] ) for the shorted patch antenna where the antenna element is wide with the ground plane underneath. The thin line type of inverted-F antennae with the ground plane to one side like A and B in the diagram are just called IFA even if they are in planar format. An author may even call an IFA of this type a printed inverted-F antenna but still reserve PIFA for the shorted patch type (e.g. Hall and Wang. [10] )

A common configuration for a shorted patch antenna is to place the shorting pin as close to one corner as possible with the feed pin relatively close to the shorting pin. In this configuration, the resonant frequency is given approximately by,

where
f0 is the resonant frequency
w, b are the width and breadth of the patch
c is the speed of light
εr is the dielectric constant of the substrate.

This formula only holds if the antenna is not affected by nearby dielectrics, such as the casing of the device. [11]

Another variation that may be encountered is the meandered inverted-F antenna (MIFA). Where there is insufficient board space to extend an antenna to the full required length, the antenna may be meandered to reduce its height while retaining its designed electrical length. [12] This can be compared to the spiralling of an antenna as found in the rubber ducky antenna. [13]

Inverted-F antennae have narrow bandwidths. A wider bandwidth can be achieved by lengthening the antenna, which increases its radiation resistance. Another solution is to place two antennae in close proximity. This works because coupled resonators have a bandwidth wider than the bandwidth of either resonator on its own. Most of the techniques for producing multi-band antennae are also effective at broadening bandwidth. [14]

Multi-band antennae

A dual-band printed inverted-F antenna from a PC Card application providing a network interface controller in the 2.4 GHz and 5.2 GHz bands Double F antenna.png
A dual-band printed inverted-F antenna from a PC Card application providing a network interface controller in the 2.4 GHz and 5.2 GHz bands

The need for multi-band antennae arises with mobile devices that need to roam between countries and networks where the frequency bands used can often be different. Perhaps the most conceptually simple design, first reported in 1997, [16] is to nest two PIFA patch antennae one inside the other. Another technique is to insert one or more spur lines into the patch, which has the effect of coupled resonators broadening the band. Other techniques rely on multiple modes being generated, which makes for a more compact design. Examples of this are the C-slot pattern, which is a similar pattern to the interdigital filter, and the tightly meandered pattern shown as, respectively, C and D in the diagram. [17]

Multi-band PIFA designs, A: nested PIFA patch antennae, B: PIFA patch antenna with two spur lines producing a tri-band antenna, C: a similar tri-band antenna with C-slots, D: tightly meandered inverted-F Multi-band PIFAs.png
Multi-band PIFA designs, A: nested PIFA patch antennae, B: PIFA patch antenna with two spur lines producing a tri-band antenna, C: a similar tri-band antenna with C-slots, D: tightly meandered inverted-F

Applications

Inverted-F antennae are widely used in compact hand-held wireless devices where space is at a premium. This includes mobile phones and tablet computers using wireless transmissions such as GSM, Bluetooth, and Wi-Fi. [18] The planar inverted-F antenna is the most frequently used internal antenna in mobile phone designs. [19]

These antennae are also of use for vehicle telematics. Vehicle manufacturers like to use antennae that follow the contours of the vehicle for style and aerodynamic reasons. Multiband PIFAs can be used to combine the antennae feeds for mobile phone, satellite navigation, and car radio. [20]

These antennas have been used for telemetry applications at military test ranges, including those supporting Inter-Range Instrumentation Group standards. [21]

An R-shaped dual-band PIFA has been proposed for use on military vehicles. The bands to be covered are 225 MHz and 450 MHz. These frequencies are in the same ratio as the mobile phone GSM bands at 900 MHz and 1.8 GHz so the design could be used for this application as well if the dimensions were scaled down to suit. [22]

Related Research Articles

Electrical length

In telecommunications and electrical engineering, electrical length refers to the length of an electrical conductor in terms of the phase shift introduced by transmission over that conductor at some frequency.

Fractal antenna an antenna that uses a fractal, self-similar design to maximize the length, or increase the perimeter, of material that can receive or transmit electromagnetic radiation within a given total surface area or volume

A fractal antenna is an antenna that uses a fractal, self-similar design to maximize the effective length, or increase the perimeter, of material that can receive or transmit electromagnetic radiation within a given total surface area or volume.

Ultra high frequency The range 300-3000 MHz of the electromagnetic spectrum

Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter. Radio waves with frequencies above the UHF band fall into the super-high frequency (SHF) or microwave frequency range. Lower frequency signals fall into the VHF or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications.

Whip antenna type of radio antenna

A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. The antenna is designed to be flexible so that it does not break easily, and the name is derived from the whip-like motion that it exhibits when disturbed. Whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer ones, made for mounting on vehicles and structures, are made of a flexible fiberglass rod around a wire core and can be up to 35 ft long. The length of the whip antenna is determined by the wavelength of the radio waves it is used with. The most common type is the quarter-wave whip, which is approximately one-quarter of a wavelength long. Whips are the most common type of monopole antenna, and are used in the higher frequency HF, VHF and UHF radio bands. They are widely used as the antennas for hand-held radios, cordless phones, walkie-talkies, FM radios, boom boxes, and Wi-Fi enabled devices, and are attached to vehicles as the antennas for car radios and two-way radios for wheeled vehicles and for aircraft. Larger versions mounted on roofs and radio masts are used as base station antennas for police, fire, ambulance, taxi, and other vehicle dispatchers.

Microstrip antenna

In telecommunication, a microstrip antenna usually means an antenna fabricated using microstrip techniques on a printed circuit board (PCB). It is a kind of internal antenna. They are mostly used at microwave frequencies. An individual microstrip antenna consists of a patch of metal foil of various shapes on the surface of a PCB, with a metal foil ground plane on the other side of the board. Most microstrip antennas consist of multiple patches in a two-dimensional array. The antenna is usually connected to the transmitter or receiver through foil microstrip transmission lines. The radio frequency current is applied between the antenna and ground plane. Microstrip antennas have become very popular in recent decades due to their thin planar profile which can be incorporated into the surfaces of consumer products, aircraft and missiles; their ease of fabrication using printed circuit techniques; the ease of integrating the antenna on the same board with the rest of the circuit, and the possibility of adding active devices such as microwave integrated circuits to the antenna itself to make active antennas.

The spurline is a type of radio-frequency and microwave distributed element filter with band-stop (notch) characteristics, most commonly used with microstrip transmission lines. Spurlines usually exhibit moderate to narrow-band rejection, at about 10% around the central frequency.

Stripline transverse electromagnetic (TEM) transmission line

Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line.

A dielectric resonator antenna (DRA) is a radio antenna mostly used at microwave frequencies and higher, that consists of a block of ceramic material of various shapes, the dielectric resonator, mounted on a metal surface, a ground plane. Radio waves are introduced into the inside of the resonator material from the transmitter circuit and bounce back and forth between the resonator walls, forming standing waves. The walls of the resonator are partially transparent to radio waves, allowing the radio power to radiate into space.

The folded inverted conformal antenna (FICA) belongs to the microstrip antenna family. The FICA placement on the handset board and its feeding mechanisms are similar to those used currently for the great majority of handsets with internal planar inverted-F antenna (PIFA) components. Whereas multiband PIFAs exhibit two resonant modes, which operate by sharing the same available antenna volume, the FICA structure is synthesised in order to sustain three resonant modes that better reuse the volume. The implementation of volume reuse allows spreading of the reactive electromagnetic energy associated with each resonant mode across the entire antenna volume. This results in FICA modes exhibiting a lower Q factor and a wider fractional bandwidth than the corresponding PIFA modes.

Mohamed Sanad is an Egyptian antenna scientist and professor in the Faculty of Engineering, Cairo University.

A shortwave broadband antenna is a radio antenna that can be used for transmission of any shortwave radio band from among the greater part of the shortwave radio spectrum, without requiring any band-by-band adjustment of the antenna. Generally speaking, there is no difficulty in building an adequate receiving antenna; the challenge is designing an antenna which can be used for transmission without an adjustable impedance matching network.

Distributed-element filter

A distributed-element filter is an electronic filter in which capacitance, inductance, and resistance are not localised in discrete capacitors, inductors, and resistors as they are in conventional filters. Its purpose is to allow a range of signal frequencies to pass, but to block others. Conventional filters are constructed from inductors and capacitors, and the circuits so built are described by the lumped element model, which considers each element to be "lumped together" at one place. That model is conceptually simple, but it becomes increasingly unreliable as the frequency of the signal increases, or equivalently as the wavelength decreases. The distributed-element model applies at all frequencies, and is used in transmission-line theory; many distributed-element components are made of short lengths of transmission line. In the distributed view of circuits, the elements are distributed along the length of conductors and are inextricably mixed together. The filter design is usually concerned only with inductance and capacitance, but because of this mixing of elements they cannot be treated as separate "lumped" capacitors and inductors. There is no precise frequency above which distributed element filters must be used but they are especially associated with the microwave band.

Metamaterial antenna

Metamaterial antennas are a class of antennas which use metamaterials to increase performance of miniaturized antenna systems. Their purpose, as with any electromagnetic antenna, is to launch energy into free space. However, this class of antenna incorporates metamaterials, which are materials engineered with novel, often microscopic, structures to produce unusual physical properties. Antenna designs incorporating metamaterials can step-up the antenna's radiated power.

Planar transmission line Transmission lines with flat ribbon-like conducting or dielectric lines

Planar transmission lines are transmission lines with conductors, or in some cases dielectric (insulating) strips, that are flat, ribbon-shaped lines. They are used to interconnect components on printed circuits and integrated circuits working at microwave frequencies because the planar type fits in well with the manufacturing methods for these components. Transmission lines are more than simply interconnections. With simple interconnections, the propagation of the electromagnetic wave along the wire is fast enough to be considered instantaneous, and the voltages at each end of the wire can be considered identical. If the wire is longer than a large fraction of a wavelength, these assumptions are no longer true and transmission line theory must be used instead. With transmission lines, the geometry of the line is precisely controlled so that its electrical behaviour is highly predictable. At lower frequencies, these considerations are only necessary for the cables connecting different pieces of equipment, but at microwave frequencies the distance at which transmission line theory becomes necessary is measured in millimetres. Hence, transmission lines are needed within circuits.

Air stripline is a form of electrical planar transmission line whereby a conductor in the form of a thin metal strip is suspended between two ground planes. The idea is to make the dielectric essentially air. Mechanical support of the line may be a thin substrate, periodical insulated supports, or the device connectors and other electrical items.

Distributed-element circuit Electrical circuits composed of lengths of transmission lines or other distributed components

Distributed-element circuits are electrical circuits composed of lengths of transmission lines or other distributed components. These circuits perform the same functions as conventional circuits composed of passive components, such as capacitors, inductors, and transformers. They are used mostly at microwave frequencies, where conventional components are difficult to implement.

In radio systems, many different antenna types are used with specialized properties for particular applications. Antennas can be classified in various ways. The list below groups together antennas under common operating principles, following the way antennas are classified in many engineering textbooks.

References

  1. Waterhouse & Novak, p. 19
  2. Hall et al., pp. 197–198
  3. Hall et al., pp. 197–198
    • Yarman, p. 67
  4. King, Harrison & Denton, 1958, 1960)
  5. Petosa, p. 62
    • Prasad & King, pp. 449, 452
  6. Hall et al., pp. 198–199
  7. Yarman, p. 68
  8. Hall et al., pp. 200, 209
  9. Sánchez-Hernández, pp. 16–22
  10. Hall & Wang, p. 96
  11. Hall et al., pp. 199–200
    • Yarman, pp. 68–69
  12. Kervel, pp. 1, 3–4
  13. Cohen, p. 43: "Viewing the rubber duck as a 3-D meander line using a helix, it's easy to see that other attempts at miniaturization are possible".
  14. Hall et al., p. 200
  15. Hall et al., pp. 221–222
    • Kin-Lu et al., pp. 223–225
  16. Liu et al., p. 1451
  17. Hall et al., pp. 203–204
  18. Hall et al., p. 197
  19. Yarman, p. 67
  20. Hall et al., p. 222
  21. Barton, 2017
  22. Ali et al., p. 29

Bibliography