An inverted-F antenna is a type of antenna used in wireless communication, mainly at UHF and microwave frequencies. 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, allowing it to be contained within the case of the mobile device, and it can be impedance matched to the feed circuit by the designer, allowing it to radiate power efficiently, without the need for extraneous matching components. 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. 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 antennas. Techniques include coupled resonators and the addition of slots.
Evolution and history
The inverted-F antenna is an evolution of the widely used quarter-wave monopole antenna, which consists of a conductive rod mounted perpendicularly above a conductive ground plane, fed at its base. The wire F-type antenna was invented in the 1940s. In this antenna the feed is connected to an intermediate point along the length of the antenna instead of to the base, and the base of the antenna is connected to the ground plane. 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 feedline impedance by setting the position of the feed point along the antenna element. 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 1 4 λ {\displaystyle {\tfrac {1}{4}}\lambda } monopole, but the disadvantage of a very low impedance, typically just a few ohms if fed at the base, while a base fed 1 4 λ {\displaystyle {\tfrac {1}{4}}\lambda } monopole has an impedance of 36.5 Ω . The inverted-F antenna combines the advantages of both these antennas; it has the compactness of the inverted-L, and the ability to match the impedance of the feed circuit (often 50 Ω on a printed circuit) like the F-type. The inverted-F antenna was first proposed in 1958 by the group at Harvard led by Ronold W. P. King. King's antenna was in wire form and was intended for use in missiles for telemetry.
Planar implementation
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. The shorted patch antenna has a wider bandwidth than the thin line type due to the greater radiation area. 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, which 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. The term PIFA is reserved by many authors (e.g. Sánchez-Hernández) for the shorted patch antenna where the antenna element is wide with the ground plane underneath. The thin line type of inverted-F antennas 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.) 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,
f 0 = c 4 ( w + b ) ε r {\displaystyle f_{0}={\frac {c}{4(w+b){\sqrt {\varepsilon }}_{\mathrm {r} }}}}
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