EP1309033A2

EP1309033A2 - An arrangement for radiating rf signals from a radio transmitter

Info

Publication number: EP1309033A2
Application number: EP02023476A
Authority: EP
Inventors: Zalman Schwartzman; Ovadia Grossman; Gadi Shirazi
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2001-11-01
Filing date: 2002-10-21
Publication date: 2003-05-07
Also published as: GB0126222D0; EP1309033A3; GB2382231B; GB2382231A

Abstract

An arrangement for radiating RF signals from a radio transmitter which comprises an antenna and an electrical isolator connected to the antenna for isolating the antenna from the transmitter, wherein the isolator includes a conducting elongate element along which RF electrical signals are fed in use, and further conductors which are not electrically connected to the elongate element, the further conductors comprising first and second conducting layers arranged with the elongate element extending between them and third and fourth conducting layers arranged with the first and second layers extending between them, the further conductors forming in use an electromagnetic shield to RF electrical signals passing along the elongate element, the antenna being connected to the elongate member of the isolator.

The layers of the isolator may be planar. The total combined effective electrical length of the first and third layers along an axis of the isolator and the total combined effective electrical length of the second and fourth layers along the axis of the isolator may be or approximate to 0.5λ, where λ is the mean wavelength of RF electrical signals to be passed in use along the elongate element. The effective electrical length of each of the first, second, third and fourth layers along the axis may be or approximate to 0.25λ.

Description

Field of the Invention

This invention relates to an arrangement for radiating RF signals from a radio transmitter and in particular to the use in such an arrangement of an electrical isolator device for current suppression. Such a device when used in combination with a radio frequency (RF) radiating devices such as antennae is useful to suppress interference from currents in an associated device.

Background of the Invention

In modern electronics and telecommunications equipment a variety of products are able to send information by radio communications. Such products include a radio transmitter and radio receiver often combined as a transceiver. Components such as antennae are employed in the radio transceivers to send and receive communications signals by RF radiation.

The electronic products often have conducting structures which in use can cause interference to RF signals required to be sent or received. For example, the electronic product may be a laptop computer in which the conducting structure is a metallic chassis usually grounded. It is known in the communications field to employ an isolator in a RF transmitter in conjunction with an antenna to suppress interference from electrical currents from metallic components in any associated device which is in proximity to the antenna. The purpose of such a device is to create a high impedance to interference currents.

Various forms of isolator are known in the prior art to block interfering currents. For example, ferrite beads are known for this purpose. Also sleeve dipoles are known which are designed to provide shielding around a feed to a dipole antenna.

There is a constant drive in the design of modern electronic equipment to make equipment more compact. In consequence, in some applications there is a tight constraint on the space available for an isolator to occupy. Prior art isolators are not suitable for use in such applications.

Summary of the Invention

According to the present invention in a first aspect there is provided an arrangement for radiating RF signals from a radio transmitter which comprises an antenna and an electrical isolator connected to the antenna for isolating the antenna from the transmitter, wherein the isolator includes a conducting elongate element along which RF electrical signals are fed in use, and further conductors which are not electrically connected to the elongate element, the further conductors comprising first and second conducting layers arranged with the elongate element extending between them and third and fourth conducting layers arranged with the first and second layers extending between them, the further conductors forming in use an electromagnetic shield to RF electrical signals passing along the elongate element, the antenna being connected to the elongate member of the isolator.

The elongate element may be a wire or strip extending substantially along an axis of the isolator.

The first and second layers may comprise substantially planar sheets or strips which are respectively in two planes which are preferably parallel with one another and parallel with a plane in which the elongate element is contained. The elongate element is preferably equidistant from the two planes.

The third and fourth layers may comprise substantially planar sheets or strips which are respectively in two further planes which are preferably parallel and with one another and preferably parallel with planes containing the first and second layers. The elongate element is preferably substantially equidistant from the third and fourth layers separated from such layers by a distance greater than that separating the elongate element from the first and second layers.

The first layer may be on the same side of the elongate element as the third layer and the second layer may be on the same side of the elongate element as the fourth layer. In this configuration, the total combined effective electrical length of the first and third layers along the axis of the isolator may be or approximate to 0.5λ, where λ is the mean wavelength of RF electrical signals to be passed in use along the elongate element. Similarly, the total combined effective electrical length of the second and fourth layers along the axis of the isolator may be or approximate to 0.5λ. Preferably the effective electrical length of each of the first, second, third and fourth layers along the axis is or approximates to 0.25λ.

Desirably, the isolator is such that in use the RF signal loss in the isolator is not greater than 0.2dB.

Desirably, two or more, desirably all of, the first, second, third and fourth conducting layers are electrically connected together at an end thereof. In practice, the connection is made at an end thereof toward which RF signals are directed in use for outward transmission. Such an end is usually connected in use to an antenna. The conductors are preferably not connected at their other end which in use is the end nearer a RF transmission signal generator which has produced the RF signals.

Desirably at least one of the conducting layers is connected to ground at the end distant from that at which the layers are connected.

Preferably, the elongate element is relatively narrow, in order to minimise loss of signal in the isolator. Preferably, the width of the first and second layers measured perpendicular to the axis of the isolator in the plane of these layers is between 0.5w and 5w, desirably between w and 3w, where w is the width of the elongate element. Preferably, the impedance of the isolator to RF signals in use is not greater than 50 ohms.

The elongate element and the first, second, third and fourth layers may be separated by layers of insulating material. The elongate element and the first second, third and fourth layers may be formed of the same conducting material. Likewise, the layers of insulating material may be formed of the same insulating material. The conducting and insulating materials employed may be materials known and used in the art. For example, the conducting and insulating materials may be materials as employed in the production of printed circuit boards. The conducting material may for example be a copper based material (e.g. a material containing at least 80% by weight copper) and the insulating material may be a ceramic material or an organic polymeric material optionally containing strengthening fibres or filler to provide a composite, e.g. a glass-fibre reinforced composite. The matrix material of such a composite may for example be an epoxy resin. Insulating materials having higher dielectric constants at operating frequencies are preferred.

The isolator of the arrangement according to the invention may be produced from multiple layers of insulating material on each of which is deposited a film of conducting material. Again, known printed circuit board technology may be employed to produce these multiple layers. The layers may be bonded together with an adhesive, e.g. an organic adhesive, to form a stack.

The isolator may in use connect a signal output conductor of a RF transmission signal generator to the antenna. Beneficially, the antenna may extend along an axis which is perpendicular to the axis of the isolator. For simplicity, compactness and convenience, the antenna may comprise a monopole antenna. However, the antenna may alternatively comprise a dipole.

The isolator of the arrangement according to the invention may be formed integrally as part of a printed circuit board. Alternatively, the isolator may be formed as a separate component. Where used in conjunction with a printed circuit board and formed separately, it may be bonded to the printed circuit board, e.g. at or near an edge thereof.

The isolator of the arrangement according to the present invention is of a new form which can have a compact shape and size beneficially allowing use of the isolator in applications where space is restricted yet still giving efficient suppression of unwanted interfering currents, especially from ground plane conductors in an associated device. In general, an isolator is more lossy as its volume increases so making the isolator more compact allows losses to be minimised. Furthermore, the isolator of the arrangement according to the invention acts as an efficient counterpoise that helps to enhance the gain performance of an antenna connected to the isolator. This beneficially allows a compact monopole antenna to be employed, if desired, having a gain which is close to that of a dipole antenna. This also allows good wideband matching to be achieved in the transmission system. The isolator for use in the arrangement according to the invention can be relatively easy to assemble and can be produced cheaply.

The RF arrangement according to the invention may find use in various applications, especially wideband applications. The arrangement is particularly suitable for use in a radio modem for a PC (personal computer), e.g. produced as a card for insertion in a PC.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Brief description of the accompanying drawings

Figure 1 is perspective view of layers used in construction of an isolator useful in an RF arrangement embodying the invention.

Figure 2 is a partly schematic partly perspective view of an isolator connected to an antenna in an RF arrangement embodying the invention.

Figure 3 is an end view of an arrrangement embodying the invention of an isolator mounted on a printed circuit board and connected to an antenna.

Description of specific embodiments of the invention

Figure 1 illustrates construction of an isolator for use in an RF arrangement embodying the invention. Five rectangular,

flat layers

1,2,3,4 and 5 having the same overall dimensions are obtained each comprising an insulating substrate carrying on its surface a metal film. A metal film M1 is deposited on the upper surface of the layer 1 and covers all of the upper surface of the layer 1. A metal film M2 is deposited on the upper surface of the layer 2 and forms a strip running along the length of the layer 2. The film M2 is deposited symmetrically with respect to a longitudinal central axis A2 of the layer 2. A metal film M3 is deposited on the upper surface of the layer 3 and forms a strip running along the length of the layer 3. The film M3 is deposited symmetrically with respect to a longitudinal central axis A3 of the layer 3. The width of the film M3 is about one third that of the film M2. A metal film M4 is deposited on the upper surface of the layer 4 and forms a strip running along the length of the layer 4. The film M4 is deposited symmetrically with respect to a longitudinal central axis A4 of the layer 4. The width of the film M4 is the same as that of the film M2. The layer 4 also has a further metal film M4A deposited on its underside covering all of the surface area of the underside of the layer 4. A metal film M5 is deposited on the upper surface of the layer 5 and covers all of the upper surface of the layer 5.

The layers 1 to 5 are mutually arranged so that they form a uniform rectangular stack. Metal films M4A and M5 are soldered together to form a metal joint between the two. The metal films M1, M2, M4 and M4A are electrically connected together by soldereing at one end which is their left hand end as shown in Figure 1. The metal film M3 is not electrically connected to any of the other metal films. None of the metal films is electrically connected to any other metal film at the end of the films which is the right hand end as shown in Figure 1. The layers 1 to 5 are bonded together by an organic adhesive, e.g. epoxy resin, applied at parts of their edges at their ends.

Figure 2 shows the completed stack labelled 10 of the layers 1 to 5. Figure 2 shows how the metal films of the layers 1 to 5 in Figure 1 are connected. A conductor 12 is soldered to the film M3. The conductor 12 is connected at its other end to a RF signal transmitter (not shown). The film M3 is soldered at its end distant from the conductor 12 to an electrical connector 14 leading to a monopole antenna 16.

Conductors

18 and 20 are soldered respectively to the metal films M2 and M4 and at their other end (not shown) are grounded. The films M1, M2, M4 and M5 are, as described earlier, connected together (not shown) at their end adjacent to the connector 14. The films M1 and M5 are unconnected at their other end.

In use, RF signals produced by a transmission signal generator (not shown) are delivered along the conductor 12 and the metal film M3 and to the antenna 16 via the connector 14. RF radiation signals are sent by the antenna 16 to a distant receiver (not shown). The metal films M2 and M4 serve as the first and second conducting layers referred to earlier and the metal films M1 and M4A serve as the third and fourth conducting layers referred to earlier.

As shown in Figure 3, the stack 10 is bonded to a board 22 which may be a printed circuit board. In this case, circuits of the RF transmission signal generator (not shown) may be provided on the same board. The bonding of the stack 10 to the board 22 may be by an organic adhesive. The antenna 16 is shown less schematically in Figure 3. It consists of a monopole conducting rod having an axis which is perpendicular to the surface planes of the layers forming the stack 10.

In an alternative embodiment of the invention, the layer 5 shown in Figure 1 may be replaced by a printed circuit board (not shown) containing metallisation in a particular part in place of the metal film M5 in Figure 1. Thus, the isolator is formed directly on a printed circuit board.

Claims

An arrangement for radiating RF signals from a radio transmitter which comprises an antenna (16) and an electrical isolator (10) connected to the antenna for isolating the antenna from the transmitter, wherein the isolator includes a conducting elongate member (M3) along which RF electrical signals are fed in use, and further conductors which are not electrically connected to the elongate element, the further conductors comprising first (M2) and second (M4) conducting layers arranged with the elongate element extending between them and third (M1) and fourth (M5) conducting layers arranged with the first and second layers extending between them, the further conductors forming in use an electromagnetic shield to RF electrical signals passing along the elongate element, the antenna being connected to the elongate member of the isolator.
An arrangement according to claim 1 and wherein the elongate element is a wire or strip extending substantially along an axis of the isolator.
An arrangement according to claim 1 or claim 2 and wherein the first and second layers comprise substantially planar sheets or strips which are respectively in two planes which are parallel with one another and parallel with a plane in which the elongate element is contained.
An arrangement according to claim 3 and wherein the elongate element is substantially equidistant from the two planes of the first and second layers.
An arrangement according to claim 3 or claim 4 and wherein the third and fourth layers comprise substantially planar sheets or strips.
An arrangement according to claim 5 and wherein the third and fourth layers are in two further planes which are parallel with one another and parallel with planes containing the first and second layers.
An arrangement according to claim 6 and wherein the elongate element is substantially equidistant from the third and fourth layers and separated from such layers by a distance greater than that separating the elongate element from the first and second layers.
An arrangement according to any one of the preceding claims and wherein the first layer is on the same side of the elongate element as the third layer and the second layer is on the same side of the elongate element as the fourth layer.
An arrangement according to any one of the preceding claims and wherein the total combined effective electrical length of the first and third layers along an axis of the isolator and the total combined effective electrical length of the second and fourth layers along the axis of the isolator is or approximates to 0.5λ, where λ is the mean wavelength of RF electrical signals to be passed in use along the elongate element.
An arrangement according to claim 9 and wherein the effective electrical length of each of the first, second, third and fourth layers along the axis is or approximates to 0.25λ.
An arrangement according to any one of the preceding claims and wherein the isolator is such that in use the RF signal loss in the isolator is not greater than 0.2dB.
An arrangement according to any one of the preceding claims and wherein two or more of the first, second, third and fourth conducting layers are electrically connected together at an end thereof.
An arrangement according to claim 12 and wherein all of the first, second, third and fourth conducting layers are electrically connected together at an end thereof.
An arrangement according to claim 12 or claim 13 and wherein the connection is made at an end of the isolator which in use is the end toward which RF signals are directed for outward transmission.
An arrangement according to any one of claims 12 to 14 and wherein the conducting layers are not connected at an end of the isolator which in use is the end nearer an RF signal transmission generator which produces RF signals.
An arrangement according to any one of claim 12 to 15 and wherein at least one of the conducting layers is connected to ground at the end distant from that at which the layers are connected.
An arrangement according to any one of the preceding claims and wherein the first and second layers are substantially planar and the width of the first and second layers measured perpendicular to an axis of the isolator in the plane of these layers is between 0.5w and 5w, where w is the width of the elongate element.
An arrangement according to claim 17 and wherein the width of the first and second layers measured perpendicular to an axis of the isolator in the plane of these layers is between w and 3w.
An arrangement according to any one of the preceding claims and wherein the elongate element and the first, second, third and fourth layers are separated by layers of insulating material.
An arrangement according to claim 19 and which comprises multiple layers of insulating material on each of which is deposited a film of conducting material.
An arrangement according to any one of the preceding claims and wherein the isolator is formed integrally as part of a printed circuit board.
An arrangement according to any one of claims 1 to 20 and wherein the isolator comprises a component which is bonded to a printed circuit board.
An arrangement according to any one of the preceding claims and which includes an RF signal transmission generator, the isolator being arranged to connect a signal output conductor of the RF signal transmission generator to the antenna.
An arrangement according to any one of the preceding claims and wherein the antenna extends along an axis which is substantially perpendicular to an axis of the isolator.
An arrangement according to any one of the preceding claims and wherein the antenna comprises a monopole antenna.