GB2380323A

GB2380323A - Helical antenna having cylindrical capacitive portion

Info

Publication number: GB2380323A
Application number: GB0123502A
Authority: GB
Inventors: Ovadia Grossman; Moshe Ben-Ayun; Mark Rozental
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2001-09-29
Filing date: 2001-09-29
Publication date: 2003-04-02
Anticipated expiration: 2021-09-29
Also published as: DE60211003D1; EP1435125A1; DE60211003T2; GB0123502D0; GB2380323B; WO2003030302A1; ATE324681T1; ES2260461T3; EP1435125B1

Abstract

An antenna for use in a mobile radio communication unit has a helical coil portion 3 located between and electrically connected to two further portions, namely a second portion 5 having a conductive stub for connection to the radio unit, and a third portion 7 comprising a conductive capacitive cylinder or curved plate. The length of the antenna is not greater than 100 mm, preferably not greater than 0.1 g . The antenna has applications in TETRA digital communications. Superior return loss and bandwidth characteristics are claimed.

Description

ANTENNA FOR USE IN RADIO COMMUNICATIONS FIELD OF THE INVENTION The present invention relates to an antenna for use in radio communications particularly for use in a mobile radio communications unit.

BACKGROUND OF THE INVENTION Mobile communications are carried out using mobile radio communications units known in the art as'mobile stations' which include a transmitter to convert messages or information of a user input mainly in the form of speech, but possibly also in the form of text data or visual images etc. , into radio frequency (RF) signals for transmission to a distant receiver, and a receiver to convert received RF signals from a distant transmitter back into information which can be understood by the user. Many components of the transmitter and receiver are common components usually forming a single transceiver unit.

In a mobile station, the function of sending and receiving an RF signal via an air interface to and from a distant transceiver is carried out by a component referred to in the art as an antenna or aerial. In general, an antenna is a device which converts an electrical signal oscillating at RF frequency into a radiated electromagnetic energy signal and vice versa.

In modern mobile communications, such as using digital technology, the RF signals generally have a high frequency, e. g. above 30MHz. For example, for systems operating according to TETRA standard procedures, a specified

operating frequency is in the range from 410MHz to 430MHz, centre frequency 420MHz. TETRA (Terrestrial Trunked Radio) is a set of standards defined by the European Telecommunications Standards Institute (ETSI).

An antenna needs to be designed so that it can transmit and receive accurately and with a strong signal in the designated frequency band. Unfortunately, this is made difficult by factors encountered during manufacture and use of a mobile station for use in modern systems as follows.

Firstly, an allowance of about +/-1% TO 2% of the centre frequency to which the antenna is tuned has to be provided as a manufacturing tolerance for the antenna.

This means that the band of frequencies to which the antenna is tuned can vary up and down the frequency range by the same amount. For the TETRA example given above the variation is +/-3MHz.

Secondly, a loss of signal known in the art as a return loss'or RL occurs in use in an antenna. RL is defined as the ratio of (i) the RF power returned by the antenna to the transmitter to (ii) the incident power from the transmitter. The more power returned the poorer is the tuning and the performance of the antenna. This loss can become greater as the frequency departs from an optimum operating frequency which usually coincides with the centre frequency to which the antenna is tuned, or the centre frequency of the designated frequency band. In general, maintaining an acceptable RL over a reasonable band of frequencies is difficult. For example, for a TETRA mobile station, if the maximum RL allowed is-lOdB, then the typical bandwidth achieved for this specification for an antenna operating in free space is 15MHz to 20MHz. For an RL of 5db, the typical bandwidth is 30 MHz.

Thirdly, an antenna and its associated transceiver circuits can be detuned by being in close proximity a detuning body such as a user. For example, for an antenna of a TETRA mobile station operating in the frequency range given above detuning can be as much as 25MHz from the centre frequency by being next to the head of the user.

Another important feature required of antennae used in mobile stations because of user preference is a short length. Typically, the maximum length desired for short antennae is 100mm, preferably not greater than, desirably less than, 50mm. For TETRA antennae this is equivalent to a typical maximum length of 0. 014A, preferably a maximum length of about 0. 07À, where A is the wavelength at centre frequency. However, in general terms, reducing the length causes a reduction in efficiency and bandwidth of the antenna. In the prior art, antennae which are small, e. g. not greater than 100mm in length, and for operation at high frequencies, e. g. greater than 30MHz, have been in the simple form of an inductive coil above ground plane connected directly to the RF feed line. Such an antenna is used for example in the currently available Motorola dl700 radio for use in mobile communications.

It is clear that in order to cope with the various loss and bandwidth demands of the antenna in a mobile station for use in an advanced system such as TETRA, an antenna is required having a short length and operating at high frequency and having a higher than usual bandwidth, e. g. higher than obtained with currently available antennae for such uses. The purpose of the present invention is to provide a novel antenna for this requirement.

SUMMARY OF THE PRESENT INVENTION According to the present invention in a first aspect there is provided an antenna for use in a radio communication device the antenna having a length of not greater than 100mm, and including a first portion comprising a conductive helical coil, wherein the first portion is located between two further portions, namely a second portion for connection to a conductor of the radio device and comprising a conductive linear stub portion and a third portion which comprises a conductive capacitive portion, wherein the portions mutually are arranged to provide at an operating frequency of the radio device an electrically resonating structure. Such an antenna will be referred to herein as the'novel antenna'.

The said three portions of the novel antenna may have a common axis.

The first portion of the novel antenna may comprise a helical or spiral coil having an axis which is co-incident with a common axis of the antenna. The first portion may have a substantial length along its axis, e. g at least 0.2 L where L is the overall lenghth of the antenna. The first portion may comprise coiled conducting wire.

The second portion of the novel antenna may include as the stub portion an elongate conducting cylindrical portion extending along a common axis of the antenna. The cylindrical portion may be a portion having an enlarged cross sectional area compared with narrower portions at the respective ends of the first portion. The cylindrical portion may comprise a right circular cylinder of enlarged

diameter. The diameter may for example be at least 0. 2L, preferably between 0. 1L and 0.3L, where L is the length of the antenna. The enlarged cylindrical portion and the coil of the first portion may have outside diameters which differ by not more than 0.7D, preferably by not more than 0. 5D, where D is the outside diameter of the coil. The enlarged cylindrical portion of the second portion has a lengthwhich is not greater than 0.6L. The enlarged cylindrical portion may include at its ends narrower conducting protrusions. The protrusions may be linear wire portions. The cylindrical portion of the second portion may have a screw thread for attachment in a conventional manner to a complementary thread provided in a receptacle in the mobile station.

The coil of the first portion may be electrically connected to the second portion at an end of the cylindrical portion, e. g. at a protrusion provided at an end of the cylindrical portion.

The third portion of the novel antenna may comprise a capacitive plate. The plate may be formed by a hollow cylinder having an axis extending along an axis of the antenna. The hollow cylinder may comprise a right circular cylinder. The cylinder may provide a curved plate having a gap between its lateral edges running parallel to a common axis of the antenna. The arc formed in cross section by the curved plate may extend for an angle of at least 270 degrees, e. g. 300 degrees or more, desirably at least 320 degrees.

Where the third portion comprises a hollow cylinder or curved plate, the third portion and the coil of the first portion may have outside diameters which differ by not more

than 0. 5D, preferably by not more than 0. 2D, where D is the outside diameter of the coil.

The coil of the first portion may be elcetrically connected to the third portion at an end of the third portion, e. g. at an end of a hollow cylinder or curved plate thereof.

The third portion may be considered as as another (wider) turn of the coil of the first portion, and preferably is connected to the coil in a manner such that the third portion and the first portion have a common axis and a similar outer envelope whereby the third portion acts as as a continuation of the coil.

The novel antenna desirably has a length of not greater than O. 1A, e. g. between 0. 05A and 0.1#, preferably between 0. 05A and 0. 075A, where A is the mean wavelength of electromagnetic radiation to be transmitted or received by the antenna in operation. The length of the first portion is desirably from 0. 025A to 0. 035A, preferably at least

0. 03A. The length of the second portion is desirably from 0. 025A to 0. 035A, preferably at least 0. 03A. The length of the third portion is desirably from O. 01A to 0. 03A, preferably from 0. 015A to 0. 025A.

In practice, the length of the novel antenna is desirably not greater than 70mm, in many cases about 50mm or less.

The novel antenna is desirably such that the arrangement of the portions gives an improved loaded and unloaded resonance Q factor, e. g. in operation a loaded resonance Q factor of the antenna of 6 or less, preferably 4 or less.

Desirably, the novel antenna has in operation a bandwidth which is at least 12%, preferably at least 15% or

or higher, that of the RF centre frequency radiated or received by the antenna. For an antenna for use in a TETRA mobile station, the bandwidth is desirably at least 50MHz.

In this context,'bandwidth'is here defined as the desired range of operating frequencies of the antenna within which the magnitude of the return loss of the antenna is not more than-5dB. This means that the antenna gain variation within the band is below 1.6db. Beneficially, as exemplified later, the novel antenna provides for a short antenna length of not greater than 100mm a significantly improved bandwidth compared with the prior art. In addition, the average gain (reduction in loss) of the antenna at operating frequencies is also unexpectedly improved.

The novel antenna is suitable for use in a radio transmitter or receiver or transceiver for mobile communications. In principle, there is no restriction on the operational frequency of the communications possible using the antenna, but most beneficial use of the antenna is likely to be found in the operational frequency range 30MHz to IGHz.

According to the present invention in a second aspect there is provided a mobile station which includes the novel antenna.

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 a front elevation partly in schematic form of an antenna embodying the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION As shown in Figure 1, an antenna 1 embodying the invention for use in a mobile station has a longitudinally extending axis 2 and comprises a first conducting portion 3, a second conducting portion 5 and a third conducting portion 7. The first, second and third portions may be made of a copper based material or other efficiently conducting material well known and used in the art. The first portion 5 and the third portion 7 are enclosed in a conventional manner in an insulating case 9, e. g. made of a moulded plastics material. The case 9 is conventional and provides mechanical and environmental protection of the antenna 1.

The first portion 3 has a length of about 0. 38L, the second portion has a length of about 0.36L and the third portion has a length of about 0.26L where L is the overall length of the conducting parts of the antenna 1. Where the antenna 1 is for use in TETRA communications at a centre frequency of 420MHz, the overall length of the conducting parts is about 0. 07A, i. e. about 50mm.

The first portion 3 comprises a coil extending along the axis 2 of the antenna 1.

The second portion 5 comprises a right cylindrical conducting body of enlarged diameter 5b having at its ends protruding wires 5a and 5c, the wire 5c extending into the case 9. The wire 5a is in use connected to feeder transmission line conductor (not shown) via a feeder structure (usually a form of spring-not shown) of a mobile station. The cylindrical body 5b of the second portion 5 may have a screw thread (not shown) for

attachment in a conventional manner to a complementary thread (not shown) provided in the mobile station.

The first portion 3 is connected to the second portion 5 at the wire 5c.

The third portion 7 comprises a curved plate of conducting material extending along the axis 2. The first portion 3 is electrically connected to the third portion 7 at an end of the curved plate thereof. The first portion 3 and the second portion 7 have approximately equal outside diameters. The curved plate forms in cross section in a plane perpendicular to the axis 2 an extended arc of greater than 270 degrees, e. g. greater than 300 degrees.

The curved plate has longitudinally extending edges 7a and 7b facing one another and a gap 7c extending between the edges 7a and 7b. The third portion 7 functions as a capacitive portion.

The purpose of the second portion 5 is as follows.

The second portion 5 provides a connection between the conductive feed line above ground plane (not shown) of the mobile station and the coil of the first portion 3 whereby the first portion 3 is elevated from the ground plane. The portion 5 allows the antenna to have a better and more uniform current distribution on the antenna and its counterpoise (the ground plane) giving an improvement in efficiency or gain of the antenna. The second portion 5 also allows the bandwidth of the antenna to be increased by providing a smaller than usual capacitance to ground.

The purpose of the third portion 7 is to provide a so called'top loading'which mainly allows a reduction in the antenna length to be achieved without a significant change in operating frequency. An antenna

top loading is known per se, e. g. as from US4857939, but has not previously been employed in the modification of high frequency short antennae with which the present invention is concerned. The curved plate of the third portion 7 replaces but performs a similar function to additional winding (s) of the coil of the first portion (in the same direction). The length of the curved plate is chosen to achieve the highest bandwidth for the antenna 1 given the length limit and the limitations for simplicity of production on its mechanical configuration. The gap 7c beneficially increases the capacitance to ground and the inductance of the third portion 7.

It may be noted that the coil windings at the top (end distant from the ground plane) of a conventional short, high frequency antenna are very inefficient in their inductance creating function in view of the very low current at the top. Replacing one or more windings at the top of the antenna by a capacitive portion, namely the third portion 7, in the antenna embodying the invention shown in Figure 1 allows one or more low current windings to be replaced and the antenna length to be reduced without substantially changing the centre frequency. The configuration of the portion 7 shown in Figure 1 provides a field distribution within the case 9 which is different from that from a simple coil and this gives an improvement in antenna losses experienced in the plastics case compared with those obtained with antennae of the prior art.

Functionally, the antenna 1 provides a resonant LCR (inductor/capacitor/resistor) circuit which together with a tuning circuit, which may be a conventional circuit, in the

mobile station (not shown) may be tuned to operate at the required centre frequency of operation of the mobile station. The inductive component L of the antenna resonant circuit is provided mainly by the coil of the first portion 3, the capacitive component is provided mainly by the third portion 7 and the resistive component is provided by all three portions.

In use, the antenna 1 is connected to transceiver circuitry of a mobile station (not shown) usually in the form of one or more printed circuits and/or a metal chassis providing the ground plane described earlier, usually via a feed structure. The transceiver circuit as in conventional circuits may include a circuit for fine tuning of the transmitted and received RF signals as mentioned earlier.

An example of an antenna 1 shown in Figure 1 was produced for use in a TETRA mobile station. The antenna had dimensions of 19mm, 18mm and 13mm respectively for the first portion 3, the second portion 5 and the third portion 7. Measurements were carried out as follows to determine the comparative benefits of this antenna referred to below as the'subject antenna'compared with a reference antenna (a commercially available antenna for the same use at 420MHz consisting essentially of a coil of length 40mm connected directly to feed line.

Firstly, various properties as summarised in Table 1 as follows were measured at a frequency of about 420MHz.

TABLE 1

Antenna type Reference Subject antenna antenna with no fine tuning 3db Return Loss 14. 6% 17.7% bandwidth 3dB in-hand 16. 9% 20.5% bandwidth Peak RL 25 db 14 db

In Table 1,3dB Return Loss (RL) bandwidth is the bandwidth for the antenna operating in air over which the RL is not greater than 3dB. The in-hand bandwidth is the bandwidth of the antenna operating in the hand of a user for which the RL is not greater than 3dB. (When the antenna is in the hand, which can be considered as a lossy material around the antenna, the gain is reduced and the bandwidth increased, as the loss is reflected as extra loss resistance in the antenna impedance.

Table 1 shows that superior bandwidth results and a superior peak RL, giving superior efficiency, were obtained with the subject antenna compared with the reference antenna. Further tuning of the novel antenna by use of a tuning circuit in the mobile station further increased the antenna bandwidth by an additional amount, typically about a further 50% above the figures given in Table 1.

Secondly, various properties of the antenna equivalent circuit were determined at a frequency of 420MHz for the subject antenna and the reference antenna. These properties are shown in Table 2 as follows.

TABLE 2

Property in equivalent circuit Reference antenna Subject antenna Resistance (ohms) 50 30 Capacitance (pF) 0. 64 2 Inductance 600J 200J Average gain -3.2 -1.4 (dBi) Loaded Q factor 8 3

In summary, the results obtained show that the subject antenna gives an improved RL loss, in other words an improved efficiency, and a significantly improved loaded Q factor and bandwidth at the relevant frequency, i. e.

420MHz, compared with the reference antenna.

Claims

CLAIMS 1. An antenna for use in a radio communication device the antenna having a length of not greater than 100mm and including first portion comprising a conductive helical coil, wherein the first coil portion is located between and electically connected to two further portions, namely a second portion for connection to a conductor of the radio device and comprising a conductive linear stub portion and a third portion which comprises a conductive capacitive portion, wherein the portions mutually are arranged to provide at an operating frequency of the radio device an electrically resonating structure
2. An antenna according to claim 1 and wherein the portions of the antenna have a common axis.
3. An antenna according to claim 2 and wherein the first portion comprises a helical or spiral coil portion whose axis is co-incident with the common axis of the antenna.
4. An antenna according to claim 2 or claim 3 and wherein the second portion comprises an elongate cylindrical portion extending along the common axis of the antenna.
5. An antenna according to claim 2, claim 3 or claim 4 and wherein the third portion comprises a hollow cylinder extending along the common axis of the antenna.
6. An antenna according to claim 5 and wherein the coil is connected to an end of the hollow cylinder.
7. An antenna according to claim 5 or claim 6 and wherein the hollow cylinder comprises a curved plate.

<Desc/Clms Page number 15>
8. An antenna according to claim 7 and wherein the curved plate has a gap between its lateral edges running parallel to the common axis.
9. An antenna according to any one of the preceding claims and wherein the antenna has a length not greater than 0. 1A, where A is the mean wavelength of electromagnetic radiation to be transmitted or received by the antenna in operation.
10. An antenna according to claim 9 and wherein the length of the first portion is from 0. 025A to 0. 035A.
11. An antenna according to claim 8 or claim 9 and wherein the length of the second portion is from 0. 025X to 0. 035X.
12. An antenna according to claim 8, claim 9 or claim 10 and wherein the length of the third portion is from 0. 015A to 0. 025A.
13. An antenna according to any one of the preceding claims and wherein the arrangement of the portions is such that in operation the loaded resonance Q factor of the antenna is 6 or less.
14. An antenna according to claim 13 and wherein the arrangement of the portions is such that in operation the loaded resonance Q factor of the antenna is 4 or less.
15. An antenna according to any one of the preceding claims and wherein the antenna has in operation a bandwidth which is at least 12% that of the centre frequency radiated or received by the antenna, wherein the bandwidth is defined as the desired range of operating frequencies of the antenna within which the magnitude of the return loss is less than 5dB.
16. An antenna according to claim 15 and wherein the bandwidth is at least 12 per cent that of the centre frequency.

<Desc/Clms Page number 16>
17. An antenna substantially as herein described with reference to the accompanying drawing.
18. A mobile station for use in mobile radio communications which includes an antenna according to any one of the preceding claims.