GB2422959A

GB2422959A - A method of variable tuning for a loop antenna

Info

Publication number: GB2422959A
Application number: GB0502406A
Authority: GB
Inventors: Phillip James Forshaw
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-02-07
Filing date: 2005-02-07
Publication date: 2006-08-09
Also published as: GB0502406D0

Abstract

This invention is a method of tuning a small loop antenna, (sometimes known as a magnetic loop antenna). For this type of antenna, the capacitance 2 is normally varied for tuning. This invention varies the inductance of the antenna to achieve tuning. An additional conductor 3 (tuning inductor) is placed in the vicinity of the antenna. This tuning inductor can be in the form of a coil or other shape that forms a closed circuit. The variation of inductance is due to the shielding effect of an electrical closed circuit, which constricts the area that the lines of magnetic flux can easily pass through. By moving the tuning inductor relative to the loop, the inductance and hence resonant frequency can be varied. Altering the size of the tuning inductor also tunes the antenna.

Description

1 2422959

A METHOD OF VARIABLE TUNING FOR A LOOP ANTENNA

This invention is a method of variable tuning for an existing type of small loop antenna (sometimes referred to as a magnetic loop antenna). Such an antenna may be used for the reception or transmission of radio signals. This type of antenna typically consists of a looped conductor a capacitor and a means of matching to a transmission line feeder. The length of the looped conductor is a small fraction typically 10 to 30 percent of a wavelength at the frequency of operation and hence forms a very compact antenna compared to a half wave dipole. The conductor is usually formed into a loop of one or more turns the ends of which are connected to a capacitor. The shape of the loop may vary and typical shapes are circular or square or octagonal. The looped conductor exhibits inductance that together with the capacitor forms a circuit that is resonant at a particular frequency. The looped conductor is henceforth referred to as the main loop.

The bandwidth over which the antenna is usable is quite narrow and effectively limits the use of the antenna to a spot frequency or a small band of frequencies close to the resonant frequency unless there is a means of varying the resonant frequency (i.e. tuning). For this type of antenna a variable capacitor is normally used to vary the resonant frequency. When a loop antenna is used for transmission such a capacitor needs to be capable of handling high voltage and high current. A balanced form of construction is also preferable. A butterfly' variable capacitor with wide-spaced vanes is therefore commonly used. Vacuum variable capacitors are also sometimes used in this application. Capacitors such as these are specialised items and are expensive to construct.

This invention varies the inductance of the antenna to achieve tuning. This variation of inductance is due to an additional conductor (henceforth referred to as a tuning inductor) that is placed in the vicinity of the antenna. This permits the use of a fixed value capacitor of simple and inexpensive construction in a tunable loop. The looped conductor consists of a coil of one or more turns in the form of a circle or square or octagon or other shape. This tuning inductor can be in the form of a loop of one or more turns in the form of a circle or square or octagon or other shape or in the form of one or more flat plates or other shapes which form an electrical closed circuit. The variation of inductance is due to the shielding effect of an electrical closed circuit that constricts the area that the lines of magnetic flux can easily pass through. This effectively alters the permeability of the magnetic circuit and hence varies the inductance. Hence, this is a form of permeability tuning. By moving the tuning inductor relative to the main loop the inductance can thus be varied. Changing the size of the tuning inductor also varies the inductance of the antenna and achieves tuning.

For example, one method of moving a tuning inductor consisting of a oneturn coil is to rotate it around an axis that is in the plane of the main loop. For this method when the tuning inductor is at right angles to the plane of the main loop its effect will be at a minimum and the antenna will be tuned to its lowest frequency. When in the same plane as the main loop the effect will be at a maximum and the antenna will be tuned to its highest frequency. Moving the tuning inductor between these positions varies the tuning of the antenna smoothly between the lowest and highest frequencies. Other methods of moving the position of the tuning inductor relative to the main loop may be used.

The tuning inductor may also be used in a loop antenna having a variable capacitor and either or both may be used to tune the antenna.

The tuned loop using the tuning inductor can be scaled for use at different frequencies. For use at high frequency and very high frequency use the conductors of the main loop and tuning inductor may typically be constructed from non-magnetic metal tubing. For use at ultra high frequency and microwaves other forms of construction may be used for the main loop and tuning inductor such as wire or metal foil on a printed circuit board or metal film on a substrate.

There are several well-established methods of matching loop antennas to a feeder.

Using a previously known method of matching such as a gamma match is recommended as it can be configured to leave the inner area of the main loop clear of obstruction. The inner area of the main loop may then be used as a location for the tuning inductor.

An array of two or more similar loop antennas using tuning inductors can be used in order to obtain greater gain or directivity than for a single antenna.

A loop antenna using a tuning inductor can be used in conjunction with conducting rods that modify the radiation pattern according to the previously known science of parasitic arrays.

A loop antenna using a tuning inductor can be used in conjunction with conducting sheet or mesh held in position to enhance or reduce radiation in a preferred direction.

A loop antenna using a tuning inductor can be used in conjunction with a conducting parabolic dish held in position to enhance radiation in a preferred direction.

A loop antenna using a tuning inductor can be affixed to items and used as part of a radio frequency identification or security system.

Supports for the components of the loop antenna and tuning inductor shall preferably be of non-conducting materials. If conducting materials are used as supports, it is recommended that care is taken in the design of the supports so they do not adversely affect the operation of the antenna.

This invention will now be described and illustrated with reference to the accompanying drawings, showing embodiments by way of example only.

FIGURE 1 shows the essential elements of this invention.

FIGURE 2 shows a recommended method of connecting and matching a feeder.

FIGURE 3 shows a first constructional embodiment of this invention.

FIGURE 4 shows a second constructional embodiment of this invention.

For clarity, supports for the main loop and tuning inductor have not been shown in Figures 1, 2 and 4.

FIGURES 1 and 2 show a tunable loop antenna using a tuning inductor. Item I is the main loop. Item 2 is the capacitor. Item 3 is the tuning inductor. Item 4 is a gamma' match, which connects and matches the antenna to a feeder FIGURE 3 shows a first constructional embodiment of this invention Item I is the main loop. Item 2 is the capacitor. Item 3 is the tuning inductor, shown able to rotate about a vertical axis. Item 4 is a gamma match to which a feeder may be connected.

Item 5 is a support which holds the main loop in place and incorporates a shaft and bearing which holds the Tuning Inductor and permits it to rotate The Tuning Inductor may be rotated manually or by a motor, allowing remote tuning.

FIGURE 4 shows a second constructional embodiment of this invention as an example of how a tunable loop antenna using a tuning inductor and gamma' match may be constructed from metal foil on a printed circuit board or from metal film on a substrate. This form of construction is suitable for scaling down to a small size for use at microwave frequencies. In the case shown, the variation in size of the tuning inductor is achieved by using a link (Item 6) on one of the open circuit loops to form a closed loop and hence selecting the size of the tuning inductor from a number of pre- set options. Alternatively, a moving tuning inductor, as previously described, could be used to provide continuous tuning. Two closely spaced conductors form the capacitor, Item 2.

An antenna as described in Figure 4 would be suitable for mass production using printed circuit techniques or by using thin films of metal deposited or etched on a suitable substrate. Where the tuned loop antenna using the tuning inductor is integrated into a more complex system, the narrow bandwidth of the antenna may be used to advantage when designing associated circuitry. In particular, it may be possible to reduce the component count for filters in the receive or transmit circuitry as the antenna itself acts as a filter.

Claims

1. This invention isa method of tuning a loop antenna consisting of a looped conductor and a fixed value capacitor by varying the inductance of the antenna using an additional conductor that is located in the vicinity of the antenna.

2. An antenna according to Claim I in which the additional conductor is referred to as a tuning inductor.

3. An antenna according to Claim I in which the looped conductor consists of a coil of one or more turns in the form of a circle or square or octagon or other shape.

4. An antenna according to Claim 1 and Claim 2 in which the tuning inductor is in the form of a coil with one or more turns in the form of a circle or square or octagon or other shape.

5. An antenna according to Claim 1 and Claim 2 in which the tuning inductor is in the form of one or more flat plates or other shapes which form an electrical closed circuit.

6. For an antenna according to Claim 1 and Claim 2 altering the spacing or orientation of the tuning inductor or its size relative to the ioop antenna varies the tuning of the loop antenna.

7. An antenna according to Claims 1, 2, 3, 4, 5 and 6 except where the capacitor is a variable capacitor and either the capacitor or tuning inductor or both may be used to vary the tuning of the loop antenna.

8. An antenna according to Claims 1, 2, 3, 4, 5, 6 and 7 can be connected to a feeder by means of a gamma match or any other suitable method.

9. An antenna according to Claims 1, 2, 3, 4, 5, 6 and 7 used in an array of two or more similar antennas in order to obtain greater gain or directivity than for a single antenna.

10. An antenna according to Claims 1, 2, 3, 4, 5, 6 and 7 used in conjunction with conducting rods that modif,' the radiation pattern according to the previously known science of parasitic arrays.

11. An antenna according to Claims 1, 2, 3, 4, 5, 6 and 7 used in conjunction with conducting sheet or mesh held in position to enhance or reduce radiation in a preferred direction.

12. An antenna according to Claims 1, 2, 3, 4, 5, 6 and 7 used in conjunction with a conducting parabolic dish held in position to enhance radiation in a preferred direction.

13. An antenna according to Claims 1, 2, 3, 4, 5, 6 and 7 affixed to items and used as part of a radio frequency identification or security system.

14. An antenna substantially as herein described and illustrated in the accompanying drawings.