GB2235337A

GB2235337A - Loop antenna

Info

Publication number: GB2235337A
Application number: GB9018000A
Authority: GB
Inventors: Edward Charles Forster
Original assignee: PHASE TRACK Ltd
Current assignee: PHASE TRACK Ltd
Priority date: 1989-08-24
Filing date: 1990-08-16
Publication date: 1991-02-27
Anticipated expiration: 2010-08-16
Also published as: DE4026852A1; GB8919283D0; GB2235337B; GB9018000D0

Abstract

A non-resonant loop antenna comprises a plurality of smaller loops LP2, LP3, LP4 connected in parallel and in phase thereby reducing the effective overall loop inductance. <IMAGE>

Description

1 1 LOOP ANTENNA This invention relates to loop antennas.

Fig. 1 shows a simple loop antenna LP1 which when placed in an alternating magnetic f ield develops a voltage e across its terminals. Fig. 2 shows the equivalent circuit of Fig. 1 and it consists of voltage generator GI in series with inductance Ll. Generator G1 delivers a voltage e but loop inductance L1 limits the current which may be delivered to a load 10 if applied directly to the terminals. Where this antenna is used for sensitive receiving purposes it is found that the diameter of the loop must be quite large but the inductance increases with the area enclosed by the loop and in practice efficiency in is delivering signal directly to a load is very low.

Fig. 3 shows how thesimple loop may be combined with an active device to provide amplification. The circuit is schematic and omits the d.c. supply and biasing components for the device. The loop comprising voltage generator G1 and series inductance L1 delivers a current i to the device, in this case transistor Q1 which is connected in common base mode with a load impedance of Ri. The input impedance of the transistor Q1 can be considered near zero for practical purposes and, assuming a current gain of unity, a current of i will also flow through Ri.

1 X 2 However, in order to have sufficient voltage e a large size loop must be used which means that the inductance Ll becomes too high to allow the circuit to produce any useful gain. In general the circuit is of limited 5 practical use.

Fig. 4, which is also schematic, shows how two loops comprising voltage generators G2 and G3 together with series inductances L2 and L3 can be used to drive two common base amplifiers in push-pull configuration and formed by transistors Q2 and Q3 with the output combined in transformer TI delivering current to the load R2. This has the well known property of reducing second order intermodulation products developed by non-linearities inherent in the active devices. The two loops comprising G2/L2 and G3/L3 are f ormed by a two turn coil centre tapped and of the same diameter as considered previously for LP1. Again this is of limited practical use owing to inefficiency. 20 The present invention provides a non-resonant loop antenna comprising a plurality of smaller loops connected in parallel and in phase thereby reducing the effective overall loop inductance. Preferred aspects of the invention are set out in accompanying claims 2 to 12.

51 The invention provides a means of reducing loop t, 3 series inductance, and thus enables a greater power output.

Arrangements embodying the invention will now be described by way of example with reference to the 5 accompanying drawings, in which:

Fig. 1 shows a simple loop antenna of conventional design; Fig. 2 shows the equivalent circuit of the loop antenna of Fig. 1; is a schematic circuit diagram of active antenna system; schematic circuit diagram of an Fig 3 conventional Fig. 4 is a alternative conventional active antenna system; Fig. 5 shows a loop antenna according to the present invention; Fig. 6 shows another. loop antenna according to the invention; Fig. 7 shows the mechanical structure of a further loop antenna according to the invention; and Fig. 8 shows a still further antenna according to the invention.

Fig. 5 shows an antenna according to the present invention, wherein a single loop is split into smaller loops LP2, LP3 and LP4 enclosing the same total area as a corresponding single loop. Since no common path Cxists for magnetic f lux lines to link any of the i 4 loops, they are very well isolated f rom each other. The output voltage is developed across points a and b of each loop and in the same phase. Thus all points a may be connected together and all points b. The inductance of each loop is 1/n times the inductance of a single loop enclosing the total area enclosed by the three loops LP2, LP3 and LP4 but when these individual loops are connected in parallel the effective inductance falls by 1/n times again. Thus the series inductance of the combination f alls by 1/n2 but the voltage f alls only by 1/n. In this case of three equal sized loops, n equals 3. It can be seen that a considerable advantage is achieved by this method.

The maximum size of the loop antenna system is limited by self resonance of individual loops due to self capacitance. It ip also desirable that the diameter of the whole system does not become a significant fraction of a wavelength (preferably not substantially more than a tenth) which would then affect the phase of the component voltages.

The antenna may be coupled to an active element, such as a common base amplifier as shown in Fig. 3, to produce a practical and efficient antenna system.

In its basic form the loop system with one common base amplifier is however unbalanced about ground and this result:s in both electric and magnetic signal 1 1P components being received. This means that the broadside nulls of the ideal magnetic loop antenna are not achieved and that the antenna is also susceptible to electrostatic coupling with local interference sources.

By arranging two segmented loops antennas LAI and LA2 schematically shown in Fig. 6 coaxially and in close proximity it is possible to feed a pushpull amplifier of the type shown in Fig. 4. This balanced loop antenna system has all the advantage of a pure magnetic loop antenna and push-pull circuitry for the active devices. In Fig. 6, each of the antennas LAI and LA2 is similar to that of Fig. 5, except that it has 4 smaller loops instead of 3. The number may be varied.

As a further alternative, a single segmented loop antenna such as that of Fig. 5 may be coupled to a push-pull amplifier; in this case, preferably no part of the loop would be earthed.

Fig. 7 illustrates an active loop antenna 70 in accordance with the present invention. The antenna 70 has an overall width of I metre, is suitable f or 50 KHz to 30 MHz signals, is mounted on a tripod 72, and has an amplifier 74 adjacent the terminals of the individual loops. The amplifier 74 is connected to a plower supply and line feed circuit 76.

6 The mechanical arrangement of the loops allows for much simpler and lighter construction than that employed in conventional single loop systems. It also allows portable antennas, collapsible in umbrella fashion, to be designed.

In the embodiment of Figure 8, the overall shape of the antenna 80 is triangular. There are six smaller, triangular loops. All the loops have terminals located at the mid-point 82 of the base of the.large triangle.

It is preferred that the terminals of the individual smaller loops be located at a common point, as in the above embodiments, to avoid the need for carefully-designed connection leads for maintaining the in-phase relationships of the signals. It may also be preferred for this connection point to be centrally located and surrounded by the smaller loops, as in the embodiments of Figs. 5 to 7.

An individual loop may be provided with a ferromagnetic or ferrimagnetic core. Also, although the individual loops in the above embodiments each include only a single turn, any loop may comprise more than one turn.

The above embodiments use the preferred arrangement whereby there is a plurality of individual Toops in a common plane. It would alternatively be 7 possible to have other arrangements such as a single individual loop in each of a plurality of separate, preferably parallel, planes. This however increases the risk of mutual coupling. In any of the arrangements of the present invention, the loops are preferably arranged so that the mutual coupling therebetween is no greater than 0.5, and preferably no greater than 0.1.

9 1 8

Claims

CLAIMS:

1. A non-resonant loop antenna comprising a plurality of individual loops disposed substantially in the same plane and connected in parallel and in phase.

2. A loop antenna as claimed in claim 1, including a common connection point at or adjacent which the terminals of the loops are all located.

3. A loop antenna as claimed in claim 2, wherein the loops substantially surround said connection point.

4. A loop antenna as claimed in any preceding claim, wherein the individual loops are of equal size.

A loop antenna as claimed in any preceding is claim, wherein the antenna has an overall loop area which is substantially filled by the areas of the individual loops.

6. A loop antenna as claimed in any preceding claim, including a further plurality of individual lloops connected in parallel and in phase, and 9 disposed in a common plane separate from but parallel and in close proximity to the plane of said f irst mentioned plurality of individual loops.

7. A loop antenna as claimed in any preceding claim, wherein the overall diameter of the area occupied by the individual loops is small compared with the wavelength of the signals to be received thereby.

8. A loop antenna as claimed in any preceding claim constructed in an umbrella fashion to be mechanically collapsible.

9. A loop antenna as claimed in any preceding claim wherein the prinQipal mechanical structure consists of radial arms.

10. An active loop antenna system comprising a loop antenna as claimed in any preceding claim having output terminals arranged to deliver signals to an active device.

11. A system as claimed in claim 10, wherein the active device forms part of a common base amplifier.

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12. A system as claimed in claim 10, including amplifiers connected to said terminals and arranged in push-pull configuration, so as to form a substantially balanced antenna with respect to ground.

13. A non-resonant loop antenna comprising a plurality of individual loops not having any substantial mutual coupling and connected in parallel and in phase.

14. A loop antenna substantially as herein described with reference to any one of Figs. 5 to 8 of the accompanying drawings.

Publish d 1991 atIbe Patent 0111ce. State House. 66/71 High Holbom. LondonWClR41?. Further copies m2ky be obtained from Sales Branch. Unit 6. Nine Mile PoinL Cwmfelinfach, Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray, Kent.