GB2246023A - Double loop antenna with a reactance element in each loop - Google Patents

Abstract

A double loop antenna suitable for a window of a motor vehicle has two loop conductors 10a, 10b which-are formed on an insulation plane (the window glass of the vehicle). A feed point 12a is provided at a contact portion of the loops 10a, 10b. A reactance element conductor 13a, 13b is coupled to each loop conductor 10a, 10b, so that the antenna impedance at the feed point 12a becomes almost a pure resistance. The antenna has good transmission/reception characteristics in a UHF band for a vehicle telephone. The invention may be applied to multi, semi or polygonal loop antennae. <IMAGE>

Description

DOUBLE LOOP ANTENNA This invention relates to a double loop antenna, and more particularly, to an antenna printed on a dielectric substance such as a window glass of a motor vehicle for use with mobile telephone.

A double loop antenna attached to a glass window of a motor vehicle for use in a UHF band is known. The double loop antenna comprises a pair of semi loop antenna elements with half wave-length conductors branched from a power feed point in both lateral directions adjacent a grounded conductor, their end terminals being grounded and the feed point being unbalanced power fed (Japanese patent laid open application No. 69704/1987 and U.S.

Patent No. 4,721,964).

Fig. 11 of the accompanying drawings shows a prior art double loop antenna 1 which is printed on a rear window glass of a motor vehicle.

The antenna 1, which is a transmission/reception antenna for an automobile telephone, has elements which are printed on a rear window glass 2 together with defogging heater wires 3 and an antenna conductor 4 for radio (FM/AM) reception. The transmission/reception antenna 1 is arranged adjacent the body of the automobile and consists of a pair of semi loop elements la and lb branched laterally from a power feed point 12a. Both end terminals ic and ld of the elements la and ib are grounded so that the feed point 12a is unbalanced power fed from a core conductor of a coaxial feeder cable 7.

Thins transmission/reception antenna 1 has a sufficient gain and an almost non-directional directivity in a UHF band ranging 850 - 950 MHz.

The transmission/reception antenna 1 has small dips in directivity for waves coming from sides of the automobile and a defect'in that gain is slightly lower than a dipole antenna.

OBJECT AND SUMMARY OF THE INVENTION It is an object of this invention to improve these problems and improve directivity and gain of the loop antenna.

According to this invention, there is provided a double loop antenna comprising twn contiguous loop conductors, power feed point means arranged therebetween or at a contact portion:of the loops, and a reactance element conductor coupled to each of the loop conductors.

Impedance at the feed point becomes close to a pure resistance by loading effect of the reactance element.

Matching loss between the antenna and the feeder cable is reduced to increase gain. Phase shift between reception waves on the loops is reduced so that dips in directivity appearing in the direction of arrangement of the loops is compensated to make non-directional characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an illustration of a conductor pattern showing an embodiment of a double loop antenna according to this invention; Fig. 2 is a Smith chart and SWR diagram for illustration of an impedance characteristic of the antenna in Fig. 1; Fig. 3 is a chart for illustration of an impedance characteristic of a prior art double loop antenna for comparison with Fig. 2; Fig. 4 is a chart showing a directivity of the antenna of Fig. 1; Fig. 5 is a chart showing a directivity of the prior art double loop antenna; Fig. 6 is a chart showing gain-frequency characteristics of the antenna in Fig. 1 and the prior art antenna; Figs. 7 - 10 are illustration showing conductors on a window glass of automobile according to modifications of mirror image antenna; and Fis. 11 is an illustration of a conductor pattern including a prior art double loop antenna for an automobile.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Fig. 1 shows a conductor pattern of a double loop antenna according to this invention. This conductive pattern may be formed by printing a conductive material on a surface of a dielectric plate or by arranging a conductive wires in a dielectric body. The loop antenna consists of two contiguous circular loop conductors 10a and 1Ob. A pair of feed terminal portions 12a and 12b are provided at interconnecting portionof the loops with a gap 11 of about 2 mm provided to space the terminals. One (12a, for example) of the terminal portions 12a and 12b is fed with a signal and the other is grounded. Reactance element conductors 13a and 13b are formed in respective loop conductors 10a and 1Ob with intervals by 180 deg. apart from the terminal portions 12a and 12b.Each of the reactance conductors 13a and 13b is formed into a U-shape by projecting a part of respective loop conductors 1Oa and 1Ob in a radial direction. The reactance conductors 13a and 13b cancel a reactance component included in a impedance of the antenna measured at the terminal 12a and 12b to result in almost a pure resistance.

The antenna in Fig. 1 is provided with a radius for each of loop conductors 10a and lOb so as to operate as a one-wavelength double looD antenna in a range of 800 - 900 MHz. At 900 MIIz for the desired greq,uencya onewavelength corresponds to 333.3 mm. The wavelength is short ened due to dielectric constant E: r of the dielectric sub- stance on which the antenna conductor is attached. When a glass ( r = 3.0) is employed as dielectric substance, shortening ratio is:

The shortened wavelength is: = = 333.3 X 0.58 = 192.5 (mm).

Therefore, the radius of the loop conductors i0a and 1Ob is: R = 192.5/2 Tr = 30.6 (mm).

In this embodiment, a conductor width is as large as 4.0 mm.

The Sizes of the reactance conductors 13a and 13b are determined so that the U-shaped channel form having a width of 9.6 mm and depth L minimizes a reactance component measured at the terminals 12a and 12b. Fig. 2 shows a Smith chart and an SWR graph for illustration of an impedance characteristic when L is fixed at 25 mm.

Fig. 3 shows as a reference a Smith chart and a SWR graph for a prior art antenna pattern having no reactance element conductors 12a and 12b.

It is shown by comparison between Figs. 2 and 3 that an antenna impedance near to the characteristic impedance ZO = 50Q (normalized impedance Z/ZO=1.0) is obtained in a band of 820 - 940 MHz by means of additional reactance element conductors 13a and 13b. In a reception band of 850 910 MHz and a transmission band of 910 - 940 MHz for mobile telephone, SWR is less than 1.5, resulting in good matching with the feeder cable in comparison with the prior art (Eig.

3).

Fig. 4 shows the directivity of the antenna conducttor in Fig. 1. Fig. 5 shows the directivity of a prior art antenna conductor having no reactance element conductors 13a and 13b. It appears by comparison between Figs. 4 and 5 that gain of the antenna in the embodiment is improved for waves coming from both sides normal to a traveling direction. In Figs. 4 and 5, a base of O dB in gain corresponds to a dipole antenna and measurement is performed with a vertically polarized wave.

Fig. 6 shows a frequency characteristic (dotted line) of average gain along a horizontal plane (0 - 360 deg.) for the antenna in the embodiment and a frequency characteristic (dashed line) for a prior art antenna pattern having no reactance element conductors 13a and 13b. As shown by the graph, the antenna pattern in the embodiment has an improved average gain by about 2 dB larger than the prior art in a band of 800 - 960 MHz. In Fig. 6, a base of O dB in gain corresponds to a dipole antenna and measurement is performed with a vertically polarized wave.

Gain and directivity of the double loop antenna in the embodiment are improved as shown in Figs. 2, 4 and 6, due to the fact that matching to a feeder cable is improved to reduce the matching loss, and phase shift between received waves on respective loops is reduced.

In the double loop antenna shown in Fig. 1, an upper half and a lower half of the conductor are symmetrical to each other, so that a mirror image antenna may be formed by employing one of halves. For this modification, as shown in Fig. 7, semiloop conductors 10a and 1Ob and Lshaped reactance element conductors 13a and 13b corresponding to the upper half of the antenna of Fig. 1 are arranged on the glass 2 adjacent ; the body 5 which is a grounded conductor. Terminals 14a and 14b are provided at both ends of the reactance element conductors 13a and 13b for grounding the conductors. A core conductor of a coaxial feeder cable 7 is connected to a power feed terminal portion 12a formed at a junction point of the semiloop conductors 10a and 1Ob.Moreover in the embodiment of Fig. 7, heater wires 3 and an antenna conductor 4 for radio reception may be provided on the glass 2 in a similar way as the prior art in Fig. 11.

As shown in Fig. 8, a ground line conductor 15 may be printed on the glass together with the semi loop conductors 1Oa and 1Ob and L-shaped reactance element conductors 13a and 13b. Both end of the L-shaped reactance element conductors 13a and 13b are connected to the ground line 15.

An outer conductor (grounded) of the feeder cable 7 is connected to the line 15.

As shown in Fig. 9, a plurality of semiloop conductors 10a and 10b may be linked with L-shaped reactance element conductors 13a and 13b attached to the ends of the semiloops. As shown in Fig. 10, a line conductor 16 may be inserted between the semiloop conductors 10a and 10b to be spaced laterally. Length of the line conductor 16 may be 1/2 of a wavelength.

A Polygon may be employed for the loop conductors 10a and 10b in stead of a circle or a half circle.

According to this invention, a reactance element conductor is provided to each of loop conductors constituting a double loop antenna to reduce the antenna impedance to a pure resistance. A good gain and non-directivity are obtained with relatively simple and small-sized antenna pattern.

Claims (10)

1. A double loop antenna comprising two contiguous loop conductors, power feed point means therebetween or at a antact portion of the loops, and a reactance element conductor coupled to each of said loop conductors.

2. A double loop antenna according to claim 1, wherein said reactance element conductor comprises a U-shaped folded conductor coupled so as to project outward from said loop conductor to make an impedance of the antenna at the feed point means as large as a pure resistance.

3. A double loop antenna according to claim 1, wherein said antenna is a transmission/reception antenna for a vehicle telephone, is formed on a window glass of the vehicle and each loop has a loop length tuned in a UHF band.

4. An antenna according to claim 1, wherein said loop conductors comprise two contiguous semi loop conductors and a grounded conductor located to close lacking portions of the semiloop conductors, and said reactance element comprises an L-shaped conductor connecting an end of the semi loop conductor to said grounded conductor.

5. An antenna according to claim 4, wherein said semiloop conductor is a half of a circular loop.

6. An antenna according to claim 4, further comprising an unbalanced power-feed coaxial feeder wire, a core conductor of which is connected to said feed point, and an outer conductor of which is grounded.

7. An antenna according to claim 4, wherein said semi loop conductors are formed on a window glass of a vehi cle and said grounded conductor is a body of the vehicle.

8. An antenna according to claim 4, wherein said grounded conductor is a grounded conductive wire arranged on an insulation plane along said semiloop conductors.

9. An antenna according to claim 4, wherein a plurality of semi loop conductors are series connected to both ends of the semiloop conductors, and the ends of the leftmost and right most elements are grounded respectively through Lshaped reactance element conductors.

10. An antenna according to claim 4, wherein a linear conductive wire having a length of about A/2 is provided to connect said semiloop conductors at its ends, said feed point being arranged at an intermediate point of said linear conductive wire.