Directional antenna for radio telephone

0001Particularly in the case of portable radio telephones, electromagnetic radiation from the antenna causes bio- / magnetic interactions with the user of a telephone. In contrast to car telephones, in which the antenna is shielded from the user by the body, the antenna is in the usual portable radio telephones in close proximity to the ear canal and the brain of the user. The electromagnetic radiation causes a resonance of water molecules, so that the affected parts of the body could warm up. This could be harmful to humans.

0002This effect is increasing in the next generation of mobile phones. These telephones operate in a frequency range of more than 5 GHz, in particular 7-8 GHz. At these high frequencies, more biological / magnetic interactions are to be expected.

0003DE-OS 31 18 818 A1 describes a microwave directional antenna with line emitters. that radiates in two directions. The use of waveguide slot antennas is proposed as a line radiator, the beams being focused by dielectric lenses located outside the line radiator. For this purpose, a line source is arranged in the focal line of the dielectric lens. The arrangement results in a narrow beam due to the focusing by the lens. On the other hand, as large a radiation area as possible should be detected in mobile radio telephones, only the head region of the cell phone user being to be excluded.

0004DE-OS 31 35 818 A1 describes a cavity resonator for use both as an antenna and a transmitter with a slotted waveguide. Depending on the frequency, it is recommended to provide a center conductor. The antenna is used as a transmit / receive antenna and is therefore basically suitable for mobile phones.

0005Such a slot antenna is also found in the<img file="EP0814533A2_D0001.tif" />Taschenbuch der Hochfrequenztechnik "by Meinke, Gundlach, edited by K. Lange and K.-H. Löcherer, Volume 2, Springer Verlag Berlin / Heidelberg, 1992, page N17, chapter The directional characteristic of the antenna has a diameter of <maths id="math0001" num=""><math display="inline"><mrow><mtext>D = 6 * λ</mtext></mrow></math><img file="EP0814533A2_D0002.tif" /></maths> a relatively wide spread, the area opposite the slot being field-free.

0006The problem with the slot antennas described is that the diameter and the slot length depend on the wavelength. In the frequency range of 7-8 GHz relevant for microwave radiotelephones, the diameter at which the desired directional characteristic occurs (at<maths id="math0002" num=""><math display="inline"><mrow><mtext>D = 6 * λ</mtext></mrow></math><img file="EP0814533A2_D0003.tif" /></maths>), are in the range of approx. 26 - 22.5 cm. This diameter is clearly too large for mobile phones.

0007In contrast, the opening angle for the mobile radio area should be as large as possible to ensure a good connection that is independent of the position of the user. However, the size must be small enough to meet the ergonomic requirements for portable radio telephones. This is not the case with conventional antenna shapes. Because of the small distance between the antenna of a portable radio telephone and the head of a user, the user must be protected from the influence of the near fields by a directional effect of the antenna.

task

0008The object of the invention was therefore to provide a directional antenna for directional radiation of electromagnetic waves, which protects against biological / magnetic interactions in the potentially endangered head area and ensures reliable operation of the radio telephones. The directional antenna should be compact so that it can be used in portable radio telephones. The directional antenna should therefore have a line resonator that consists of an inner conductor and an outer conductor that surrounds the inner conductor. The directional antenna is fed on one or both ends of the inner conductor.

invention

0009The object is achieved by the directional antenna according to claim 1.

0010It has been shown that a strong directivity, but also a relatively large spread, can be achieved with a dielectric lens. Then there is the effect that the space in the head region of the user offset by 180 ° from the main beam direction is almost field-free. The spreading width can be varied using the optical principle through the lens shape. The focal length of the lens is of particular importance. The directional beam can be expanded if the focal line is not in the center of the cavity.

drawings

0011The invention is illustrated by the following drawings. Show it:<dl id="dl0001" compact="compact"><dt>Figure 1:</dt><dd>a schematic view of the directional antenna in longitudinal section;</dd><dt>Figure 2:</dt><dd>a schematic view of the directional antenna in cross section;</dd><dt>Figure 3:</dt><dd>a representation of the beam path of the directional antenna in cross section;</dd><dt>Figure 4:</dt><dd>a representation of the field strengths in the vicinity of the antenna;</dd><dt>Figure 5:</dt><dd>a representation of the relative electric field strength at a distance of one meter from the antenna.</dd></dl>

Embodiment

0012As shown in the longitudinal section in FIG. 1, the directional antenna 1 essentially consists of a line resonator 2 with an inner conductor 3 and an outer conductor 4. The outer conductor 4 has upper and lower end faces and thus represents a resonator body. The outer conductor 4 is via a certain sector open. It thus allows electromagnetic waves to be emitted in a given direction. A delay lens 5 is introduced into the opening of the outer conductor 4. It causes an additional bundling of the emitted waves. The size can be reduced by filling the outer conductor 4 or resonator with a dielectric 6, for example ceramic or plastic.

0013FIG. 2 shows the preferred embodiment of the directional antenna 1 in a coaxial, round shape. The delay lens 5 has a concave shape on its outer surface. The inner surface of the delay lens is circular, shaped according to the circumference of the outer conductor 4.

0014The beam path of the directional antenna 1 is shown in FIG. 3. While the waves in the interior of the resonator body 4 reflect between the latter and the inner conductor, the waves which radiate onto the opening of the outer conductor 4 are brought into a parallel, directed beam by the delay lens 5. The space behind the resonator body 4 is radiation-free. In the case of a portable radio telephone, the antenna would therefore have to be oriented such that the user's head is behind the resonator body 4 and the beams are directed away from the head by the delay lens 5.

0015The characteristics of the directional antenna 1 can be seen from FIGS. 4 and 5.

0016FIG. 4 shows the field strength in the vicinity of the antenna. Similar to FIG. 1, the antenna is shown in longitudinal section. The field strength is proportional to the length of the arrows. The large arrows show that the field strength is greatest in the area of ​​the delay lens 5. On the right side of the illustration, behind the outer conductor 4, the field strength is negligible.

0017FIG. 5 shows the relative electric field strength at a distance of one meter from the antenna. The resonator height, ie the height of the outer conductor 4, was assumed to be a multiple of half the wavelength of the waves in the dielectric 6 for the frequency range from 7-8 GHz. The resonator diameter, ie the diameter of the outer conductor 4, is also a multiple of half the wavelength of the waves in the dielectric 6. It can be seen that the field strength is relatively large in the clockwise direction from 90 ° to 270 °. On the other side of the antenna, viewed from 90 ° to 270 ° counterclockwise, the field strength is only 0.2 times the field strength of the opposite area (at 0 °). The antenna is therefore suitable for considerably reducing the harmful radiation in the area of ​​the head of a telephone user.