Antenna formed of a strip-like resonance element over a base plate

Technical field

The invention relates to an antenna, formed by a strip-shaped resonance element above a base plate, and to a handheld radio and a transceiver with such an antenna.

State of the art

The devices for wireless communication can be built smaller and smaller. However, it is unsatisfactory if the antenna has to protrude from the housing due to its physical size. This not only limits the design options with regard to the shape of the housing, it also has to ensure that the antenna cannot break off.

In principle, it would therefore be desirable to fully integrate the antenna in the housing. The problem is that this leads to much stronger interactions between the antenna and the other parts of the handset, since the latter are very close to the radiating antenna parts.

In the specific application (e.g. as a cordless telephone under the DECT standard), various technical specifications (bandwidth, radiation characteristics, 50Ω adjustment, etc.) must be observed. The specific device design (geometry) also has a significant influence on the electrical properties of the antenna. It must be possible to take these influences into account when dimensioning and manufacturing an antenna. An antenna is therefore desirable, which can be adapted to the specific circumstances by varying certain parameters.

Presentation of the invention

The object of the invention is to provide an antenna that can be integrated in a handset (handset or handset) due to its small geometric dimensions and electrical tunability. By varying the dimensioning parameters, the resonance frequency and terminating resistance should be able to be adjusted.

The solution to this problem results from the features of claim 1. Essentially, the antenna thus consists of a strip-shaped resonance element and a base plate, the resonance element being connected at one end to the base plate by a short-circuit element inclined to the base plate. The short-circuit element is therefore not perpendicular, but at an acute angle (ie β <90 °) to the base plate. Such an antenna can be easily accommodated in a typically rounded housing.

In order to keep the height of the antenna in the end region of the resonance element as low as possible (e.g. in rounded housings), the angle is chosen to be as small as possible. If the angle is too acute, however, technical difficulties can arise. The angle is therefore advantageously set at 20 ° or more.

For reasons of ease of dimensioning, the resonance element is preferably rectangular, the circumference corresponding to approximately half a wavelength of a resonance oscillation. Depending on the application, the resonance element can also have a different shape. In order to tune the antenna in a specific environment, the circumference can also be selected to be somewhat larger or smaller than half the wavelength.

The short-circuit element is preferably formed in one piece on the end of the resonance element. The antenna is then essentially formed by a metal strip which is connected at one end to the base plate (for example soldered on). The second end of the strip-shaped resonance element is preferably bent towards the base plate and ends at a certain distance from it. An antenna designed in this way can be optimally accommodated in a housing wall of a handheld radio that is curved over the base plate. In this context, arc-shaped resonance elements with constant or continuously changing curvature are of particular interest.

To feed the resonance element, a feed is provided through the base plate at a certain distance from the electrical short circuit between the base plate and the resonance element.

The antenna geometry according to the invention has various parameters which can be varied in order to adapt the terminating resistor and the resonance frequency. This includes the length of the resonance element, the distance between the feed and the electrical short circuit, and also the distance between the upper end of the resonance element and the base plate. The resonance element has e.g. B. a length of about 0.15 to 0.25λ, the free end a distance of about 0.01 to 0.02λ, the feed a distance of about 0.01 to 0.1λ from the short circuit (λ = wavelength of a resonance oscillation).

In the case of a handheld radio according to the invention (e.g. a cordless telephone, which conforms to standards in a frequency range above 1 GHz), the antenna is arranged together with (shielded) electronic circuitry and possibly a supply part (accumulator) on a common circuit board so that when the handheld radio is used, the antenna is outside a handle range and is arranged on a side of the circuit board facing away from the user. However, the antenna is arranged in the housing so that it is not shielded by the hand when using the phone. The electromagnetic radiation is also directed away from the user's head.

Further advantageous embodiments and combinations of features result from the detailed description and the entirety of the claims.

Brief description of the drawings

The drawings used to explain the exemplary embodiments show:<dl id="dl0001"><dt>Fig. 1</dt><dd>A schematic representation of an antenna according to the invention in side view;</dd><dt>2a, b</dt><dd>a schematic representation of an antenna according to the invention with microstrip feed in plan view and in side view;</dd><dt>Fig. 3</dt><dd>a schematic representation of a cordless handset with an antenna according to the invention;</dd><dt>Fig. 4</dt><dd>a schematic representation of a diversity antenna arrangement.</dd></dl>

Ways of Carrying Out the Invention

In Fig. 1, a flat base plate 1 and a resonance element 2 are shown schematically. An elongated, preferably rectangular metal strip serves as the resonance element 2 and is electrically connected on one side to the base plate 1 via a short-circuit element 3.

1, the resonance element 2 is more or less uniformly curved (eg in the manner of a circular arc segment) and has a free end 4. The latter has a certain distance A from the base plate 1. In practice, this is Resonance element 2 z. B. hook-shaped, the short-circuit element 3 rises relatively steeply from the base plate 1, after which the resonance element - after a knee-like bend - runs slightly downward against the base plate 1.

The control takes place through a coaxial connection 6, the inner conductor of which is guided to the resonance element 2 via a pin 5 penetrating the base plate 1. The pin 5 is z. B. connected to the longitudinal edge of the resonance element 2. The inner conductor of the coaxial connector 6 can also serve as pin 5.

The short-circuit element 3 is plate-shaped and is at an acute angle (z. B. β = 30 °) to the base plate 1. If λ denotes the wavelength of the resonance vibration, then the geometric dimensions z. B. be as follows: length of the resonance element L = 0.22λ, distance between pin 5 and short-circuit element 3 D = 0.04λ, height of the pin h = 0.02λ, total height H = 0.033λ and distance of the end 4 from the base plate 1 A = 0.015λ. It goes without saying that the dimensions can vary depending on the circumstances. They are chosen so that the antenna can function in principle in the manner described below.

In principle, the resonance element 2 (conductor strip) is a λ / 4 resonator. Its current maximum is at the point of contact with the base plate. The currents in the conductor strip each flow in the opposite direction to the currents in the adjacent base plate piece. The maximum electric field strength occurs above the slot at the open end 4. The radiation diagram has both vertical and horizontal polarization components. Each polarization component has zeros in some directions, which, however, are covered by the radiation from the other polarization components present there. The radiation maximum is in the direction perpendicular to the base plate 1, but the exact characteristic is influenced by the shape of the base plate 1 and the immediate surroundings (e.g. the electronics or an adjacent housing) of the antenna. Due to the simple resonance structure, the bandwidth is relatively narrow (approx. 5%).

The antenna according to the invention has some changeable geometric parameters which allow the antenna to be optimally tuned even in a complicated environment, so that a bandwidth of 5% is appropriate for many applications. For this simple resonance radiator, the impedance curve in the Smith Chart approximately forms a circle, which can be adjusted by moving the control (pin 5) in the longitudinal direction (L) of the conductor strip (2) so that it affects the real impedance axis at about 50Ω. One possibility of fine tuning on the finished antenna is offered by the approach of the free end 4 to the base plate 1. Opening or closing the distance A changes the adaptation, the resonance frequency and the bandwidth to a small extent.

If the distance D of the pin 5 from the short-circuit element 3 (ie from the electrical short-circuit of the conductor strip with the base plate) is increased, the circle in the Smith chart shifts downward with the resonance frequency unchanged. The range is getting a little bigger.

Conversely, if the feed distance D is reduced, the circle in the Smith chart shifts upwards with the resonance frequency unchanged. The bandwidth is getting a little smaller.

If the distance A of the free end 4 to the base plate 1 is reduced, the resonance frequency drops, the circle in the Smith chart shifts somewhat downward, and the bandwidth becomes significantly smaller.

Conversely, increasing the distance A results in an increase in the resonance frequency. The circle in the Smith chart shifts slightly upwards while increasing the bandwidth.

2a, b shows an antenna arrangement according to the invention with a so-called microstrip feed. The base plate 7 is mounted on a (non-conductive) substrate 8.

A strip line 9 is, for. B. perpendicular to the longitudinal axis of the resonance element 11 on the underside of the substrate to the lower end of a pin 10. The pin 10 goes vertically through the substrate 8 and the base plate 7 upwards to the resonance element 11.

The antenna according to the invention is particularly suitable for use in cordless telephones and very generally for systems with moderate bandwidth requirements (e.g. DECT standard). The small dimensions, in particular the flat design in the area of ​​the longitudinal ends (see reference numerals 3 and 4) of the resonance element 2, allow easy integration into flat, elegant handsets. Due to the high flexibility in the adaptation, the antenna can be adapted to the inevitable interaction with the components located in the immediate vicinity (electronics, shielding housing or accumulator).

A handset is shown schematically in FIG. 3 for illustration. The antenna 12 and (for example shielded) electronics 13 are arranged on a common printed circuit board 14. On one side of the electronics 13 opposite the antenna 12, e.g. B. an accumulator 15 is provided. The whole thing is in a flat and z. B. preferably housed rounded housing 16.

Listening and speaking shell are on the (not visible in Fig. 3) <img file="EP0806810A2_D0001.tif" />the lower side of the housing 16. The grip area 21 for the hand is formed in the area of ​​the electronics 13 and the accumulator 15 on the housing 16. It consists, for example, of suitably shaped depressions for the hand or fingers. when using the telephone, the electronics 13 and the accumulator 15 are in the hand, while the antenna 12 is outside the palm of the hand, so the antenna radiates away from the user's head. This not only minimizes any risk of damage to the user from the electromagnetic radiation, but also an interaction between the antenna 12 and the human body (antenna detuning).

As shown in FIG. 3, the housing can be built very flat even with the curved rear side 17, because the ends of the resonance strip of the antenna pass into the plane of the base plate in a virtually fluent manner.

As can be seen from FIG. 4, the antenna according to the invention can also be used very simply in a diversity arrangement (for example in a base station of a cordless telephone). Two antenna strips 19, 20 are then provided on a base plate 18. They are preferably arranged at right angles to one another with their longitudinal axes. The antenna strips 19, 20 can, for. B. be placed near two different sides of the base plate 18. In this way, space, polarization and angle diversity can be exploited.

In summary, it can be stated that the modest dimensions of the antenna according to the invention make it ideal for use in handsets and base stations in the frequency range above 1 GHz. The large number of adaptation options can be used to avoid lossy adaptation circuits and to integrate the antenna in a densely built-in device. The radiation maximum directed away from the base plate reduces the undesired interaction between the body parts of the user or the objects located nearby and the antenna. The antenna is also suitable for diversity receiver structures.