The present invention relates to an antenna coupling device for coupling radio frequency signals from a communication device having an internal first antenna.
Some older types of mobile telephones have been equipped with a coaxial connector to which a conductor to a second antenna can be attached, simultaneously disconnecting the first antenna of the telephone. However, the trend towards smaller, lighter and cheaper mobile telephones has led to new models which do not offer this facility. If connection to a second antenna is desired, an electromagnetic coupler must be used, though this solution results in inevitable losses. For the first, the couplers work in the near field of the first antenna impairing the drift of the telephone which may cause losses. For the second, part of the electromagnetic energy cannot be picked up by the coupler, this results in radiation inside the car.
Different models of couplers are needed to fit different types of telephones depending on the first antenna. A complication is that operation at two frequency bands is required.
Most of the telephones from the last decade and some new ones are equipped with short top loaded monopole antennas or short helix antennas protruding from the top of the mobile telephone device. Couplers to such antennas have been described in several patents, e.g., in SE 500 983, SE 503 930, U.S. Pat. No. 5,619,213, JP 82 79 712, SE 504 343, U.S. Pat. No. 5,668,561 and WO 98/25323. A common feature of these solutions is that they use coils. The electromagnetic coupling relies mainly upon the magnetic component of the near field. A different solution involving a meander pattern has been presented in SE 506 726 and SE 507 100. The electromagnetic coupling depends in this case as well upon the electric as the magnetic component of the field.
Recently many mobile telephones have been equipped with internal antennas. A common type is the slot antenna and especially popular is the planar inverted F (PIFA) antenna. The near field patterns of such antennas vary to a greater extent than those of monopoles and helices. Consequently, couplers have to be individually designed for each type of mobile telephone with internal antenna. A coupler well suited for some n-band (n greater than 1) internal PIFA antennas, making use mainly of the electric component of the near field, has been presented in SE 0002575-9. One disadvantage with this coupler is that the n frequency bands are not independent of each other, due to the fat that the coupler only has one branch.
The document EP 0 999 607 discloses an antenna coupler comprising a planar conductive antenna element, which is essentially similar to the planar conductive antenna element in the mobile telephone. Additionally the antenna coupler comprises a piece of dielectric material for holding the conductive antenna element, and a first ground plane which is conductive, essentially continuous and essentially parallel to the conductive antenna element. This antenna coupler is intended to be tilted in relation to the antenna element in the mobile telephone with an angle, xcex1. One disadvantage with this solution is that it implies a great distance between the coupler and the antenna element. This fact reduces the coupling factor. another disadvantage is that this solution takes up too much space.
It is an object of the present invention to solve the above mentioned problems.
According to the present invention there is provided an antenna coupling device for coupling radio frequency signals from a communication device having an internal first antenna. The communication device is operable in n frequency bands, where n greater than 1 and n is an integer. The antenna coupling device comprises a port connected/connectable to a transmission line. A conductive surface of said antenna coupling device has a geometric shape in the form of a tree structure connected to said port. The tree structure comprises a number, m, of branches, where mxe2x89xa7n. The tree structure comprises at least one branch bix for each frequency band i of said communication device, wherein i is an integer and 1xe2x89xa6ixe2x89xa6n, and x is an integer and 1xe2x89xa6xxe2x89xa6k(i), and the total number, m, of branches satisfy the following expression             ∑              i        =        1            n        ⁢          k      ⁡              (        i        )              =  m
wherein k(i) is a function of i, which only can obtain an integer value and is the total number of branches for a frequency band i.
A main advantage with this antenna coupling device is that it is capable of operating in n independent frequency bands. This facilitates the work when designing an antenna coupling device.
A further advantage in this context is achieved if at least one branch bix for each frequency band i fulfils the condition; a length of said branch bix, as measured form said port to a free end of said branch bix is not less than about xe2x85x9 of xcexi, where xcexi is the wavelength in the medium at the frequency band i.
Furthermore, it is and advantage in this context if said at least one branch bix for said frequency band i of said communication device is/are placed, when said antenna coupling device is in operation, above a domain i of said internal first antenna, wherein a current causing electromagnetic fields in said at least one branch bix is intended to pick up a considerable part of an electromagnetic wave in said frequency band i.
A further advantage in this context is achieved if said domains are at least in part disjoint.
Furthermore, it is an advantage in this context if each branch bix has a constant width.
A further advantage in this context is achieved if the widths of at least two branches bix are equal.
Furthermore, it is an advantage in this context if at least one of said branches bix has a variable width along said branch bix.
A further advantage in this context is achieved if at least one of said branches bix has a part in the form of a meander line.
Furthermore, it is an advantage in this context if different branches bix can intersect each other.
A further advantage in this context is achieved if further branches can be used to improve matching of impedance to a characteristic impedance of said transmission line.
Furthermore, according to one embodiment it is an advantage in this context if said antenna coupling device has an open ground plane.
A further advantage in this context according to another embodiment is achieved if said antenna coupling device has a closed ground plane.
Furthermore, according to one embodiment it is an advantage in this context if said tree structure of said antenna coupling device is placed on a printed circuit board.
A further advantage in this context according to another embodiment is achieved if said tree structure of said antenna coupling device is in the form of plating.
Furthermore, according to one embodiment it is an advantage in this context if said tree structure of said antenna coupling device is in the form of conducting ink.
It should be emphasised that the term xe2x80x9ccomprises/comprisingxe2x80x9d when used in this specification is taken to specify the presence of stated features, steps or components but does not preclude the presence of one or more other features, integers, steps, components or groups thereof.
Embodiments of the invention will now be described with a reference to the accompanying drawings, in which: