Wireless mail delivery systems are known that feature a peripheral unit and a central unit, wherein the peripheral unit has eg a monopole antenna that is to be mounted relatively freely, thereby enabling the emitted signals to unimpededly reach the receiver in the central unit. These prior art mail delivery systems suffer from the drawback that the freely mounted antenna is vulnerable. Thus, it would be expedient to arrange the antenna in close connection with the surface of the mailbox and thereby reduce the vulnerability of the mailbox. This will have the intended effect in connection with a mailbox of non-conductive material, such as plastics or wood, but in connection with a mailbox of a conductive material, such as metal, the antenna will be influenced to a substantial degree, and the system will not work as intended.

It is an object of the invention to provide a wireless mail delivery system, wherein the peripheral unit is compact and can be arranged in close connection with a surface of a mailbox and it will operate essentially independently of the material the mailbox is made of, ie independently of whether the mailbox is made of a conductive or a non-conductive material, eg metal, plastics or wood.

This object is accomplished by a mail delivery system according to the invention, wherein the peripheral unit has an antenna comprising a U- shaped, electrically conductive antenna element having a first branch and a second branch, and with a space between the first branch and the second branch, and of which the first branch is plate-shaped whereby it constitutes an electrical neutral plane for the second branch, and such that the antenna has a first resonance frequency; and wherein, by arrangement of the peripheral unit on an electrically conductive face, the antenna has another resonance frequency, where the difference between the first and the second resonance frequency is smaller than or equal to the bandwidth of the antenna; and the electrical radio-frequency signal has a frequency between the first and the second resonance frequency.

Such peripheral unit can be arranged with the plate-shaped branch of the antenna facing towards the mailbox and in quite close abutment thereon, and irrespective of whether the mailbox is of a conductive or a non-conductive material, it will only insignificantly influence the transmission properties of the antenna.

In the following, a preferred embodiment of the mail delivery system according to the invention will be described with reference to the drawing, wherein: Figure 1 is a schematic view of a mail delivery system according to the invention; Figure 2 is a longitudinal sectional view through a preferred embodiment of a peripheral unit of the mail delivery system according to the invention; and Figure 3 is a plan view of the peripheral unit shown in Figure 2.

Figure 1 shows a mail delivery system according to the invention comprising a peripheral unit 100 for arrangement in connection with a mailbox, and a central unit 200. The peripheral unit 100 comprises a mail detector 101 detecting receipt of mail in the mailbox. The mail detector emits a signal to a generator 102 generating a radio-frequency signal that is transmitted to an antenna 103. The antenna 103 emits a corresponding wireless electromagnetic signal. A central unit 200 comprises an antenna 202 receiving the electromagnetic signal from the peripheral unit 100. The signal received is transmitted to a receiver 202 that performs a suitable signal processing and emits a signal to an indicator 203. The indicator 203 is able to give a visible signal, eg in the form of a flashing lamp, a text or a graphic symbol in a display or an audible sound signal. Hereby the user is informed that mail is received in the mailbox. The central unit has a reset function 204 enabling the user to zero the indicator 203.

Figures 2 and 3 show a peripheral unit 100 comprising a printed circuit card 104 with the emitter 102 mounted on the circuit card. The transmitter 102 comprises all electric and electronic components for operating the peripheral unit 100, and as shown these components may be arranged on both sides of the circuit card. One or more batteries 105 provide the transmitter 102 with electric energy. The transmitter 102 is connected to an antenna 103 that comprises a U-shaped, electrically conductive antenna element with a first branch 103a and a second branch 103b, a space being provided between the first branch 103a and the second branch 103b. This space is necessary for the functioning of the antenna. The first branch 103a is plate-shaped and has a width of a few percent of the wave length, eg 3 %, which means approximately like the width of the circuit card 104 and a length corresponding to one quarter of a wave length, while the second branch 103b is an electrically conductive path on the circuit card 104, and the first branch constitutes an electric neutral plane for the second branch. The antenna 103 has two feed terminals 106 and 107 at a distance from each other that are

determined by the frequency area in which the transmitter and the antenna are to operate. The feed terminal 106 is common for the two branches 103a and 103b, while the feed terminal 103b is connected only to the first branch 103a. All of this is enclosed in a sealed housing 108 which can, by suitable means, be attached to a surface of a mailbox 300, wherein the first branch 103a of the antenna is situated close to the mailbox 300.

The shown peripheral unit 100 has a mail detector 101 being herein a movement sensor. A movement sensor is expedient in that it does not have to be connected to anything outside the housing 108, and therefore the housing can be sealed and be made suitably weatherproof. Then the entire unit 100 can be arranged on the flap or wicket of the mailbox or on some other part of the mailbox that will be opened when mail is delivered to the mailbox. A number of movement sensors are known that can be used, eg those in which a movable part connects or disconnects an electric switch or wherein the electric state of the sensor is changed in some other way when the sensor is moved. The transmitter 102 reacts to this change in the movement sensor by generating electric radio-frequency signals that are transmitted to the antenna 103 that thereby emits corresponding electro- magnetic signals that can be received by the central unit.

Preferably, the radio-frequency signals are coded with a in individual code for the mail delivery system. Hereby it is ensured that a plurality of mail delivery systems according to the invention can be used in close proximity of each other while providing great reliability that signals from a peripheral unit is recorded only by the corresponding central unit.

If a peripheral unit is damaged or lost, eg stolen, or if the batteries are deplete of electric power, the user may obtain a new peripheral unit without an ensuing need to also replace the central unit. As described, the new peripheral unit is actuated in that the magnet 111 is removed and the new

peripheral unit is arranged within the service area of the antenna of the central unit. The central unit is set to a"learning state", and the mail detector is actuated, whereby the peripheral unit emits an individually coded wireless signal. This signal is received by the central unit which is configured not to respond immediately thereto; rather, in the leaning state the central unit will remember the code for the new signal and in the time to come react to signals having the new code from the new peripheral unit. In this manner one single central unit will also be able to operate along with two or more peripheral units having individual codes, and on its display the central unit will be able to indicate in which of the associated mailboxes mail is received.

Other kinds of mail detector 101 can be used: a switch that is activated when mail is received, either by movement of the flap or the wicket or by the mail received influencing, either by its weight or in some other way, the switch.

Non-touch sensors can also be used, eg where the received mail interrupts the optical path between a light source and a light-sensitive element.

The shown movement sensor 101 is arranged in an opening in the circuit card 104 to protrude through the circuit card and into the space between the two branches of the antenna, and in particular occupies only little space on that side of the circuit board that faces away from the antenna. In a similar manner the transmitter 102 is also distributed on both sides of the circuit card. Hereby it is ensured that the peripheral unit can be manufactured to be compact and have small height.

The batteries 105 are flat, preferably of the button-cell type having throughout a conductive metal surface, except from a thin partition between the poles of the batteries. Lithium batteries of the type CR2032 or CR1620 are suitable as they are flat and have long longevity. The batteries 105 are also arranged in the space between the two branches 103a and 103b of the antenna, and

they are coupled electrically to the second branch 103b of the antenna, whereby the batteries constitute a part of the antenna.

A dielectric material 109 is arranged between the batteries 105 and the first branch 103a of the antenna, where it imparts mechanical stability to the structure, but also influences the antenna properties. The dielectric material can be arranged elsewhere in relation to the antenna and may occupy more or less space than shown. The dielectric material may be eg PTFE, PP or PP.

In the electric coupling from the batteries 105 to the transmitter 102, an electric switch 110 is inserted that can be interrupted by a magnetic field, preferably a reed switch. When supplied to the end user, a magnet 11 is secured to the outside of the peripheral unit in proximity of the reed switch 110, whereby it is interrupted, and thus the transmitter 102 is not supplied with voltage from the batteries. When it is taken into use, the user has to remove the magnet, whereby the voltage supply to the transmitter is established. Hereby it is ensured that the peripheral unit is supplied to the end user with unused batteries.

When the peripheral unit is arranged on a mailbox of electrically non- conductive material, eg plastics or wood, the antenna has a first resonance frequency which is determined exclusively by the structure of the antenna and the above-referenced electronic components, batteries and the dielectric material. When the peripheral unit is arranged on a mailbox of electrically conductive material, eg metal, with the first branch 103a of the antenna in close proximity of the metal plate of the mailbox, this metal plate will affect the properties of the antenna, and thereby another resonance frequency is imparted to the antenna; the difference between the first and the second resonance frequency being more or less equal to the bandwidth of the antenna. The transmitter 102 transmits with a frequency between the first and second resonance frequencies of the antenna, ie within the bandwidth of the antenna.