Technical field

The present invention proposes a simple and robust solution to switch between antennas .

State of the art and background of the invention

An antenna selector selects one of two or more antennas. The selector contains a switch having one common terminal that electrically connects to one of two or more other terminals. Mostly a two state switch is used, meaning one input can connect to one of two outputs. It can be done mechanically by a switch inside a connector. In normal case the connector switch is in a state that transfer the electrical signal between one input and one output but when an external mating connector is plugged into the connector switch it switch state and the signal transfers from the input to the external mating connector. Mechanical switches for radio frequencies are mainly coaxial and it is a complex component that in general is expensive and only exists in a few shapes.

Electrical switches carry out the same function but instead of using a mechanical force to switch state it switch state by a electrical signal. Also a electrical switch can perform the switch mechanically. Those switches are called MEMS. The most common switches select states by transferring the signal through transistors than either can be on or off depending witch route the signal shall transfer.

To have small and cheap antennas in the different nodes in a wireless communication system is a common desire both in the planning and set-up phase. Embedded antennas have those properties. But the functionality of the embedded antennas depend a lot of the location.

If a node containing an embedded antenna is located in such environment where the radio communication don't operate sufficient, such as basements, in a tin cabinet or other places that shields the radio signals, it is desirable that a better antenna can be connected to that an antenna can be placed at a different spot and connect by a cable to the node. To easily be able to switch between an embedded or external antenna one of abovementioned switches is needed.

Today the mechanical switches integrated in a coaxial connector where the signal is transferred to the external antenna is most common. These connectors are very expensive and exist primarily to small and brittle connectors that aren't appropriate in applications such as industrial.

Another option is to use an electronically controlled switch that transfers the radio signal from the embedded antenna to the external antenna. Today an extra action is needed to set the switch to correct state. It can be done by a manual operation to switch state for the electronic switch. This is an extra operation that easily can be forgotten.

Therefore it is desirable with a cheap but more secure and robust system to select between an embedded and external antenna .

Description of the invention

The present invention offers a robust and safe selection between antennas. In the starting position, the switch that selects between the antennas is in a state giving connection between the radio transmitter/receiver and the terminal which is connected to an antenna, denominated antenna nr.l. This antenna can be of any kind. The other terminal of the switch

is connected to a connector that is unconnected to any antenna and it therefore has high resistance between its terminal and ground. By a resistor to power supply, VCC, and due to high resistance to ground, this terminal will have a sufficient voltage that the switch will remain in the state that transfer the radio signal to antenna nr.l. If an antenna, denominated antenna nr.2, having resistance low enough between its terminal for radio signal and ground, is connected to the connector, the DC voltage will drop below the switch threshold level and the radio signal will be transferred to antenna nr.2 instead of to antenna nr.l. No extra operation to control the switch is needed and the connector to the external antenna can be of arbitrary type. Common types of coaxial connectors are SMA, SMB, FME or other appropriate for the application.

The circuit schematic depends of the selected electrical switch and its demands and threshold voltage for switching, number of control signals, filter to not attenuate the radio signal, etc.

Description of drawings

Figure 1: shows the antenna selector by prior art mechanical switch.

Figure 2: shows antenna selector by prior art electrical switch.

Figure 3: shows present invention antenna selector when antenna nr.2 isn't connected.

Figure 4: shows present invention antenna selector when antenna nr.2 is connected.

Preferred embodiments

Figure 1 shows a mechanical switch (24) that change state when an external connector (14) connects. Figure 2 shows a electrical switch that by manual operation (23) change state. In this case arbitrary connector can be used but a extra operation is needed to change the state of the switch.

Figure 3 show an example of a electrical schematic of a antenna selector according to present invention. According to figure 3 the antenna selector is included in a block (22) that can be placed on printed circuit board (PCB) . According to figure 3 is the antenna nr.l (10) inserted in the block and can consist of an antenna mounted on or inserted in the PCB. The antenna nr.l doesn't need to be placed on the PCB but can also be connected by cable or connector. The antenna nr.2 (11) has a connection (12) between the inner and outer conductor of its cable (13) or connector (14) in such way that the resistance between the inner and outer conductor is sufficient small that the switch (17) switch state when antenna nr.2 connects to the connector (15) and by that the radio signal (16) by the switch (17) reach the antenna nr.2 (11) that excites the signal.

Present invention is appropriate both when antenna nr.l is external or embedded version. It is common that the antenna nr.l is embedded to cut the cost.

Ofetn the antenna nr.2 is embodied in such way that it can be considered as electrically short circuit. F-antennas, slot antennas, PIFA, loop and some other antennas are in general electrical short circuit that is, having a negligible electrical resistance between its connection terminals.

These antennas can either be mounted direct on a connector (14) but are also often connected by cable (13). The cable (13) and connector (14) is often of coaxial type but can also be out of other variants of cable and connectors.

The antenna nr.l (10) can be electrical short circuit such as F-antenna, slot antenna, PIFA or loop antenna. The antenna nr.l can also consist of an electrically open antenna that is, not having electrical connection between the radio signal and its ground. Examples of open antennas are monopole and dipole antennas. By electrical connection means a connection a directed current (DC) can pass through.

The radio signal (16) has a schematic symbol that in general symbolizes a signal generator. When the system is reciprocal it is obvious that (16) also can be a receiver of radio signal and also other components used at transmitting and/or reception of radio signals.

To switch state of the switch (17) more components are needed to get desirable functionality and performance. The switches control signal (21) according to figure 3, have a voltage of or near the antenna selectors supply voltage, VCC, when the antenna nr.2 isn't connected. This voltage is transferred through the filter (19) to the connector (15) . In general it is necessary to add a DC-block to the switch by a capacitance (18) . It is often practice to add a resistor (20) to the VCC to reduce the current when connecting antenna nr.2

Figure nr 4 shows the antenna selector when antenna nr.2 (11) is connected. By the connection the control signal to the switch will drop and when it is below the threshold voltage the switch (17) changes state so the radio signal from (16)

passes the switch (17) and is feed to antenna nr.2 (11) instead of antenna nr.l (10).

To reduce the losses of the radio signal it is good engineering practice to add a filter (19) so the radio signal is blocked from the resistance (20) and control signal (21) . The filter can be designed in different ways and figure 3 and 4 shows a filter containing three components. More and fewer components can be used. The design ought to be in such way that the DC current can pass to get correct function for the antenna selector.

In figure 3 and 4 the switch (17) is controlled by one signal only (21) . Switches often require two control signals and also having them inverted in such way that if (21) having high state, that is a voltage near the VCC level, the other control signal shall have low level, that is voltage near ground level. This can be embodied by adding an inverter having input (21) and output to the other control signal. An inverter can be ought of a simple transistor or a logic circuit.

A person skilled in the art realizes that same properties can be achieved by changing the electrical schematic of figure 3 and 4.

The switch in the antenna selector can consist of different types. Common switches are electrical containing transistor of different kind but also mechanical switched of MEMS or diods is common. As connector the coaxial is most common. SMA, SMB and FME are names of different coaxial connectors but the connector doesn't need to be coaxial.