This invention relates to an antenna configured to be installed in an unattended ground sensor.

Unattended ground sensor units are used as part of security systems for infrastructure and personnel protection. Typically a number of unattended ground sensor units are provided around the perimeter of a site to be protected or along another boundary. Each unattended ground sensor unit comprises at least one sensor for sensing a physical phenomenon in the region of the unit and often transmission means for transmitting signals away from the unit in dependence on the output of the sensor. Thus, for example, the sensor unit may include a seismic sensor for detecting vibrations in the ground which are indicative of a passing person, vehicle or other object. Similarly the sensor unit may include acoustic sensors, magnetometers, heat and light sensors and so on, each for a similar purpose of detecting the presence of a person or object or movement of a person or object in the region of the unit. Typically unattended ground sensors are connected together in a network and, for example, connected to a central control unit where the output of each of the sensor units can be monitored and determinations made regarding the presence or movement of people and/or objects in or around the site which is to be monitored.

Unattended ground sensors typically have a whip antenna to allow them to communicate with one another and/or a central control unit. Whip antennas have been used because they are a simple design and can provide an omnidirectional radiation pattern. However, the present inventor has recognised that whip antennas may have certain drawbacks for unattended ground sensors. Firstly, whip antennas generally extend vertically from a sensor which means that they can be difficult to disguise or camouflage. Secondly, the whip antennas and/or external RF connector can be damaged, especially in the type of hazardous environment where unattended ground sensors may be deployed. Thirdly, whip antennas can be affected by wind, causing small movements of the ground sensor; these movements can be misinterpreted as a seismic event and may be the cause of false alarms/detections.

It would be desirable to provide an antenna for an unattended ground sensor unit which is more covert, insofar it is relatively unobtrusive once installed, but yet is robust and easy to deploy. The present invention is aimed at providing unattended ground sensor units which address at least some of these issues.

According to the present invention there is provided an antenna configured to be assembled in an unattended ground sensor, the antenna comprising: a first conductor having a first planar surface; a second conductor having a second planar surface, wherein the second planar surface is substantially parallel to the first planar surface; an antenna conducting pin connected to the first conductor and extending between the first and second conductors and spacing them apart; and one or more shorting pins connected to the first conductor and extending between the first and second conductors, wherein the one of more shorting pins are electrically connected to both the first and second conductors. In this way, a top-loaded antenna can be provided. This is advantageous because a top-loaded antenna is generally shorter than a whip antenna for a comparable application with a similar efficiency. This allows an unattended ground sensor that is more covert and can be more easily camouflaged. A top-loaded antenna can also be accommodated internally within an

unattended ground sensor which means that the antenna is less vulnerable to deliberate or accidental damage. For example, a vehicle could be driven over an unattended ground sensor including such an antenna without causing damage. A top-loaded antenna is also less likely to be affected by wind, especially if the antenna can be located entirely within the ground sensor housing.

The antenna conducting pin is preferably arranged so that electrical currents can be directed through it to the first conductor. The antenna conducting pin is preferably insulated from the second conductor, which is normally at a ground potential. It has been determined by experiment that the inclusion of one or more shorting pins can increase the antenna efficiency. Specifically, the shorting pins may help in matching the antenna to the driving circuit. Improved antenna efficiency may be determined by measuring the voltage standing wave ratio (VSWR). The antenna conducting pin may extend through a bore in the second conductor, and an electrical insulator may be provided between the second conductor and the antenna conducting pin.

The antenna may comprise at least two shorting pins arranged on opposite sides of the antenna conducting pin. For example, the shorting pins may be diametrically opposite one another, either side of a tubular antenna conducting pin. This arrangement has been found to improve antenna efficiency for a given antenna volume. The one or more shorting pins may be made of a different material to the antenna conducting pin. In one embodiment the shorting pins are made of the same material as the first and second conducting surfaces. The shorting pins may be made of brass and the antenna conducting pin may be made of copper. It has been determined by experiment that this configuration can improve antenna efficiency for a given antenna volume. Preferably there is an antenna controller electrically connected to the conducting pin so that electrical currents can be directed through the conducting pin to the first conductor. The antenna controller may be provided in a circuit board and the circuit board may be attached to the second conductor and arranged on the opposite side to the first conductor. By attaching the circuit board to the underside of the second conductor it is possible to create a robust and compact antenna arrangement.

The antenna may include a solid dielectric spacer arranged in the space between the first and second planar surfaces. Preferably the solid dielectric has a higher dielectric constant than air. This can allow the separation of the first and second planar surfaces to be reduced further, which is advantageous as it can reduce the physical size of the antenna. The solid dielectric may also improve the robustness of the antenna by resisting any mechanical forces that could otherwise bend or warp the first and second planar surfaces. The solid dielectric may be a material such as Polytetrafluoroethylene (PTFE).

The solid dielectric preferably comprises a bore through which the conducting pin can extend. This arrangement can facilitate the assembly process so that the solid dielectric can be fitted onto the conducting pin.

The solid dielectric may include a keying feature that can be engaged to resist its rotation. This may be especially important when there is more than one conducting pin because the solid dielectric would not be able to rotate without breaking the pins. By providing the keying feature the solid dielectric can be rotationally locked to prevent any damage to the conducting pins.

The antenna may include a clamp for engaging the solid dielectric. Preferably the clamp can resist any movement of the solid dielectric in a vertical direction (i.e. in a direction that is substantially perpendicular to the first and second planar surfaces). The clamp may also be arranged to lock together all of the components of the antenna. In one arrangement the clamp may be built into the cap of an unattended ground sensor.

The solid dielectric may comprise a flange that can be engaged by the clamp. The clamp may also include a keying feature that can engage with the keying feature on the solid dielectric. Thus, the clamp can prevent vertical and rotational movement of the solid dielectric. It can be important to prevent the solid dielectric from rotating to prevent stress on the shorting pins which can be soldered to the first and second conductors.

Preferably the frequency of operation is in the range of 800-1000MHz.

Preferably still the antenna is designed to operate in the range of 825- 950MHz. The antenna may be used at different frequencies in different regions of the world. In certain arrangements the frequency of operation could be below 825MHz. As the frequency reduces the size of the components in the antenna may need to increase in order to achieve the same efficiency. In a mobile/portable device this may mean that a clamp arrangement needs to secure a higher mass.

The transmission range of the antenna may be designed to be at least twice the detection range. Thus, if two ground sensors are separated by the transmission range then any seismic activity between the sensors can be detected by at least one of them.

According to another aspect of the invention there is provided an unattended ground sensor comprising a housing for accommodating the antenna as described above. The unattended ground sensor is preferably arranged to be affixed to the ground so that any seismic vibrations can be detected. By enclosing the antenna within the housing the antenna can be protected. This can improve the reliability of the unattended ground sensor in comparison to a sensor with an external whip antenna. It may also be helpful in camouflaging the unattended ground sensor.

Preferably the unattended ground sensor further comprises at least one sensor for sensing a physical phenomenon in the region of the unit and the antenna is arranged to transmit information regarding the sensed physical phenomenon. The unattended ground sensor unit may comprise a memory for storing data in dependence on the output of the sensor. The sensor unit may comprise a head portion and body portion with a threaded exterior surface. The head portion may comprise engagement portions for engaging with a tool for rotatingly driving the sensor unit during installation in the ground. Preferably the sensor is a seismic sensor. The separation between the cap and the first conductor can be optimised to minimise antenna detuning.

According to another aspect of the invention there is provided a method of assembling an unattended ground sensor comprising the steps of:

establishing electrical connections between an antenna controller and an antenna, wherein the antenna comprises a first conductor having a first planar surface, a second conductor having a second planar surface, wherein the second planar surface is substantially parallel to the first planar surface, an antenna conducting pin connected to the first conductor and extending between the first and second conductors and spacing them apart, and one or more shorting pins connected to the first conductor and extending between the first and second conductors, wherein the one of more shorting pins are electrically connected to both the first and second conductors; fitting the antenna controller and the antenna within a housing of the unattended ground sensor; and fitting a cap to the housing so that the antenna is enclosed. According to yet another aspect of the invention there is provided an antenna comprising: a first conductor having a first planar surface; a second conductor having a second planar surface, wherein the second planar surface is substantially parallel to the first planar surface; a conducting pin connected to the first conductor and extending between the first and second conductors; a solid dielectric spacer arranged in the space between the first and second planar surfaces; and a clamp for engaging the solid dielectric, wherein the solid dielectric includes a keying feature that can be engaged to resist its rotation.

The solid dielectric can allow a top-loaded antenna to be produced with a reduced size for a given volume and efficiency. The solid dielectric can minimise antenna de-tuning, and it can also improve the robustness of the antenna by resisting any mechanical forces that could otherwise bend or warp the first and second planar surfaces. The clamp can be provided to resist any movement of the solid dielectric in a vertical direction (i.e. in a direction that is substantially perpendicular to the first and second planar surfaces). The clamp may also be arranged to lock together all of the components of the antenna. For example, the clamp may lock the solid dielectric to one of the conductors. In one arrangement the solid dielectric may be provided with a flange and the clamp may be provided to engage with the flange. Once installed the clamp may lock the solid dielectric to a housing structure. The second conductor may also be clamped to the housing structure by the clamp.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic side view of an unattended ground sensor unit in an embodiment of the invention;

Figure 2 is a section on line A-A of the unattended ground sensor unit shown in Figure 1 ;

Figure 3 is an exploded view of the unattended ground sensor unit shown in Figure 1 ; Figure 4A is a perspective view of a solid dielectric component for use in an antenna in an embodiment of the invention;

Figure 4B is a top view of the solid dielectric component shown in Figure 4A; Figure 4C is a side view of the solid dielectric component shown in Figure 4A; Figure 4D is a section on line A-A of the solid dielectric component shown in Figure 4C;

Figure 5A is a perspective view of a retention ring for use in an antenna in an embodiment of the invention;

Figure 5B is a top view of the retention ring shown in Figure 5A;

Figure 5C is a side view of the retention ring shown in Figure 5A;

Figure 5D is a bottom view of the retention ring shown in Figure 5A; and Figure 6 is a cross-sectional view of an antenna for assembly in an unattended ground sensor, in an embodiment of the invention. Figures 1 -3 show an unattended ground sensor unit 30 for use as part of a security or surveillance system. The unattended ground sensor unit comprises a body portion 6 and a head portion 5. The body portion 6 and head portion 5 together form a housing within which other components of the sensor unit are disposed. In the present embodiment a battery 7 for powering the sensor unit is located in a compartment within the body portion 6. Also provided within the ground sensor unit is a sensor 3, and an antenna 2. In the present embodiment the sensor 3 and antenna 2 are disposed in the head portion 2 of the unit. This means that the antenna 2 is arranged to be above ground level when the unit is installed in the ground. A screw thread is provided on and around an external surface of the body portion 6. Thus, if the sensor unit is rotated about the main axis whilst the sensor unit is inserted into the ground the thread will tend to draw the sensor unit into the surrounding ground at least up to a point where the head portion 2 comes in contact with the surface of the ground. Thus when installed, or deployed, the body portion 6 is located below ground level. The head portion 2 is provided with engaging recesses 26 (see Figure 1 ) to aid in the act of rotating the sensor unit around the main axis for insertion of the sensor unit into the ground. In some cases a deployment tool may be provided for engaging with these recesses to aid in insertion of the unit.

As is apparent from Figure 3, the antenna 2 is accommodated mainly in the head portion 5 of the unit. The antenna includes a base conductor 100 that is attached to a first side of a printed circuit board 201 . Antenna control circuitry is provided on a second side of the printed circuit board 201 and the sensor 3 is also connected to the printed circuit board. A retention ring 4 is provided to clamp the various components of the antenna 2 together. Locking rings 8, 9 are provided to clamp the head portion 5 and the body portion 6 together once all of the internal components have been assembled. In some arrangements a single locking ring may be provided.

Further detail of the antenna 2 is apparent from Figure 6. The antenna 2 comprises a base conductor 100 and a top conductor 102, both of which are circular plates, made of copper and arranged in a horizontal plane (with respect to the normal operating orientation of the unit). An antenna rod 204, made of brass, is electrically connected to the top conductor 102. A hole is provided in the base conductor 100 and the antenna rod 204 extends through the hole to be connected to antenna control circuitry on a printed circuit board 104 on the reverse side of the base conductor 100. An insulating ring 106 is provided around the antenna rod 204 where it extends through the base conductor 100 so that the antenna rod 204 is electrically insulated from the base conductor 100.

The base conductor 100 is connected to ground, and two shorting pins 205 are provided between the top conductor 102 and the base conductor 100. The shorting pins 205, made of copper, are provided on diametrically opposite sides of the antenna rod 204. A dielectric spacer 202 is provided between the base conductor 100 and the top conductor 102. The dielectric spacer 202 includes bores to accommodate the antenna rod 204 and the shorting pins 205.

In operation, the antenna control circuitry is arranged to excite a current in the antenna rod 102, and this current flows to the top conductor 102. The shorting pins 205 carry the current to the base conductor 100, where the current is earthed. It has been determined by experiment that this arrangement provides an improved antenna efficiency for the application of an unattended ground sensor.

Further detail of the dielectric spacer 202 is shown in Figures 4A-D. The dielectric spacer 202 is generally cylindrical with bores 108 to accommodate the antenna rd 204 and the shorting pins 205. A flange 1 10 is provided at the bottom of the spacer 202, with respect to the normal operating orientation of the antenna 2, and the flange 1 10 includes a keying cut-out 1 12. The flange 1 10 and the keying cut-out 1 12 are intended to be engaged by the retention ring 4, further details of which are apparent from Figures 5A-D.

The retention ring 4 is designed to fit over the dielectric spacer 202, and the ring 4 includes a surface 1 14 with gripping teeth 1 16 for engaging the flange 1 10. A keying engagement 1 18 is also provided for engaging the cut-out 1 12 in the flange 1 10 and resisting any rotational movement of the spacer 202. When the retention ring 4 is fitted in place any vertical or rotational movement of the spacer 202 can be resisted preventing damage to the solder connections between the rod and the conductors and the shorting pins and the conductors.