Structure for shielding an antenna from radio interference

Transcription

1 Structure for shielding an antenna from radio interference Category: Communications Reference: TDO0060 Broker Company Name: STFC Innovations Ltd Broker Name: Matthew Edwards Telephone: Abstract: A world renowned organization has developed a structure that is capable of shielding an antenna from multipath and electromagnetic interference. The shield was primarily designed to protect Global Navigation Satellite Systems (GNSS) reference stations from interference that is generated in the local vicinity. This novel system has been developed due to the fact that the reference stations are often located in urban environments and so are in close proximity to a variety of different objects that can scatter waves and cause interference. If the antenna is not shielded, this interference would impact on the quality of the GNSS service data and reduce the positioning accuracy of the entire system. Description: Antennas are mounted in a variety of locations and, as such, a number of these are located in close proximity to other objects. This means that they receive not just the desired transmitted signal but also waves that have scattered off these nearby objects. The shied that has been developed allows for this problem to be overcome in a sensible and practical manner. The antenna is mounted within the shielding structure to achieve both isolation from unwanted signals and to synthesise a controlled environment. The isolation is achieved via reflection, diffraction and absorption of the local, unwanted interference. The synthesis of a controlled environment is created by the suppression of any reflections from interior surfaces of the shield. When both of these features are combined, unwanted signals are prevented from reaching the antenna phase centre. The shielding structure lowers the background signal significantly. For a GNSS reference station, this performance enhancement links directly to the improvement of the quality and accuracy of the service data that is provided to mobile GNSS receivers. The figures below show the shield from different angles so that a comprehensive understanding of the design can be achieved.

2 Figure 1 Figure 1 shows a vertical cut along the central axis of the shield with the antenna at the centre. As can be seen in Figure 1, the shield is constructed in such a way that the antenna is below the local horizon. In the case of GNSS signals coming from satellites above the Earth most of the unwanted interference comes from below this local horizon. The interior surfaces of the shield are also coated in an absorbing material to greatly reduce the intensity of any unwanted interference. To protect from moisture ingress, a coating made from a plastic material may be used to cover the shield, including the opening to the cavity. The exact coating obviously depends on the specific operational environment. Figure 2

3 Figure 2 shows a top and slant view of the shield, again with the antenna at the centre. Figure 3 Figure 3 shows the various dimensional aspects of the shield. The shield has a set of angular and dimensional parameters that determine the optimum performance. As can be seen in Figure 1, the angles δ, γ and ε are shown. The angle δ is created by the upright section of the structure intersecting the baseplate. This angle can be between 60 o and 90 o, but as close to 90 o as possible is preferable. The angle γ is formed between an imaginary plane parallel to the baseplate and the diagonal created by the inner wall. This angle will be smaller than δ and preferably in the range 0 o to 30 o. An optimum has been observed at 20 o by the inventors. A third angle ε is defined as the angle between the sloping top surface of the shield and an imaginary plane parallel to the baseplate. This angle should be in the range 2 o to 10 o and an optimum has been observed at 5 o. Figure 3 demonstrates the different dimensional ratios of the shield structure. The radius and height of the cavity are denoted r and h respectively. The radius of the opening of the cavity is defined by dist_c. The height of the diagonal inner wall section on a vertical plane is denoted h_up. There are three dimensional ratios that are critical to the design of the shield. R1 is the ratio between the radius of the opening, dist_c, and the maximum radius of the cavity itself, r. R1 = dist_c / r and should fit into the range 0.35 < R1 < 0.7. An optimum is at R1 = 0.54 R2 is the ratio between the height of the cavity, h, and the diameter of the cavity, d=2r. R2 = h / (2r) and should fit into the range 0.22 < R2 < An optimum is at R2 = 0.34 R3 is the ratio between the height of the diagonal section of cavity, h_up, and the height of the entire cavity, h. R3 = h_up / h and should fit into the range 0.15 < R3 < 0.3. An optimum is at R3 = 0.22

4 The absolute size of the structure is dependent on the wavelength λ that corresponds to the operational frequency of the antenna. For example, for GNSS ground stations, one operating frequency is approximately 1.4GHz with a corresponding wavelength λ of ~21cm. For this shield the diameter, d=2r, is in the range from 9λ to 15λ. This can be narrowed down to 11λ < d < 13.5λ with an optimum at d = 12.4λ. Due to the fact that the shield is large compared to the wavelength that corresponds to the operating frequency of the antenna, the far-field approximation is applicable. Moreover, the antenna and shield do not affect one another s electromagnetic properties. This implies that the shield can be used to protect all antennas of a given operating frequency from interference, as long as the antenna can fit within a diameter d= 1.25λ. This is an important feature as it means that existing antennas do not need to be redesigned. The shield has been optimized to achieve all of this while keeping overall size and weight to a minimum. Innovations and advantages of the offer: The size and weight of the shield are at the minimum that can still deliver the desired results. It can be fitted onto existing antennas. The design is scalable in relation to the wavelength of the desired signal. Improves signal quality when the direction of the transmission is known. Especially useful in urban environments. Has a relatively simple design. Further Information: N/A Application: While being applicable to any GNSS base station application, there are other sectors in which this technology could be applied. The key applications for this shield are where the frequency and direction of the transmitted signal are known but there are objects near to the antenna that can create interference. The technology would be applicable in areas such as: Aerospace communications Marine surveillance Tracking of freight within a warehouse/dockyard scenario While originally designed for operation in the radio frequency domain, this band is not exclusive. The shield is applicable to other kinds of electromagnetic waves and antenna configured to work at these frequencies. Space Heritage: It was used as a shielding structure around GNSS ground station antenna. It stops unwanted interference from objects in the vicinity local to the antenna.

5 Broker comments: The key aspect of this technology is that the design is directly related to the wavelength of the desired signal, meaning it can be applied to a vast range of antenna systems. This gives it a wider range of end applications which makes it also attractive to companies looking to develop it through the ESA BIC programme. This technology description was downloaded from