Abstract:
A dipole antenna, comprising: a dipole arrangement comprising at least a pair of antenna arms, each antenna arm having a feed end and a distal end, the feed ends positioned in proximity to each other; a feed structure, coupled to said dipole arrangement, comprising a balun for providing the antenna with a balanced feed; wherein, each antenna arm comprises: a conductive end plate, located at the distal end of the respective antenna arm; and an inductive coil, located at the feed end of the respective antenna arm.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from United Kingdom Patent Application No. 1300513.7, filed on Jan. 11, 2013, and United Kingdom Patent Application No. 1301436.0, filed on Jan. 28, 2013. Each of these prior applications is herein incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    Various embodiments relate to electrically-short high-power antennas and to the use of such antennas to generate high electric field strength at short distances from the antenna. 
         [0003]    When roads are subjected to subzero temperatures, in the presence of moisture, ice can form on the road surface. This is undesirable as it reduces the performance of vehicles using the road, and can result in accidents. Despite advances in technology, it is common for roads to be defrosted by spreading salt or other material on the road surface in order to lower the melting point of the ice on the road surface. There is a need for more technically advanced and environmentally friendly methods for defrosting road surfaces. 
       BRIEF SUMMARY 
       [0004]    Various embodiments provide a dipole antenna, comprising: a dipole arrangement comprising at least a pair of antenna arms, each antenna arm having a feed end and a distal end, the feed ends positioned in proximity to each other; a feed structure, coupled to said dipole arrangement, comprising a balun for providing the antenna with a balanced feed; wherein, each antenna arm comprises: a conductive end plate, located at the distal end of the respective antenna arm; and an inductive coil, located at the feed end of the respective antenna arm. 
         [0005]    Further features of embodiments are described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Embodiments will now be described, by way of example only, and with reference to the accompanying drawings, in which: 
           [0007]      FIG. 1  is a perspective view of an antenna arrangement in accordance with various embodiments; 
           [0008]      FIG. 2  is schematic circuit diagram of a model of the antenna of  FIG. 1 ; 
           [0009]      FIG. 3  shows the orientation of the antenna of  FIG. 1  with respect to the plots shown in the following Figures; 
           [0010]      FIGS. 4A to 4D  shows the near-field electric field distribution of the antenna shown in  FIG. 1 ; and 
           [0011]      FIGS. 5A to 5D  show the near-field magnetic field distribution of the antenna shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    An antenna arrangement  100  in accordance with various embodiments is shown in  FIG. 1 . The antenna arrangement  100  includes an antenna  101  and a feed structure  102 . Also shown in  FIG. 1  is supporting structure  103 . Antenna  101  is an electrically-short high-power dipole antenna. The dipole is formed by antenna arms  104 A,  104 B. 
         [0013]    In use, the supporting structure  103  may be positioned on a vehicle, such as a car or truck. The antenna arrangement  100  is designed to be mounted on the front of such a vehicle, such that the antenna arrangement is positioned a predetermined distance above the ground. In use, the electric field generated by the antenna  101  is directed towards the ground, as will be explained in more detail below. In  FIG. 1 , supporting structure  103  includes a back plate, which is a reflector, as will be described in more detail below. It will be appreciated that the supporting structure may take other forms, such as an array of horizontal supporting sections. 
         [0014]    The antenna arrangement  100  is typically positioned so that the antenna  101  is positioned around 1 m to 1.5 m above the ground. Moving the antenna  101  closer to the ground would increase the field strength on the ground, but the field would be more localised. The impendence of the antenna, and hence its efficiency, would also be reduced. Moving the antenna  101  away from the ground would decrease the field strength. 
         [0015]    Each antenna arm  104 A,  104 B includes a coil  105 A,  105 B and a conductive end plate  106 A,  106 B. The coils  105 A,  105 B may have a value of around 5 μH (micro-Henries) each. The conductive end plates  106 A,  106 B are formed at distal ends of the antenna arms  104 A,  104 B. The coils  105 A,  105 B are formed at the proximal ends of the antenna arms. The conductive end plates  106 A,  106 B are coupled to a feed point of the feed structure  102  by conductive wires  107 A,  107 B. Towards the proximal end of each antenna arm  104 A,  104 B, the coils  105 A,  105 B are formed by the conductive wires  107 A,  107 B. As can been seen in  FIG. 1 , the conductive end plates  106 A,  106 B, conductive wires  107 A,  107 B and the coils  105 A,  105 B share a common primary axis. The antenna arms  104 A,  104 B arranged such their primary axes are aligned, with one arm mirroring the other, in terms of the arrangement of its components. In  FIG. 1 , each coil  105 A,  105 B is wound in the same direction and consists of six full turns. Although the antenna arms  104 A,  104 B are shown in a fixed position, they may be mounted on brackets that allow them to be lifted upwards. This may be useful if ground clearance is an issue, so that the antenna arrangement may be lifted away from any objects or ground protrusions. 
         [0016]    The conductive end plates  106 A,  106 B are rectangular in shape and are arranged such that the short sides of each plate are connected to the conductive wires  107 A,  107 B. The conductive end plates  106 A,  106 B include a primary plane, which is aligned with a surface of the rectangle. In use, the antenna arms  104 A,  104 B are arranged such that this plane is substantially aligned with the horizontal, and hence the surface of the ground which is positioned below the antenna. 
         [0017]    The feed structure  102  includes a coaxial transmission line  108 . One end of coaxial transmission line  108  is coupled to an RF input which is located on the other side of the supporting structure, and which is not shown in  FIG. 1 . As shown in  FIG. 1 , the coaxial transmission line  108  abuts the back plate of the supporting structure. Where the coaxial transmission line  108  meets the back plate, a connection is made to the RF input and a matching circuit (also not shown in  FIG. 1 ). The other end of the coaxial transmission line  108  is the feed point for antenna arms  104 A,  104 B. 
         [0018]    At the feed point end of coaxial transmission line  108 , the conductive wire  107 B of antenna arm  104 B is connected to the outer sleeve of coaxial transmission line  108 . Feed structure  102  also includes cylindrical conductor  109 . The cylindrical conductor may be hollow or solid, depending on the antenna design. Cylindrical conductor  109  is positioned in parallel with, and adjacent to, coaxial transmission line  108 . At the feed point end of cylindrical conductor  109 , the conductive wire  107 A of antenna arm  104 A is connected to the cylindrical conductor. Furthermore, the cylindrical conductor  109  is connected to the inner conductor of coaxial transmission line  108  by coupling member  110 . At the feed point end of the coaxial transmission line  108 , the cylindrical conductor  109  and the outer conductor of the coaxial transmission line are connected by high voltage capacitor  111 . The high voltage capacitor  111  may have a value of around 180 pF. The cylindrical conductor  109  and the outer conductor of the coaxial transmission line  108  are connected together at the end distal from the feed end (i.e. adjacent to the supporting structure back plate). This connection is a low impedance connection. 
         [0019]    Supporting structure  103  includes supporting arms  112 A,  112 B. Supporting arms  112 A,  112 B are connected to the conductive wires  107 A,  107 B of antenna arms  104 A,  104 B using connectors between coils  105 A,  105 B and conductive end plates  106 A,  106 B. The other ends of the supporting arms  112 A,  112 B are connected to reflector  113 . The supporting arms  112 A,  112 B are made from non-conductive material. 
         [0020]    The reflector  113  minimises currents induced on the structure (for example a vehicle) to which it is mounted. The reflector  113  also forms part of the support structure  103 , which supports the radiating sections of the antenna  101 . Although the antenna arrangement  100  could be mounted directly to a vehicle instead of via the support structure  103 , it is preferable to mount the antenna arrangement via a support structure, because significant currents are induced in the support structure. Therefore, the use of a separate reflector  113  minimises the currents induced on the vehicle itself. A further advantage of a separate reflector is that the antenna may be optimised to the reflector  113 , and the complete assembly may be readily transferred form one vehicle to another vehicle. The complete antenna arrangement  100  therefore becomes platform independent. 
         [0021]      FIG. 2  is an equivalent circuit diagram of a model of antenna  101 . The model shows electrical equivalents to the components of antenna arrangement  100 . The model includes a matching circuit  200 , a twin-line balun  201  and a radiating section  202 . Also shown is RF input  203 . The matching circuit  200  represents the matching circuit, which is referred to in connection with  FIG. 1 . The matching circuit  200  includes transmission line  204 , which represents coaxial transmission line  108 . Transmission line  204  includes two conductors, which are equivalent to the inner conductor and outer conductor of coaxial transmission line  108 . In particular, a first conductor  205  of transmission line  204  represents the currents flowing in the inner conductor of coaxial transmission line  108 . A second conductor  206  represents currents flowing on the inner surface of the outer conductor of coaxial transmission line  108 . The first conductor  205  of transmission line  204  is fed by RF input  203 . The second conductor  206  of the transmission line  204  is coupled to ground. This is not shown in  FIG. 1 . Matching circuit  200  also includes matching capacitor  207 , which will have parasitic resistance  208  and parasitic inductance  209 . The value of the matching capacitor will depend on the impedance to which the antenna  101  is being matched to, but may be a 1000 pF variable capacitor. The matching circuit  200  consists of the matching capacitor  207  and the transmission line  204  which are used to provide matching for the antenna  101 . The matching capacitor  207  is located on the rear side of the reflector  113 , and is not shown in  FIG. 1 . 
         [0022]    Radiating section  202  is the electrical equivalent to the antenna arms  104 A and  104 B. The antenna resister  210  represents the radiation resistance of the antenna, together with the resistive losses associated with using finite conductivity materials for the antenna structure. The antenna inductor  211  and antenna capacitor  212  model the resonant behaviour of the radiating antenna structure. The antenna capacitor  212  is representative of the interaction between the conductive end plates and the ground. Similarly, the antenna inductor  211  is representative of the coils  105 A and  105 B. 
         [0023]    Balun  201  includes transmission line  213 . A first conductor  214  of the transmission line  213  is equivalent to the cylindrical conductor  109 . A second conductor  215  of transmission line  211  is equivalent to the outer surface of the outer conductor of the coaxial transmission line  108 . The two conductors of transmission line  213  appear short circuited in  FIG. 2  because the cylindrical conductor  109  and coaxial transmission line  108  outer conductor are both connected together at the reflector  213 . Balun  201  also includes capacitor  216 , which is representative of high voltage capacitor  111 . The balun also includes parasitic inductor  217  and parasitic resistor  218 , which are representative of parasitic inductance and resistance generated by high voltage capacitor  111 . 
         [0024]    In the following, a description of the principles underlying the above-described antenna is provided. A dipole antenna will naturally resonate when it is just under one half of a wavelength long (around 0.48, depending upon the dipole&#39;s diameter). When a dipole is such a length, it will tend to have a large bandwidth, which means that its impedance does not change rapidly with frequency. A half-wave dipole has an impedance at resonance of around 70 ohms. It is fairly simple to transform this to 50 ohms, either through the design of the antenna or an external matching circuit. Reducing the length of a half-wave dipole will reduce its impedance, reduce its bandwidth and increase its Q. Mounting the antenna parallel to a material (the ground in this case) will alter the antenna&#39;s impedance. The amount the impedance is altered depends upon the material properties (εr &amp; σ) and the distance between the antenna and the material. 
         [0025]    The electrical performance of an antenna is closely related to its size in wavelengths. When the space is several wavelengths long, then a wide variety of antennas and antenna arrays may be used and the radiation bandwidth will be large. However, the performance of all antennas will be severely reduced when the space becomes electrically small. By definition, an electrically small antenna is one when ka&lt;1, where the wave number, k is 2π/λ, and the parameter ‘a’ is half the length of the antenna&#39;s longest dimension. In the limit, the radiation quality factor, Qr tends to 1/(ka)3, as ka tends to zero. It is possible to calculate the theoretical limit for the Q of an antenna and hence its bandwidth for any VSWR (voltage standing wave ratio) and antenna efficiency. 
         [0026]    It should be noted that a lossy or inefficient antenna will have a larger bandwidth than an efficient antenna of the same size. Most antennas require a reasonable efficiency, typically greater than 70%, but the exact figure depends upon the antenna&#39;s application. However, some wideband antennas are deliberately made lossy to improve their bandwidth. The present application requires a very efficient antenna and any losses must be minimised. 
         [0027]    An antenna will have an input impedance: 
         [0000]        Zin=Rin+j·Xin   (1)
 
         [0028]    For an electrically-small dipole, the real part, Rin will be small and Xin will be large and negative. The purpose of loading the antenna is to reduce the magnitude of Xin to ideally zero. The loading may be performed either capacitively or inductively. The minimum value of capacitance is required at the end of the dipole arms and the minimum value of inductance is near to the centre of the antenna. A mixture of capacitive and inductive loading may also be utilised for the design presented. 
         [0029]    The input resistance to the antenna is: 
         [0000]        Rin=Rloss+Rr   (2)
 
         [0030]    Rr is the radiation resistance, which will be of the order of a few ohms for the electrical length of the design presented. The loss resistance, Rloss, will increase the input resistance. However, the high efficiency requirement (98%) of the antenna means that the loss resistance must be of the order of tens of milliohms. The skin depth is around 0.02 mm in copper, which dictates the use of large diameter conductors. 
         [0031]    The design of the antenna described in connection with  FIGS. 1 and 2  is optimised for high power operation and a specified near-field distribution. High power operation means that the antenna design accounts for both high current sections and high voltage sections. The current in an electrically short dipole is maximum near the feed point or the centre of a dipole. Conversely, the voltage is maximum near to the end of the dipole. The high current density near the centre means that any surface resistance causes both unwanted losses and heating of the components. The loss in these areas is addressed in the current design by the selection of appropriate materials with adequate dimensions. The finite losses give rise to heating and adequate cooling is included in the design by forced air cooling. The cooling air is fed through the transmission lines  108  and  109 . The voltage at the centre of the design is moderate and increases significantly at the ends of the dipole. 
         [0032]    The operating frequency of the antenna  101  may be 13.56 MHz. At this frequency, high power off-the-shelf ruggedized RF power supplies are readily available, and may be used as the RF power source  203 . 
         [0033]    The loss requirement of the antenna  101  dictates that high conductivity metals should be used. For example, suitable materials for the antenna are copper and aluminium. At higher microwave frequencies, silver plating may be used to reduce the losses, however at the frequency of the proposed design, any plating would have to be relatively thick. 
         [0034]      FIGS. 3 ,  4 A to  4 D and  5 A to  5 D show various plots relating to the near-field distribution of the antenna  101  shown in  FIG. 1 .  FIG. 3  shows the orientation of the antenna  101  with respect to the plots of  FIGS. 4A to 4D  and  5 A to  5 D.  FIGS. 4A to 4D  show the near-field electric field distribution for the antenna  101 .  FIGS. 5A to 5B  show the near-field magnetic field distribution for the antenna  101 .  FIG. 4A  shows the Ex field on the ground about the centre line of the antenna.  FIG. 4B  shows the Ey field on the ground about the centre line of the antenna.  FIG. 4C  shows the Ez field on the ground about the centre line of the antenna.  FIG. 4D  shows the total electric field on the ground about the centre line of the antenna. 
         [0035]      FIG. 5A  shows the Hx field on the ground about the centre line of the antenna.  FIG. 5B  shows the Hy field on the ground about the centre line of the antenna.  FIG. 5C  shows the Hz field on the ground about the centre line of the antenna.  FIG. 5D  shows the total magnetic field on the ground about the centre line of the antenna. 
         [0036]    The antenna arrangement  100  is DC grounded for safety. The DC ground is provided by the twin-line balun  201  described above. The outer surface  215  of the coaxial transmission line  108  forms one arm of the twin-line balun  201 . At high frequencies, the RF currents effectively flow on the surface of a structure. In fact, the current density reduces as you move away from the surface and is negligible at distance of 5 skin depths. At high frequencies, the skin depth is fractions of a millimetre, and therefore any finite thickness tube could carry independent currents on its inside surface and its outside surface. The design of the balun  201  is such that the antenna feed currents flow on the inside surface of the outer coaxial transmission line  108 . The balun currents flow on the outside surface of the outer conductor of the coaxial transmission line  108 . The balun is a parallel wire transmission line that is short circuited at one end. The other end is connected to the feed point of the antenna  101 . 
         [0037]    The balun  201  is electrically shortened using high voltage capacitor  111 . The balun  201  is connected across the feed point of the centre fed dipole. The input impedance of the balun  201  is required to be very high, and ideally infinite, so as to prevent loading of the antenna  101 . The balun  201  would normally be designed to be one quarter of a wavelength long with a short circuit at one end. The low impedance short circuit would be transformed to a high impedance point by the action of the quarter wavelength long transmission line. In the present embodiment, there is insufficient physical space for a transmission line one quarter of a wavelength long, so the transmission line  108  is electrically loaded at its input with a lumped element, such as a capacitor  111 . The capacitor reactance resonates with the impedance of the electrically short short-circuited transmission line giving rise to a high input impedance. The capacitor  111  must be a high voltage component due to the power input to the antenna. 
         [0038]    As described above, distributed inductance loading is provided at the centre of the dipole using coupled coils  105 A,  105 B, having the same direction of winding. Spacing between the coil elements is designed for high power operation. An electrically-short dipole (i.e. one that is much less than one half of a wavelength long) requires either inductive or capacitive loading (or both) to make the antenna resonate and its input impedance real. The value of inductance is minimised if the inductors are located close to the centre of the dipole and conversely, the size of any capacitance is minimised if the capacitors are placed at the ends of the dipole. Any practical inductor has a physical length which means a value larger than minimum is required. The inductors for this design are physically long in order to prevent voltage breakdown between adjacent coils when operated at high powers. This large size tends to further increase the amount of inductance required. The orientation and winding direction of the two coils  105 A,  105 B is selected to maximise the coupling between the two coils so that the total inductance is minimised. 
         [0039]    The coil size and position is selected for generation of the magnetic component. The large size of the coils required for high power operation, together with both coils being wound in the same direction, yields a much higher magnetic component under the centre of the antenna than would be the case under a normal design. This field also has beneficial properties. 
         [0040]    The antenna arms  104 A,  104 B are end-loaded using conductive end plates  106 A,  106 B to electrically shorten the antenna  101  and provide significant electric field in the Z direction. The conductive end plates  106 A,  106 B provide the capacitive loading that permits the electrically-short antenna  101  to resonate. The conductive end plates  106 A,  106 B for antenna  101  are orientated so that the electric field is directed towards the ground rather than between each plate. This tends to increase the area required for each plate. The electrically-small antenna  101  has a high quality factor which gives rise to a large voltage at the ends of antenna arms  104 A,  104 B. The large voltage on the large conductive end plates  106 A,  106 B provides a very high electric field under the antenna  101 . 
         [0041]    End loading the antenna  101  to couple energy to the ground reduces the quality factor of the antenna. Because the conductive end plates  106 A,  106 B are directed towards the ground, significant electric field is directed towards and coupled into the ground. The high coupling with the ground and the lossy nature of the ground serve to reduce the Q of the antenna. In order to prevent corona discharge when operating at high powers, the shape of the conductive end plates  106 A,  106 B needs to be considered. The voltage at the ends of the antenna arms  104 A,  104 B will be approximately Q times the feed voltage, where Q is the Quality factor of the antenna  101 . An electrically-small antenna, with inductive and capacitive loading to make it resonate, will have a large Q, and the voltage at the ends of the antenna arms  104 A,  104 B will be considerable. In order to prevent corona discharge, the ends of the conductive end plates  106 A,  106 B are shaped with a suitably large radius of curvature. Any other features must be blended to avoid points or sharp protrusions. 
         [0042]    The conductive end plates  106 A,  106 B of the antenna  101  will be at a high voltage in use. The conductive wires  107 A,  107 B between the coils  105 A,  105 B and conductive end plates  106 A,  106 B will be at a lower voltage. The higher voltage conductive end plates  106 A,  106 B will effectively shadow the conductive wires  107 A,  107 B near to where the wires join the conductive end plates  106 A,  106 B. This means that features such as supporting brackets for supporting arms  112 A,  112 B may be fixed to the conductive wires  107 A,  107 B close to the conductive end plates  106 A,  106 B with less chance of corona discharge. 
         [0043]    In an alternative embodiment, the conductive end plates could be arranged so that field is constrained between the plates. However, the above-described embodiment results in more energy being coupled to the ground. 
         [0044]    In one or more embodiments, the balun  201  is electrically shortened using the high voltage capacitor  111 . The current in the conductors is at a maximum at or adjacent a connection with the feed structure. The voltage in the conductors is at a maximum at the distal ends each respective conductor. The feed structure is arranged such that the antenna feed currents flow on the inside of the coaxial feed line  108 . The balun currents flow on the outside surface of the coaxial feed line  108 . The capacitive plates  106 A,  106 B and inductive coils provide loading which makes the antenna&#39;s input impedance real. The orientation and winding direction are selected to maximise coupling between coils so total inductance is minimised. The capacitive plates electrical shorten the antenna and provide significant E field in the z direction. The end plates may be 100 mm thick and the radius may by 50 mm. 
         [0045]    The present invention is not limited by the aforementioned description, and any practical variations within the spirit and scope of the claims are permissible.