Abstract:
A low profile or very low profile antenna having a finely tunable frequency response is provided. The antenna is easily tuned to have a precise frequency response while its protective radome is in place. The antenna generally includes a first antenna element, a second antenna element, a ground plane, and a tuning mechanism. The tuning mechanism may be accessed while the radome of the antenna is in place. The antenna is particularly well-suited for use in shipping applications, where a very low antenna profile is desirable.

Description:
FIELD OF THE INVENTION 
     The present invention relates to radio frequency tunable antennas. In particular, the present application relates to very low profile antennas having one or more tuning elements. 
     BACKGROUND OF THE INVENTION 
     Antennas are used to radiate or receive radio wave signals. The transmission and reception of radio wave signals is useful in a broad range of activities. For instance, in the overland shipping industry, it is desirable to be able to track the exact location of goods or materials while in transit from a central location. In addition, it is desirable to be able to direct the performance of certain functions with respect to such goods from a central location. However, it is important that any antenna used in connection with such an application have a low profile so that it does not interfere with the normal functioning of, for example, the trailer or shipping container to which the antenna is affixed. 
     Increasingly, raw materials and finished goods are shipped by tractor trailer. A typical tractor trailer, or semi, consists of a tractor to provide motive force, and a trailer of approximately 40 feet in length that is supported at a front end by the tractor. Tractors and trailers are equipped with a standard hitching mechanism to allow interchangeability between a large number of tractors and trailers. In addition to being towed by tractors, trailers may be placed on specially adapted rail cars. Therefore, in a typical scenario, a trailer may be loaded with goods at a factory and hitched to a tractor for transport to a rail facility. The trailer may then be uncoupled from the tractor and placed on a rail car. The rail car may then carry the trailer to another rail depot where the trailer may be lifted from the rail car and interconnected with a second tractor for transport to its final destination 
     Because of the large carrying capacity of modern trailers, and because of the large number of goods being transported in such trailers, shipping companies and the owners of goods and materials transported by tractor trailers require precise and up to date information regarding the location of their goods and materials while they are in transit. Existing methods for tracking tractor trailers include noting the location of the trailers as reported by the drivers operating the tractor to which a trailer is attached. However, such information is only as timely as the last report received from the driver. Also, such a system relies entirely upon the driver to accurately report the position and condition of the goods or materials. Furthermore, such systems offer no way for a company to track the whereabouts of a shipment that has been hijacked or stolen from a temporary storage facility. 
     One proposed method of tracking the location of trailers or shipping containers combines a global positioning satellite (GPS) system with a radio transmitter. Such a system allows position information as determined by the GPS receiver to be communicated to the radio transmitter, which in turn transmits the position information to a central site. Also, the transmitter may be used to communicate information regarding such things as the ambient temperature inside the trailer. Systems combining GPS receivers and radio transmitters may additionally include a radio receiver. The provision of a radio receiver allows certain commands to be transmitted from the central control site to individual trailers. Therefore, commands may be sent from the central control site to, for example, operate the refrigeration unit on trailers equipped to carry refrigerated goods. With such a system, the shipping company need not rely on the driver to operate the refrigeration unit. Another example of the usefulness of a radio receiver on a trailer is to allow central control of the locking and unlocking of a trailer. Yet another example of the advantages offered by a transmitter and receiver on a trailer is the provision of revised routing instructions to the driver through a computer interconnected to the antenna. 
     Where a radio antenna is to be provided on a trailer, it is desirable that such an antenna have a low or very low profile. In a typical trailer having an enclosed space for containing the items to be transported and thereby protecting them from theft and the elements, the trailer is as tall as possible given the constraints imposed by overpasses, tunnels, and applicable laws. Accordingly, any antenna structure affixed to such a trailer must not add appreciably to the trailer&#39;s height. In addition, such trailers must be capable of reliable operation in all types of weather. Therefore, it is desirable that an antenna affixed to the exterior of a trailer be protected from the elements. A further desirable attribute of an antenna to be placed on the exterior of a trailer is that it not have a deleterious effect on the aerodynamic drag of the trader. All of these requirements are met by an antenna having a low or very low profile. Furthermore, these requirements are advanced by placing the antenna within an enclosure. 
     With the increasing concern for saving fuel, the aerodynamics of tractor trailers have received more and more attention. One commonly adapted measure to improve the aerodynamics of enclosed trailers is to provide a sloping top surface. Therefore, trailers are commonly provided with a top that rises approximately one inch from the leading edge of the trailer to approximately 14 feet behind the leading edge of the trailer. Accordingly, at the front edge of the trailer, there is an area that is approximately one inch below the highest extent of the trailer. Therefore, if an antenna unit having a height of about one inch or less were provided, it would not add appreciably to the height of the trailer. In addition, an antenna having a height of one inch or less would have little effect on the aerodynamics of the trailer. An antenna having a small height with respect to the operating wavelength is generally known as a very low profile antenna (VLPA). 
     Although placing the antenna within an enclosure or radome protects the antenna from the elements and helps maintain the aerodynamics of the trailer, these enclosures affect the tuning of the antenna. Also, the placement of an antenna on a large surface, such as on the top of an enclosed trailer, also affects the tuning of the antenna. Therefore, it would be advantageous to provide a low profile or very low profile antenna that could be tuned with its radome in place. Furthermore, it would be desirable to provide such an antenna that could be tuned with consideration given to the effect the antenna&#39;s ultimate operating environment has on its tuning. 
     Antennas having a low or very low profile generally have a very high Q value. An antenna with a high Q value has relatively high sensitivity over a relatively narrow range of frequencies. Therefore, for such an antenna to be adequately sensitive over its intended useful frequency range, it must be precisely tuned. Generally, the operating frequency of a patch antenna element can be altered by altering the length of the element or by altering the height of the element above the ground plane. 
     According to an existing method for tuning antennas, the length of the antenna elements are trimmed. However, this method of tuning antennas is tedious and time consuming. Also, this method produces a large amount of waste, resulting in a messy assembly area. Furthermore, this method of tuning antennas is irreversible; an antenna that has been over-trimmed cannot be made to meet the required scations, and must generally be discarded. Additionally, such designs are incapable of being tuned with an associated radome in place, and therefore require that the person tuning the antenna anticipate the effect that installation of a radome will have on the useful frequency range of the antenna. 
     Other existing antenna designs that provide a tuning mechanism to obtain optimal performance have required a large number of additional components to provide tunability. For example, some such designs provide an additional ground plane, the distance of which from the antenna element is adjustable using screws and wing nuts. However, the addition of such components results in an antenna having a greatly enlarged size and complexity. In other designs, dielectric material is placed in close proximity to the antenna element to vary the load on that element and thereby tune the antenna. Such designs introduce additional complexity and result in unnecessary electrical losses. These designs are also too large for use in very low profile antenna applications. 
     Another existing design incorporates ribbon antenna elements located adjacent to a ground plane. One end of the ribbon-shaped element is affixed to the ground plane, while the other is held above the ground plane by a nonconductive screw. The height of the element above the ground plane may be varied by adjustment of the screw. This design requires that the ribbon element be flexible, to prevent binding of the tuning screw during the tuning process. In addition, these designs do not provide a way to tune the antenna with a radome in place. Furthermore, this design provides no protection against short circuits when the tuning screw is adjusted so that the ribbon element is in very close proximity to the ground plane. When used in an environment with significant amounts of vibration, the ribbon element can move away from the head of the tuning screw, thus allowing the antenna to lose its tuning. 
     For the above-stated reasons, it would be advantageous to provide a low profile or very low profile antenna that is capable of being tuned with high precision. In addition, it would be advantageous to provide such an antenna that is highly resistant to de-tuning or failure due to vibrations in the antenna&#39;s operating environment. Concomitantly, such an antenna must be reliable, inexpensive to manufacture, and easily tuned. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a very low profile antenna is disclosed having tunable elements. The antenna includes a first antenna element, a second antenna element, a ground plane, and at least one tuning element. The tuning element operates by varying the height of at least one end of the first and/or second antenna elements over the ground plane. The distance between the tuning elements may also be varied. 
     The antenna elements are generally planar, and typically have a width that is at least about 80% of the element&#39;s length. Each of the antenna elements is provided with a tuning element at a first end. At a second end, the antenna elements are electrically and mechanically interconnected to the ground plane. The distance between the first and second antenna elements is about {fraction (1/400)} th  of the wave length of the signal at the operating frequency of the first antenna element. Similarly, the distance between the second antenna element and the ground plane is about {fraction (1/400)} the wavelength of the signal at the operating frequency of the second antenna element. In a preferred embodiment of the present invention, the assembly including the ground plane and the antenna elements is less than 1″ in height. 
     Generally, there is a progression in size from the first antenna element to the ground plane. Thus, the first antenna element has a length and a width, and the second antenna element has a length and a width that are slightly larger than those of the first antenna element. Similarly, the ground plane has a length and a width that are slightly larger than those of the second antenna element. According to one embodiment, the width of the antenna elements and the ground plane are at least about 80% of their lengths. 
     In addition to the direct mechanical interconnection between the first and second antenna elements and the ground plane at a second end, and the tuning elements provided at a first end of the first and second antenna elements, the elements and the ground planes may be spaced apart using nonconductive mechanical spacers. According to one embodiment, reception and transmission signals are communicated between the antenna and the associated transceiver by a coaxial feed line. This feed line may be electrically interconnected to one of the antenna elements. The feed line may also be interconnected to another of the antenna elements through a jumper cable. The length of the jumper cable is selected to match the impedance of the antenna element to which it is interconnected with the impedance of the feed line and the transceiver circuitry. 
     The antenna can be provided with a radome to protect the antenna from the elements when mounted on a trailer or other shipping container. The radome also ensures that the antenna does not degrade the aerodynmic efficiency of the trailer. The antenna may be tuned by accessing the tuning elements through the bottom of the ground plane while the radome is in place. In one embodiment, the antenna is tuned with its radome in place, and while positioned on a second ground plane having dimensions that approximate those of the top of a trailer. According to this embodiment, the tuning element or elements are accessed through both the first and second ground planes while the antenna radome is in place. Once the antenna has been tuned, the tuning elements may be permanently secured in position by gluing or welding. According to one embodiment of the present invention, the antenna, with the radome in place, is less than or equal to about 1″ in height. 
     Based on the foregoing summary, a number of salient features of the present invention are readily discerned. A very low profile antenna can be provided having a frequency that is tunable while its radome is in place. The antenna of the present invention may also be tuned in an environment that approximates that of its ultimate operating environment, enabling the antenna to be very finely tuned. Additionally, the antenna of the present invention has a very low profile and can be unobtrusively mounted on the exterior of a trailer or other shipping container. In another embodiment, where first and second tuning elements are used as separate transmit and receive elements, they can each be separately tuned to have a very precise frequency response. 
     Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a very low profile antenna in accordance with the present invention, installed on a trailer, 
     FIG. 2 is a top perspective view of an antenna system in accordance with an embodiment of the present invention; 
     FIG. 3 is an exploded perspective view of an antenna system in accordance with the present invention; 
     FIG. 4 is a perspective view of a tuning element in accordance with the present invention; 
     FIG. 5 is a partial side view of an antenna system according to an embodiment of the present invention illustrating the interconnections between the antenna feed line and the first and second antenna elements; 
     FIG. 6 is a partial side view of an antenna system according to an embodiment of the present invention showing the tuning elements in a first position; 
     FIG. 7 is a partial side view of an antenna system according to an embodiment of the present invention showing the tuning elements in a second position; and 
     FIG. 8 is a bottom perspective view of an antenna system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention, an antenna system having a very low profile and having tunable elements is provided. 
     With reference to FIG. 1, an antenna system  104  according to the present invention is shown affixed to the top of a trailer  103 . As shown in FIG. 1, the antenna system  104  is generally encased in a radome, and has a very low profile. When located as illustrated in FIG. 1, the antenna system  104  does not add to the height of the trailer  108 . This is because the trailer  108  is, for aerodynamic purposes, approximately one inch lower at its front end  112  than at the rear  118 . However, even when affixed to a trailer  108  not having a sloped top section, the antenna system  104  does not add appreciably to the height of the trailer. This is because the antenna system  104 , in a preferred embodiment, has an overall height of less than about one inch. Also, it can be seen from FIG. 1 that the antenna system  104  is rectangular in shape, is without protrusions and is mounted flat against the top of the trailer  108 . 
     Referring now to FIG. 2, the antenna system  104  of the present invention is illustrated with the radome removed. The antenna system  104  generally comprises a ground plane  204 , a relatively rigid first antenna element  208  and a relatively rigid second antenna element  212 . The ground plane  204  is substantially planar, and is rectangular in shape. Directly above and substantially parallel to the ground plane  204  is the second antenna element  212 . The second antenna element  212  is substantially planar in shape, and features a first end portion  216  that is spaced apart from and generally parallel to the ground plane  204 . At a second end portion  220  the second antenna element  212  turns down to join the ground plane  204  at tab  224 . Accordingly, the second antenna element  212  is mechanically and electrically interconnected to the ground plane  204  at one end  220 . 
     The first antenna element  208  has a first end portion  228  that is spaced apart from the second antenna element  212 . In general, this first antenna element  208  is substantially parallel to the second antenna element  212 . At a second end portion  232  of the first antenna element  208 , the first antenna element  208  turns down to meet the second antenna element  212  and the ground plane  204  at tab  236 . Accordingly, the first antenna element  208  is mechanically and electrically interconnected to the second antenna element  212  and the ground plane  204  at one end  232 . 
     Also illustrated in FIG. 2 is the feed line  252 , which interconnects the antenna system  104  to the transceiver (not shown). The feed line  252  passes through a hole  256  formed in the ground plane  204 . After entering through the hole  256 , the feed line  252  is held in position by a clamp  260  affixed to the ground plane  204 . Clamp  260  also electrically interconnects the ground plane to the coaxial shield of the feed line  252 . The feed line  252  then terminates at a first transmission connector  264 , which conducts transmission signals to the first antenna element  208  from the feed line  252 . A jumper  268  is also connected to the first transmission connector  264 . The jumper  268  communicates radio frequency signals received by the second antenna element  212  through a second transmission connector  272  to the feed line  252 . The length of the jumper  268  is chosen to facilitate the matching of the impedance of the second antenna element  212  to the impedance of the feed line  252  and the impedance of the transceiver (not shown). By providing matched impedances, the antenna system  104  can provide an antenna having low reflection losses for both the first antenna element  208 , which is used to transmit radio frequency signals in the illustrated embodiment, and the second antenna element  212 , which is used to receive radio frequency signals in the illustrated embodiment. Furthermore, the antenna system  104  is capable of doing so even where the first antenna element  208  and the second antenna element  212  have different operating frequencies and different physical properties and dimensions. For example, in one embodiment, the operating frequency of the first antenna element  208  may be in the range of about 148-150 MHz and the operating frequency of the second antenna element  212  may be in the range of about 137-138 MH. 
     Referring now to FIG. 3, the radiating antenna elements  208  and  212  and the ground plane  204  are illustrated in an exploded view. As shown in FIG. 3, it can be seen that the rectangular antenna elements  208  and  212  and the rectangular ground plane  204  are aligned such that their sides are substantially parallel to one another, and such that they occupy substantially parallel planes. Also illustrated in FIG. 3 is one method of affixing the second end portions  220  and  232  of the antenna elements  208  and  212  to the ground plane  204 . Specifically, the clamp  260  receives fasteners  308 , which pass through the ground plane  204 , the tab  224  of the second antenna element  212 , and the tab  236  of the first antenna element  208 , to secure these components to one another. 
     A first tuning element  312  and a second tuning element  316  are located to engage the first antenna element  208  and the second antenna element  212  respectively, at holes  320  and  324  in the first end portions  216  and  228  of those elements  208  and  212 . In the illustrated embodiment, the holes  320  and  324  are located in a corresponding corner of the first portions  216  and  228  of the first  208  and second  212  antenna elements. However, the tuning elements  312  and  316  may be placed anywhere on the antenna elements  208  and  212  that is sufficiently distal from the interconnection of the antenna elements  208  and  212  to the ground plane  204  that adjustments to the tuning elements  312  and  316  are effective in varying the heights of substantial portions of the antenna elements  208  and  212  above the ground plane  204 . 
     Referring now to FIG. 4, the design of the tuning elements  312  and  316  is illustrated in detail. The tuning elements  312  and  316  generally include a shaft  404 , threads  408 , a slotted tip  412 , a first enlarged head  416  and a second enlarged head  420 . The slotted tip  412  of the tuning element  312  and  316  is adapted to receive a screwdriver when fine tuning of the antenna is desired. Alternatively, the slotted tip  412  may comprise a hexagonal head for turning by a socket, or any other known method for providing a fixture that can be turned by a tool or directly by hand. The first enlarged head  416  of the antenna tuning element  312  and  316  is separated from the second head  420  by an extension of the shaft  404 . The distance between the heads  416  and  420  is determined by the thickness of the antenna element  208  and  212  that is to be received between them. According to one embodiment, the first head  416  is provided with a slot or recess or other shape for interconnection with a tool adapted to impart rotation to an element. The provision of such a fixture is useful in the assembly of the antenna  104 . In a preferred embodiment, the tuning element  312 ,  316  is constructed from a nonconductive material. For example, the tuning elements  312 , 316  may be constructed from nylon. 
     Referring again to FIG. 3, the slotted holes or openings  320  and  324  comprise first enlarged portions  328  sized to allow a first head  416  of the antenna tuning elements  312  and  316  to pass through. The holes  320  and  324  are further provided with a narrowed portion  332  having a width that is approximately equal to the diameter of the shaft  404  of the tuning elements  312  and  316 . A clearance hole  336  is provided in the first end portion  216  of the second antenna element  212  to allow the first tuning element  312  to pass through the second antenna element  212  and engage the first of two threaded holes  340  provided in the ground plane  204 . The second tuning element  316  engages the second hole  340  in the ground plane  240 . 
     The first and second antenna elements  203  and  212  are spaced apart from each other and from the ground plane  204  by non-conductive spacers or bushings  344 . In the illustrated embodiment, the spacers  344  are generally located in a second corner of the first portion  216  and  223  of the first  208  and second  212  antenna elements. Fasteners  308  are passed through the holes  348  in the first  208  and second  212  antenna elements to engage the ground plane  204  at the provided holes  304 . 
     Referring now to FIG. 5, the transmission feed line  252  and its interconnections to the first  208  and the second  212  antenna elements are shown. The antenna feed line  252  passes through holes  504  and  508 , formed in the first  208  and second  212  antenna elements, and proceeds between the second antenna element  212  and the ground plane  204  to a point generally towards the middle of the antenna system  104 . The antenna feed line  252  is interconnected to the first antenna element  208  through a first terminal connector  512  that is connected to a first transmission connector  264 , which passes through a hole  516  formed in the second antenna element  212  to connect to the first antenna element  208 . In the illustrated embodiment of the antenna system  104  of the present invention, the first transmission connector  264  carries transmission signals from the transceiver through the feed line  252  and to the first antenna element  208  for transmission. Also interconnected to the first transmission connector  264  is a second terminal connector  520 , which is interconnected to a jumper cable  268 . The jumper cable  268  terminates in a third terminal connector  524 , which is interconnected to the second antenna element  212 . In the illustrated embodiment of the antenna system  104  of the present invention, the second antenna element  212  is adapted to receive radio frequency signals and to communicate those signals to the transceiver (not shown) through the jumper cable  268  and the feed line  252 . 
     Because the first antenna element  208  and the second antenna element  212  are, in one embodiment, adapted to transmit or receive in different frequency ranges, they differ from one another in size. This results in the antenna elements  208  and  212  having differing characteristic impedances. In any electronic system, it is desirable to match the characteristic impedances of the various components to reduce losses and therefore reduce power demands in the system. According to the present invention, the characteristic impedance of the first antenna element  208  is matched to the characteristic impedance of the feed line  252  and the transceiver. The jumper  268  in effect alters the characteristic impedance of the second antenna element  212  seen by the feed line  252  and the transceiver. By carefully choosing the length of the jumper cable  268 , the characteristic impedances of the first antenna element  203 , the second antenna element  212 , the feed line  252  and the transceiver can all be matched. 
     Referring now to FIG. 6, the first  312  and second  316  tuning elements are shown in elevation in a first position. It can be seen that the first antenna element  208  is held between the first  416  and second  420  heads of the first tuning element  312 . By screwing the tuning elements  312  and  316  in or out of the threaded hole  340  provided in the ground plane  204 , the height of the antenna elements  208  and  212  will be changed in relation to the ground plane  204 . By adjusting the tuning elements  312  and  316  individually, the height of a corresponding  20  one of the antenna elements  208  or  212  can be adjusted with respect to the ground plane  204  and to the other antenna element  208  or  212 . In this first position, illustrated in FIG. 6, the first end portions  216  and  228  of the antenna elements  208  and  212  can be seen to be substantially parallel to the ground plane  204 . Also, in this first position, the first end portions  216  and  228  are substantially perpendicular to the tuning elements  312  and  316 . 
     Also illustrated in FIG. 6 is the radome  604  of the antenna system  104 , shown partially cutaway. The radome  604  generally rises from the ground plane  204  to envelope the antenna elements  208  and  212 . At the interface between the radome  604  and the ground plane  204 , the joint is preferably made water tight by application of a sealant or adhesive. 
     In FIG. 7, the first  312  and second  316  tuning elements are illustrated in a second position. As shown in FIG. 7, the first antenna element  208  and the second antenna element  212  are not parallel to each other along their first portions  216  and  228 . The first portions  216  and  228  are also not parallel to the ground plane  204 . Furthermore, the first end portions  216  and  228  of the antenna elements  208  and  212  are not perpendicular to the tuning elements  312  and  316 . According to one embodiment of the present invention, at an extreme of adjustment, the first end portions  216  and  228  are at an angle of from 85° to 95° to the tuning elements  312  and  316 . This is a result of a bending of the first  208  and second  212  antenna elements by adjusting the first  312  and second  316  tuning elements. This is in contrast to the first position of the first tuning element  312  and second tuning element  316  illustrated in FIG. 6, in which the first portions  216  and  228  of the antenna elements  208  and  212  are substantially parallel to one another and parallel to the ground plane  204 , and substantially perpendicular to the tuning elements  312  and  316 . 
     The first position of the antenna tuning elements  312  and  316  illustrated in FIG. 6 represents a starting point for the adjustment of the operative frequencies of the tuning elements  208  and  212 . Because the operating frequency of a patch antenna such as the antenna system  104  the present invention is determined by the height and length of the antenna element, that frequency can be adjusted by altering the height of the element above the ground plane. Where there is more than one antenna element, such as in the antenna system  104  illustrated in FIG. 6, the operating frequencies of each of the antenna elements  208  and  212  must be individually tuned. Furthermore, adjustments to the height of one of the antenna elements  208  and  212  above the ground plane  204  also changes the distance of that element from the other antenna element, affecting the operating frequency of that other element. Accordingly, the actual tuning of a multiple element antenna is an iterative process in which the individual elements are alternately tuned until the operating frequencies of each of the elements is satisfactory. 
     At extremes of adjustment, the distance between the antenna elements  208  and  212  and/or the ground plane  204  may become quite small. Such a configuration is illustrated in FIG. 7, where the first  208  and second  212  antenna elements are very close together. In particular, it can be seen that the first portion  216  of the second antenna element  212  turns about the second head  420  of the first tuning element  312  before extending to meet the second tuning element  316 . The second head  420  of the first tuning element  312  thus prevents the first  208  and second  212  antenna elements from being shorted to one another as a result of the adjustment. A short circuit between the antenna elements  208  and  212  would render them useless at their intended operating frequencies. 
     Although the relationship illustrated in FIG. 7 is an extreme, short circuit protection is provided by the unique design of the antenna tuning elements  312  and  316 . Specifically, the two head  416  and  420  design of the tuning elements  312  and  316  positively prevents the antenna elements  208  and  212  from moving relative to the tuning elements  312  and  316  Accordingly, the antenna system  104  provides a tunable antenna system that is reliable in environments having a significant amount of vibration. 
     Referring now to FIG. 8, a bottom perspective view of an antenna constructed in accordance with an embodiment of the present invention is illustrated. In FIG. 8, the antenna tuning elements  312  and  316  are shown extending slightly from the ground plane  204 . Therefore, it is evident that the antenna tuning elements  312  and  316  are accessible even when the radome  604  is affixed to the ground plane  204 . Also illustrated in FIG. 8 are the bottoms of the fasteners  308 . In order to tune the antenna system  104 , the feed cable  252  is interconnected to a test transceiver (not shown) and radio wave signals are transmitted and received by the antenna system  104 . An operator turns the first tuning element  312  to adjust the frequency at which the first antenna element  208  is most sensitive. The second tuning element  316  is then turned to adjust the operating frequency of the second antenna element  212 . Because adjusting the tuning of the second antenna element affects the tuning of the first antenna element, the operable frequency of the first antenna element is checked and readjusted if necessary. Then, the second antenna element&#39;s operable frequency is re-checked and adjusted as necessary. This process continues until both elements are tuned to the desired operating frequencies. 
     In a preferred embodiment, the antenna system  104  is placed on a second ground plane (not shown) having dimensions approximating that of the top of a trailer. Holes corresponding to the positions of the antenna tuning elements  312  and  316  are provided in the second ground plane to permit access to the tuning elements  312  and  316 . The antenna system  104  is then tuned as described above. By tuning the antenna system  104  on this second ground plane, the operating frequencies of the antenna elements  208  and  212  can be more precisely determined and adjusted. This is because the provision of the ground plane allows the test environment to closely approximate the actual operating environment of the antenna system  104 . 
     After the radome  240  has been glued or otherwise affixed to the ground plane  204 , sealing the antenna system  104  against intrusion by water or dust, and the antenna system has been tuned, the antenna tuning elements  312  and  316  can be permanently fixed in position. The antenna tuning elements  312  and  316  can be permanently fixed using glue or ultrasonic welding. 
     According to an embodiment of the present invention for use with a first antenna element  208  having an operating frequency of 148 to 150 MHz and a second antenna element  212  having an operating frequency of 137 to 138 MHz the ground plane  204  is about 465 mm in length, and about 380 mm in width. The first antenna element  208  is about 500 mm length, and about 380 mn in width. The second antenna element  212  is about 530 mm in length, and about 380 mm in width. The first antenna element  208  is about 5.3 mm from the second antenna element  212 , which is about 5.3 mm from the ground plane  204 . In a preferred embodiment, the antenna elements  208  and  212  and the ground plane  204  are constructed from an electrically conductive material. In a more preferred embodiment, the antenna elements  208  and  212  and the ground plane  204  are constructed from aluminum. The radome  604  of the present invention is preferably constructed from a material that is transparent to radio frequency waves. In a preferred embodiment, the radome  604  is constructed from an ABS/PVC composite sheet. 
     In accordance with the present invention, a very low profile antenna that can be very precisely tuned is provided. The invention in its broader aspects relates to a low profile antenna that can be very precisely tuned. The antenna is suitable for use in any application requiring an antenna having a high sensitivity over a narrow range of frequencies, and a low or very low profile. The apparatus can be easily and accurately tuned and is designed to operate reliably, even in an environment such as the exterior of a trailer. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best mode presently known of practicing the invention and to enable others skilled in the art to utilize the invention in such or in other embodiments and with various modifications required by their particular application or use of the invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.