Abstract:
An antenna system with at least one tunable dipole element with a length adjustable conductive member disposed. The element is made of two longitudinally aligned, hollow support arms made of non-conductive material. Disposed longitudinally inside each element is a length adjustable conductive member electrically connected at one end. Each conductive member is stored on a spool that is selectively rotated to extend the conductive member into the support arm. The support arms are affixed at one end to a rigid housing. During use, the conductive members are adjusted in length to tune the element to a desired frequency. The antenna can be optimally tuned at a specific frequency for maximum gain, maximum front-to-back ratio, as a bi-directional antenna, and to provide a desired feed point impedance. An electronic control system allows the length of the conductive members to be manually or automatically adjusted to a desired frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of U.S. Provisional Patent Application Ser. No. 60/743,874, filed Mar. 28, 2006, the entirety of which is incorporated by reference herein. 

   BACKGROUND 
   1. Technical Field 
   The present invention relates to the field of radio antennas, and to wide frequency coverage vertical, dipole and parasitic array antennas. More particularly, the present invention relates to an adjustable antenna element, and to antenna systems employing one or more such adjustable elements. 
   2. Description of the Prior Art 
   Antenna systems employing a single antenna having adjustable-length elements providing excellent performance over a wide frequency range are known in the art. Examples of such antenna systems are the antenna systems manufactured and sold by Steppir Antennas of Issaquah, Wash., and include dipole, vertical, and yagi antennas. 
   A limiting factor in prior-art antennas is that, as the frequency of operation of the antenna becomes lower, the physical length of the antenna element must increase to allow it to resonate at the selected frequency. For example, in the case of a yagi antenna having two or more elements extending outward from a boom support arm, element lengths of up to 70 feet are necessary for operation at frequencies in the 40-meter band (7.0 through 7.3 MHz). For operation in the 80 meter band (3.5 through 4.0 MHz) element lengths are up to 140 feet. Of course, elements such as loading coils can be used to shorten the physical lengths of the antenna elements, but they degrade the performance of the antenna. 
   Mechanical considerations for constructing such antennas become more complicated as the element lengths increase as the operating frequency decreases. Considerations such as mechanical stress and wind survivability make the design of such adjustable antenna systems more challenging when long element lengths are necessary. 
   BRIEF DESCRIPTION 
   Disclosed herein is an adjustable antenna element for use in an antenna system of the type that employs adjustable-length conductive members that are deployed in hollow support arms and use a means such as a stepper motor for adjusting the length of the two conductive members inside the support arms. 
   The present invention is particularly suitable for use in antenna systems of the type disclosed in U.S. Pat. No. 6,677,914. The entire disclosure of U.S. Pat. No. 6,677,914 is expressly incorporated herein by reference. 
   The antenna element of the present invention comprises a hollow support arm formed from non-conductive material and having a first section extending in a first direction for a first length, a curved transition section and a second section extending in a second direction for a second length. As presently preferred, the first and second sections are essentially equal in length and the curved transition section is formed as a 180° radial curve, although other angles and shapes could be used. A length-adjustable conductive member is disposed in the hollow support arm. A length-adjuster is configured to adjust the length of the conductive member disposed in the hollow support arm. If the antenna element is to be a driven element, a transmission-line is electrically coupled to the conductive member. 
   In the preferred embodiment, the conductive member is adjusted by employing two spools located inside the housing unit in which the conductive member is wound. During use, the conductive member is selectively wound and unwound from a spool so that the conductive member moves inside the support arm. At least one motor is provided inside the housing unit that rotates the spool to precisely control the length of the conductive member inside the support arm. 
   According to one particular embodiment of the invention, the conductive member is formed from a beryllium-copper strip that travels out into a rigid, hollow fiberglass support tube. The fiberglass tube has a 180° “sweep” tube with a radius (12″ has been found to be suitable) that the tape follows around to an identical fiberglass tube that then guides the tape back towards the other end of the support tube. By doing this, the overall length of the antenna element required for operation of a yagi antenna on 40 meters has been reduced from 64 feet to down to a mere 38 feet. 
   In one embodiment of antenna systems where the adjustable antenna element is to be attached to a boom support structure, both ends of the hollow support arm are disposed in the same horizontal plane. One end of the hollow support arm is attached to the boom by a housing that contains the length adjuster apparatus. The other end of the hollow support arm is mechanically attached to the boom. In another embodiment of antenna systems where the adjustable antenna element is to be attached to a boom support structure, one end of the hollow support arm is attached to the boom by a housing that contains the length adjuster apparatus. The other end of the hollow support arm is disposed at a vertical position either above or below the boom and is mechanically attached to the boom using a suitable support bracket. 
   In an antenna system according to the present invention that is configured as a dipole antenna, first and second hollow support arms are formed from a non-conductive material and each have a first section extending in a first direction for a first length, a curved transition section and a second section extending in a second direction for a second length. As presently preferred, the first and second sections are essentially equal in length and the curved transition section is formed as a 180° radial curve but other angles and shapes could be used. First and second length-adjustable conductive members are disposed, respectively, in the first and second hollow support arms. A length-adjuster is configured to adjust the lengths of the first and second conductive members disposed in the first and second hollow support arms. If the antenna element is to be a driven element, a transmission-line coupler is electrically coupled to the first and second conductive members. 
   In an antenna system according to the present invention that is configured as a yagi antenna, at least one element of the antenna is comprises first and second hollow support arms formed from a non-conductive material, each having a first section extending in a first direction for a first length, a curved transition section and a second section extending in a second direction for a second length. As presently preferred, the first and second sections are essentially equal in length and the curved transition section is formed as a 180° radial curve but other angles and shapes could be used. First and second length-adjustable conductive members are disposed, respectively, in the first and second hollow support arms. A length-adjuster is configured to adjust the lengths of the first and second conductive members disposed in the first and second hollow support arms. If the antenna element is to be a driven element, a transmission-line coupler is electrically coupled to the first and second conductive members. The other elements of the yagi antenna may be configured in accordance with the present invention or may be configured as disclosed in U.S. Pat. No. 6,677,914. 
   This method is a form of linear loading that is done at the end of the element instead of the middle as is common in most antennas currently on the market. By doing the loading at the tip the very important high current area of the element near the middle is avoided. This is especially important when the element is used as a yagi element because it preserves the pattern (F/R) and gain much better than linear loading at the middle of the element. When used as a single element dipole this element is only −0.15 db below a full size dipole. The tape must be about 10% longer than it would be if it was laid out straight, but this presents no problems in the implementation or use of this element. 
   One significant advantage of folding the antenna element in this manner is that it lowers the impedance from the usual 50-70 ohms (depends on height above ground) of a full-length dipole down to 25 ohms. This allows using a 2:1 un/balun to match the dipole on 40 meters with 1.0:1 SWR without any switching by relays or otherwise. On 30 meters the tape forming the conductive member is barely around the sweep so it acts very much like a full length dipole and has an impedance of between about 50 ohms and about 70 ohms, resulting in a 2.0:1 SWR. However, it has been discovered that, in a yagi antenna employing the elements of the present invention, when fully extended the yagi director element intended for 20 meter and higher operation interacts with the dipole and lowers the SWR to 1.8:1 and also results in 2.68 dbi of gain and 1 db F/R. 
   As an illustrative example, using the antenna element of the present invention provides a stand-alone dipole antenna that can resonate at frequencies spanning wavelengths between 40 meters and 6 meters using an element having a length of only 38 feet. Such an element presents only 4 square feet of windload and its gain is only 0.15 db below a full size dipole on 40 meters. 
   The above antenna system is especially advantageous when configured as a Yagi-style antenna that can be optimally tuned at a specific frequency for maximum gain, maximum front-to-back ratio, and to provide a desired feed point impedance at the driven element. This allows a very large continuous range of frequencies to be covered with excellent performance and a very low voltage-standing-wave-ratio (VSWR) while using only one feed line. By using length adjustable elements and a shorter boom, the antenna system is able to achieve better performance than prior art antenna designs. Also incorporated into it is a Yagi-style antenna, enabling it to be quickly adjusted to change the direction of maximum signal strength 180 degrees by changing the length of the designated director to make it function as a reflector and conversely changing the length of the reflector to make it function as a director. In should also be understood that the antenna system can also function as a bi-directional style antenna by adjusting the reflector element to function as a director. 
   An electronic control system is provided that manually or automatically adjusts the length of the conductive members inside the antenna driven and parasitic elements to receive or transmit a desired frequency. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       FIG. 1  is a drawing depicting an adjustable antenna element according to the present invention. 
       FIG. 2  is a drawing depicting a dipole antenna employing adjustable antenna elements according to the present invention. 
       FIG. 3  is a drawing depicting an illustrative yagi antenna employing at least one adjustable antenna element according to the present invention. 
       FIG. 4  is a drawing depicting the adjustable antenna element of  FIG. 1  oriented in a vertical direction and showing the effects of element spacing and curvature diameter on characteristic impedance in free space. 
   

   DETAILED DESCRIPTION 
   Those of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons. 
   Referring first to  FIG. 1 , an illustrative antenna element  10  according to the present invention includes a hollow support arm including a first section  10 , a curved transition section  14 , and a second section  16 . While it is presently preferred that first and second sections  12  and  16  be substantially straight, this is not a requirement. The hollow support arm is formed from a non-conductive material such as fiberglass. 
   The loop element is mechanically a more complex element than a straight element is and is subject to torques imparted by wind forces that can distort, bend, or kink the return loop. To prevent this would require the loop to be made of a suitable material such as fiberglass or something at least that strong. Fabricating a half circle hollow fiberglass tube is very expensive. Another solution according to the present invention is to take the stress off of the half-circle “sweep” by molding clamps of a material such as polycarbonate that firmly grip the tip of each straight element tip and allow a solid rod formed from a material such as fiberglass to connect the tips together through the plastic clamps thus taking all mechanical stress off of the sweeps. This allows the sweep to be made of inexpensive, flexible plastic tube such as polyethylene. 
   A length-adjustable conductive member  18  is disposed within the hollow support arm. Conductive member  18  may be formed from a material such as, but not limited to, beryllium copper, and may be advantageously formed as a perforated strip as disclosed in U.S. Pat. No. 6,677,914. 
   The conductive member  18  is mechanically coupled to a length adjuster  20  that functions to adjust the length of conductive member  18  that is disposed in the hollow support arm. The length adjuster  20  may be disposed in a suitable housing in order to provide mechanical support for the antenna element  10 . As is known in the art, adjusting the length of the conductive member  18  may be accomplished by winding a perforated beryllium copper strip on a reel and causing it to wind and unwind from the reel and into and out of the hollow support arm by means of, for example, a stepper motor driving a sprocketed wheel that engages the perforations in the beryllium copper strip as taught in U.S. Pat. No. 6,677,914. The length adjuster  20  may be controlled by a controller  22  in the manner taught by U.S. Pat. No. 6,677,914. 
   Referring now to  FIG. 2 , an illustrative antenna element  30  according to the present invention includes a first hollow support arm  32  like that shown in  FIG. 1 . A second hollow support arm  34  extends in a direction opposite from that of support arm  32 . 
   Each of support arms  32  and  34  include a length-adjustable conductive member  36  disposed within them. Conductive members  36  may be formed from a material such as, but not limited to, beryllium copper, and may be advantageously formed as a perforated strip as disclosed in U.S. Pat. No. 6,677,914. 
   The conductive members  36  are both mechanically coupled to a length adjuster  38  that functions to adjust the length of conductive members  18  that is disposed in the hollow support arms  32  and  34 . The length adjuster  38  may be disposed in a suitable housing in order to provide mechanical support for the hollow support arms  32  and  34 . As is known in the art, adjusting the length of the conductive members  36  may be accomplished by winding a perforated beryllium copper strip on a reel and causing it to wind and unwind from the reel and into and out of the hollow support arm by means of, for example, a stepper motor driving a sprocketed wheel that engages the perforations in the beryllium copper strip as taught in U.S. Pat. No. 6,677,914. The length adjuster  38  may be controlled by a controller  40  in the manner taught by U.S. Pat. No. 6,677,914. Persons of ordinary skill in the art will recognize that antenna element  30  is a dipole antenna whose operating frequency can be adjusted by changing the lengths of the conductive members  36  disposed within the support arms  32  and  34 . 
   Referring now to  FIG. 3 , a yagi type antenna  50  may be constructed using one or more adjustable antenna elements according to the present invention. These adjustable antenna elements may be used by themselves or in conjunction with adjustable antenna elements as disclosed in U.S. Pat. No. 6,677,914. Yagi antenna  50  is shown formed on boom  52  as is known in the art. A first adjustable antenna element  54 , like the adjustable antenna element shown in  FIG. 2  is used as the driven element. Its length is controlled by length adjuster  56 , which is disposed in a housing that is used to mechanically secure the adjustable antenna element  54  to the boom. The distal end of the antenna element  54  may be mechanically fastened to the boom by means of a suitable clamp  58  as shown in  FIG. 3 . A transmission-line matching network may be used to couple the driven element  54  to a transmission line as is well known in the art. 
   A second adjustable antenna element  60  like the antenna element shown in  FIG. 2  is placed at the end of the boom furthest away from the driven element  54 . Its length is controlled by length adjuster  62  which is disposed in a housing that is used to mechanically secure the adjustable antenna element  60  to the boom. The distal end of the adjustable antenna element  60  may be mechanically fastened to the boom by means of a suitable clamp  64  as shown in  FIG. 3 . 
   A third adjustable antenna element  66  is mounted to the boom  52  at the end opposite to the end near which adjustable antenna element  54  is mounted. Its length is controlled by length adjuster  68  which is disposed in a housing that is used to mechanically secure the adjustable antenna element  66  to the boom  52 . 
   Finally, a third adjustable antenna element  70  is mounted to the boom  52  between adjustable antenna elements  54  and  60 . Its length is controlled by length adjuster  72  which is disposed in a housing that is used to mechanically secure the adjustable antenna element  70  to the boom  52 . 
   Persons of ordinary skill in the art will appreciate that the spacing between adjustable antenna elements  54 ,  60 ,  66 , and  70  will depend on the particular frequency range over which the antenna will be used and may easily be determined by persons of ordinary skill in the art using any one of a number of available antenna modeling software programs as is known in the art. 
   Such skilled persons will also appreciate that fewer or a larger number of length-adjustable elements may be included in a yagi antenna according to the principles of the present invention and that one or more fixed-length parasitic elements may also be disposed on boom  52  to interact with elements  54 ,  60 ,  66 , and  70  at particular frequencies as disclosed in U.S. Pat. No. 6,677,914. 
   In one embodiment of the present invention, the total length from end to end of the elements of  FIG. 2  is 38 feet, and the radius of curvature of the curved transition section is 12 inches, the yagi antenna of  FIG. 3  may act as a two element antenna on 40 meters and 30 meters and any frequency in between, and as a four-element antenna on bands from 20 meters to 6 meters or higher. 
   In addition, a second feed line may be coupled to adjustable antenna element  60  to allow reversing the directivity of the antenna pattern without having to rotate the boom  52 . 
   Length adjusters  56 ,  64 ,  68 , and  72  are coupled to controller  74 . During operation, the operator may use the controller  74  (and, for some but not all functions, the controllers  22  and  40  of  FIGS. 1 and 2 , respectively) to perform the following functions:
         1. Single button band selection, including the ability to scroll through the band in segments of approximately 100 kHz.   2. Continuous adjustment of the antenna over its entire frequency range using simple up/down buttons (not shown).   3. Adjustment of the antenna by sensing the VSWR.   4. 180-degree direction change (Yagi version only) by changing the director to a reflector and changing the reflector to a director via a single button control, thus allowing very fast (less than 2 seconds) direction changes.   5. Bi-directional operation (Yagi only).   6. Store different antenna designs in the microprocessor memory that maximize gain only, front-to-back ratio only, or VSWR only.       

   Referring now to  FIG. 4 ,  FIG. 4  a drawing depicts the adjustable antenna element of  FIG. 1  oriented in a vertical direction and shows the effects of element spacing and curvature diameter on characteristic impedance in free space. As depicted in  FIG. 4 , the antenna element of the present invention can be configured as a quarter-wavelength monopole vertical antenna. 
   When used as a quarter-wavelength monopole vertical antenna, the element of  FIG. 4 , some sort of counterpoise should be used. This may be in the form of one or more ground or above-ground radial elements as is well known in the antenna art. 
   As also may be seen from an examination of  FIG. 4 , the length L of the first and second sections  12  and  16  of the antenna element  10 , as well as the diameter of curvature of the curved transition section  14 , will affect the characteristic impedance of the antenna. Public domain antenna modeling software known as NEC-4, available from many sources as is known in the art, was used to model the antenna element of  FIGS. 1 and 4 , using exemplary lengths L of 38 feet and 46 feet, and exemplary curvature diameters D of 24 inches and 48 inches. 
   As shown in the table in  FIG. 4 , an antenna element  10  according to the present invention having a length L of 38 feet and a curvature diameter of 24 inches has a characteristic impedance of 18.7 ohms in free space. An antenna element  10  according to the present invention having a length L of 38 feet and a curvature diameter of 48 inches has a characteristic impedance of 23.7 ohms in free space. An antenna element  10  according to the present invention having a length L of 46 feet and a curvature diameter of 24 inches has a characteristic impedance of 39.1 ohms in free space. Finally, an antenna element  10  according to the present invention having a length L of 46 feet and a curvature diameter of 48 inches has a characteristic impedance of 44.4 ohms in free space. 
   From the above data, it may be seen that the characteristic impedance of an antenna element according to the present invention may be controlled by varying the length L of the first and second sections  12  and  16  and the curvature diameter of curved transition section  14 . 
   While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.