A transmission line such as a coaxial cable, which is connected to an antenna such as a Yagi antenna, may radiate energy that seriously degrades both the gain and radiation pattern of the antenna. Ideally, equal and opposite currents flow from the coaxial cable into the terminals of a balanced antenna. However, in practice, energy radiates from the outer conductor (the shield) of the coaxial cable. A common technique used to minimize such radiation is to connect a balun (which stands for “balanced to unbalanced”) in the circuit which forces the coaxial cable to feed equal and opposite currents into the antenna. A balun is basically a wire wound transformer or a specially connected distributed transmission line, and results in mechanical and electrical disadvantages. A mechanical disadvantage is that the balun must be protected from moisture and from exposure to the sun, and the balun can be a bulky part that produces a large bulge in the antenna. An electrical disadvantage is that the balun introduces losses, thereby reducing antenna gain and therefore its efficiency. The balun also increases manufacturing cost. A change to an antenna which minimized radiation from the transmission line such as a coaxial cable, in a simple and low cost manner which avoided the disadvantages of a balun, would be of value.
In accordance with one embodiment of the present invention, an antenna, and in particular a Yagi antenna, is provided which minimizes radiation from a transmission line such as a coaxial cable that carries signals to or from the antenna, in a manner that avoids the need for a balun, to thereby reduce the cost of the antenna and make it more compact and efficient. A Yagi antenna is provided of the type that has a flat Yagi structure, with a boom that extends in front and rear directions and with a plurality of directors that extend laterally across the boom. The antenna is improved by providing a balancing tab that extends laterally from one side of the boom. A coaxial cable is coupled to the Yagi structure by an electrical coupling that includes a folded dipole formed by a metal plate. The metal plate is mounted on a dielectric separator that lies facewise adjacent to a rear portion of the Yagi structure. The metal plate forms a loop having laterally long front and rear loop parts, the rear loop part having a gap. The inner and outer coax conductors are connected to the rear loop part at opposite sides of the gap. The balancing tab lies a short distance forward of the front loop part, on the same side of the boom as the side of the gap to which the coax cable outer conductor is connected.
The metal plate that forms the loop, and the separator, can be mounted on the Yagi structure by a pair of simple screws, and the metal plate that forms a loop projects only slightly from the plane of the Yagi elements. This results in a simple and easily constructed antenna that is compact.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.
The directors 28 includes a rearmost director 32, which lies forward of the reflector 30, and that is referred to as the first director. Progressively more forward directors are referred to as the second, third, etc. directors and are of equal lateral lengths, or are of progressively smaller lengths. A rear end part 34 of the Yagi antenna, which lies in a rear portion 35 of the Yagi structure, is a supported region where the antenna is normally supported. The entire Yagi structure 12 is formed from a plate of conductive material, such as aluminum.
As shown in
As shown in
Despite the attempt at the balanced transfer of signals by the electrical coupling 50 between the coaxial cable and the Yagi structure, currents flow in a manner that results in radiation from the outer conductor 44 of the coaxial cable. Previously, such imbalance was compensated for by the use of a balun (short for balanced to unbalanced) which is in the form of a wound transformer, or by a specially connected transmission line. Such a balun has considerable bulk, and results in a less compact, less efficient and more expensive antenna.
In accordance with one feature of the present invention, applicant greatly minimizes the radiation of signals from the outer conductor 44 of the coaxial cable, by the use of a balancing tab 90. The balancing tab is in the form of an electrical conductor that extends from one side of the boom and that lies a small distance from the loop 54 of the electrical coupling. The balancing tab has a lateral length D (
Applicant found that the effects of unwanted transmission radiation in a Yagi antenna could be suppressed by a tab when applicant accidently touched the boom of a Yagi antenna with a screwdriver while monitoring the performance of the antenna, and found that the performance was improved. Further experimentation showed that the tab preferably has the shape, position and size shown in the drawings. The tab is most easily added by constructing the Yagi structure of a plate of metal with the tab being part of the plate. The tab 90 extends from the same side 22 of the boom as the side 72 of the loop gap where the outer conductor of the coaxial cable is connected, for coupling between the loop and balancing tab. The tab functions by inducing currents in the outer conductor (shield) of the coaxial cable, that cancel the undesirable shield currents caused by an unbalanced cable being connected to the balanced antenna. Applicant prefers to not have a tab extend from the opposite side 20 of the Yagi structure, although a much different length tab could extend there with optimal performance achieved by changing the spacing between the different length tab and the coupling assembly 50.
The Yagi antenna is constructed by first forming the Yagi structure 12 (including the balancing tab) of a metal plate such as of aluminum. The screw holes 84 are drilled in the mounting ears 80. 82. The folded dipole loop 54 is placed in the dielectric housing 72 and the conductors of a coaxial cable are soldered in place. The combination of loop and housing is placed facewise against one face of the Yagi structure, with the holes in the housing aligned with holes in the mounting ears. A pairs of screw fasteners 86 are projected through the ears and dielectric spacer portion and threaded into threaded holes 94 in the loop. A cover 100 of the housing is then closed. A small covering 102 lies over the screw heads.
Applicant has constructed and successfully tested the antenna shown in the figures, which was designed for transmissions in the 1900 MHz cell phone band. The antenna had a boom length of 330 millimeters, as measured from the rear of the reflector 30 to the front end of the boom, and a boom width B of 6 mm. Although the lateral length of the directors varies slightly, their length E is about 64 mm, their longitudinal dimensions C are about 6 mm, and they project about 29 mm from each side of the boom. The balancing tab 90 had the same longitudinal length C as the directors, and projected a distance D of 12 mm from one side of the boom. This projection distance D is less than half of an average lateral length E of the directors, is less than the lateral projection G of the directors from the boom, and is preferably no more than half the director projection G. The Yagi structure was formed of a plate of aluminum having a thickness of 2 mm to provide sufficient strength. The distance A between the balancing tab and the conductive loop was 3 mm, and is preferably no more than one centimeter.
The dipole structure radiates energy, but the radiated energy is degraded by emissions from the cable outer conductor 44. Applicant counters such emissions from the cable outer conductor by providing a balancing tab 132 that is elongated and that extends parallel and adjacent to the second dipole part 126. Applicant notes that the dipole parts 124, 126 correspond to opposite sides 62A, 62B of the rear loop part of the folded dipole of
Thus, the invention provides an antenna, and especially a Yagi antenna, which minimizes radiation from a portion of the connected transmission line such as the outer conductor of a coaxial cable. Such radiation is suppressed by a balancing tab. The balancing tab lies forward of a dipole or electrical coupling loop and in the case of a Yagi projects from a side of the Yagi boom that is closest to the outer conductor of the coaxial cable. The coaxial cable is coupled to the Yagi structure by a folded dipole in the form of a conductive loop that is elongated in a lateral direction, and which can be made of a metal plate. The coaxial conductors of the coax cable are connected to opposite sides of a rear loop part, at opposite sides of a gap therein. The loop is mechanically connected to the Yagi structure with the plane of the metal plate of the loop lying parallel to a face of the Yagi structure plate, and with a dielectric spacer of a housing lying between them.
Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.
Number | Name | Date | Kind |
---|---|---|---|
4218686 | Blonder | Aug 1980 | A |
5898410 | DeMarre | Apr 1999 | A |
6326922 | Hegendoerfer | Dec 2001 | B1 |
6483476 | Cox | Nov 2002 | B2 |
20060066441 | Knadle et al. | Mar 2006 | A1 |