Yagi antenna with balancing tab

An antenna minimizes radiation from the outer conductor (44) of a coaxial cable (40) that is coupled to the antenna by providing a balancing tab (90) that lies near the connection (72) of the cable outer conductor (44) to the antenna. One antenna includes a Yagi structure (12) constructed of a plate of metal that forms a long boom (20) and a plurality of directors (28) extending laterally across the boom, with an electrical coupling loop mounted at a rear region of the Yagi structure. The coupling loop includes a folded dipole in the form of a metal coupling plate that forms a loop (54) with a gap (70), the loop having a laterally elongated front loop end (60). The Yagi structure is formed with a balancing tab (90) that lies forward of the loop, that has a lateral length (D) less than half that of one of the directors, and that extends from only one side (22) of the boom which is the side to which the outer coax conductor (44) is connected to the loop. The metal coupling plate lies facewise adjacent to a face (24) of the Yagi plate, with a dielectric spacer (76) between them.

BACKGROUND OF THE INVENTION

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.

SUMMARY OF THE INVENTION

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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-3illustrate a Yagi antenna10(named after the inventor, Yagi Hidetsuga) which is a highly directional antenna for electromagnetic radiation, with the particular antenna illustrated having been constructed for use in the 1900 MHz cell phone frequency band. The Yagi antenna includes an electrically conductive Yagi structure12having an elongated boom14that is elongated in front and rear directions F, R and that has laterally L opposite boom sides20,22and opposite boom faces24,26. The Yagi structure also has a plurality of directors28that are each elongated in the lateral directions L and that each extends across the boom and that each has parts of equal length on laterally opposite sides of the boom. The Yagi structure also includes a reflector30that lies rearward of the directors28and that generally has a greater lateral length than the directors. The antenna radiates power in the forward direction F and receives radiation moving in the rearward direction R.

The directors28includes a rearmost director32, which lies forward of the reflector30, 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 part34of the Yagi antenna, which lies in a rear portion35of the Yagi structure, is a supported region where the antenna is normally supported. The entire Yagi structure12is formed from a plate of conductive material, such as aluminum.

As shown inFIG. 4, high frequency signals are carried to and from the Yagi structure by a transmission line such as coaxial cable40, that has inner and outer conductors42,44. The signals are coupled between the cable and Yagi structure by a coupling assembly50that includes a folded dipole52in the form of a loop54. The loop has front and rear loop parts60,62and has loop sides64,66that connect together each pair of adjacent ends of the front and rear loop parts. The rear loop part has a gap70, and the coaxial conductor inner and outer conductors are connected to opposite gap sides72,74of the rear loop part, with solder connections being shown.

As shown inFIG. 5, the front and rear loop parts60,62each lie adjacent to a face24of the Yagi structure. The loop54is formed of a metal plate with one face64of the metal plate lying facewise (their adjacent faces are parallel and adjacent, or close) to the face24of the Yagi structure, with an insulative, or dielectric spacer or spacer part76lying between them. The dielectric spacer76is part of a dielectric housing78that surrounds the electrical coupling, and together the housing and coupling form the coupling assembly50. As shown inFIG. 4, the Yagi boom is formed with at least one mounting ear, and preferably with a pair of mounting ears80,82. The mounting ears have holes84that receive fasteners86(FIG. 5) that project though the Yagi boom ears, though the spacer part76and though holes in the loop. The metal plate that forms the loop has about the same thickness as the Yagi structure and of the spacer part76, so the thickness of the antenna with the electrical coupling is of the same order of magnitude as the Yagi structure alone. As a result, the Yagi antenna is compact. The metal plate that forms the loop54lies in a mount region85(FIG. 3) that lies between the reflector30and first director32.

Despite the attempt at the balanced transfer of signals by the electrical coupling50between the coaxial cable and the Yagi structure, currents flow in a manner that results in radiation from the outer conductor44of 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 conductor44of the coaxial cable, by the use of a balancing tab90. 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 loop54of the electrical coupling. The balancing tab has a lateral length D (FIG. 4) that is less than the average length of the directors28and less than the average length of projection of a director from the boom, and lies forward of the front loop part60. The separation distance A (FIG. 5) between them is of the same order of magnitude as the separation of the loop54and Yagi structure12, and the tab lies rearward of the first director32.

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 tab90extends from the same side22of the boom as the side72of 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 side20of 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 assembly50.

The Yagi antenna is constructed by first forming the Yagi structure12(including the balancing tab) of a metal plate such as of aluminum. The screw holes84are drilled in the mounting ears80.82. The folded dipole loop54is placed in the dielectric housing72and 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 fasteners86are projected through the ears and dielectric spacer portion and threaded into threaded holes94in the loop. A cover100of the housing is then closed. A small covering102lies 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 reflector30to 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 tab90had 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.

FIG. 6illustrates a greatly simplified and inefficient balanced antenna120which includes a balanced structure in the form of a dipole structure122connected to a transmission line in the form of a coaxial cable40. This antenna will radiate, although far less efficiently than if a Yagi structure were coupled to it as inFIGS. 1-5. The dipole structure includes two conductive dipole parts124,126that extend in opposite radial directions from the antenna axis130. The cable inner coax conductor42is connected to the radially inner end of the first dipole part124. The cable outer coax conductor44is connected to the radially inner end of the second dipole part126. The parts are shown mounted on an insulative board134.

The dipole structure radiates energy, but the radiated energy is degraded by emissions from the cable outer conductor44. Applicant counters such emissions from the cable outer conductor by providing a balancing tab132that is elongated and that extends parallel and adjacent to the second dipole part126. Applicant notes that the dipole parts124,126correspond to opposite sides62A,62B of the rear loop part of the folded dipole ofFIG. 4.

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.