Abstract:
Antenna traps without a separate capacitor component are disclosed. The traps are tuned by the capacitance between bifilar coils employed as the trap inductor. Simplicity, low cost, and ease of fabrication are the advantages of this trap. Two methods for winding a trap antenna from a continuous wire that becomes both antenna segments and resonant traps are also disclosed.

Description:
This application is a continuation in part of my application Ser. No. 222,241, filed Jan. 2, 1981 which is a continuation-in-part of my application Ser. No. 162,928, filed July 17, 1980. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to improvements in the art of constructing antenna traps which are used to provide multiband operation on a single antenna. A trap is a parallel resonant circuit inserted in an antenna which offers a high impedance to currents flowing in the antenna at the trap&#39;s resonant frequency, separating the inner portion of the antenna between the feedline and the trap from the remainder of the antenna. The inner portion is of a length to be resonant at the trap frequency and is an efficient absorber and radiator of radio waves of that frequency and nearby band of frequencies. Many traps can be incorporated into a single antenna, enabling the antenna to be used on many bands. Traps are well known and used in many types of antennas, such as dipoles, vertical monopoles, parasitic beams, and the like. 
     PRIOR ART 
     Typical trap constructions include both an inductor and a capacitor to establish a parallel resonant circuit, though in some antennas made from tubing the capacitor is incorporated into the structure as a coaxial rod or tube inside and insulated from the antenna tubing. In this invention, the trap capacitor is eliminated as a separate component and the capacitance between series-connected bifilar coil windings is employed to resonate the coil&#39;s inductance to the desired trap frequency. Carlson, in U.S. Pat. No. 3,465,267 has employed the interwinding capacitance between bifilar coils to produce a parallel resonant circuit in his generalized circuit component, and Matsumoto, in U.S. Pat. No. 3,560,895 has used the capacitance between bifilar coils to tune an interstage transformer to a resonant frequency. Neither of these prior art devices is suitable for antenna trap use because of mechanical support and electrical connector deficiencies. In my application Ser. No. 06/162,928 an antenna trap is disclosed that utilises the capacitance between inner and outer conductors of coaxial cable as the trap capacitor, thus eliminating a separate component in a trap in which the outer coaxial cable braid is used as the trap inductor. This invention discloses another novel structure that does not need a separate capacitor in an antenna trap, that is realized with ordinary insulated wire. 
     SUMMARY OF THE INVENTION 
     In this invention, a parallel resonant trap circuit is constructed from insulated wire bifilar coils, without employing a separate capacitor. The capacitance between the windings is electrically in parallel with the coil incuctance to tune the trap. As more turns of wire are wound into the bifilar coils, both the inductance and capacitance are increased, lowering the trap frequency and providing a convenient way of preparing traps of different resonant frequency. The traps are of very simple construction and low cost. A further provision of this invention are methods whereby these bifilar traps may be included into a trap antenna system made from a continuous wire, without requiring any electrical connections within the traps or between the traps and the antenna segments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of one embodiment of the bifilar antenna trap. 
     FIG. 2 is a schematic diagram of an alternate embodiment of a bifilar antenna trap. 
     FIG. 3 is a perspective view of one embodiment of this invention that utilizes the coil connection illustrated in FIG. 2. 
     FIG. 4 is a perspective view of an alternate embodiment of this invention that utilizes the coil connection illustrated in FIG. 1. 
     FIG. 5 shows two alternate coil winding configurations in cross-section. 
     FIG. 6 shows two perspective views of the winding of a bifilar trap so that the coil wire is continuous with the antenna wire on either side of the trap. 
     FIG. 7 shows a perspective view of a bifilar trap for use in antennas made from metal tubing. 
     FIG. 8 shows eight perspective views of the winding of a bifilar trap onto a trap insulator made from hollow tubing with slots in it. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, bifilar coils 3 and 4 are included between antenna segments 1 and 2. A cross-connection 5 joins opposite ends of the bifilar coils 7 and 8, joining the two coils in series so that their inductances reinforce or aid one another, rather than oppose one another. This connection makes one large coil out of the two smaller coils as to magnetic or inductive effects. The usual distributed capacity between adjacent turns is greatly increased by the bifilar construction, since the turns from different coils that are close to one another, such as the turns 8 and 9 at the left ends, have much greater rf voltage between them than do adjacent turns in a single coil. This capacitance between bifilar coils is in parallel with the combined coil inductance and forms a parallel resonant circuit with the bifilar coils. In this embodiment, antenna segments 1 and 2 are connected to the ends of only one of the bifilar coils, coil 3. The high impedance at resonance still functions to disconnect unwanted antenna segments from the resonant one, even though the antenna is connected across only part of the trap resonant circuit. At lower frequencies the single coil offers less impedance as a loading coil than both bifilar coils would. This is an advantage where wide bandwidth is desired, since large loading inductors restrict the bandwidth of an antenna. 
     In FIG. 2 a similar pair of bifilar coils is shown, together with the cross-connection 5, as in FIG. 1. The antenna segments 1 and 2 in this configuration are connected to the ends of the overall inductor formed by the two bifilar coils in series. This antenna connection does not affect the trap resonant frequency except to a minor degree, but places the two coils in the antenna as loading coils at lower frequencies. This arrangement is an advantage in applications where the greatest loading or shortening of the physical length of the trap antenna is desired. 
     It will be appreciated that the ratio of inductance to capacitance within the trap can be controlled by changing the number of turns of wire in the second coil 4 of FIG. 1, since these coils need not have the same number of turns. In addition, the amount of loading inductance that the trap will exhibit at lower frequencies can be adjusted by changing the location of the antenna connections to the bifilar coils. 
     In FIG. 3 a bifilar antenna trap is shown wound on an insulator 10, with bolts 11 securing antenna wires 1 and 2 to the trap insulator and also holding connecting lugs 12 in contact with the antenna wires. Lugs 12 are also connected to ends 9 and 6 of the bifilar coils. Terminals 13 are also mounted in the trap insulator, connected to coil ends 7 and 8 and the ends of cross-connection wire 5. Coils 3 and 4 are shown of different color to aid in identifying them. The white coil 3 starts at the left end of the insulator, connected to antenna segment 1, and finishes at the right end at 7 and is joined by cross-connection wire 5 back to the starting end of black coil 4, which finishes at 6 and is there connected to antenna segment 2. Electrically this trap is shown in FIG. 2. It should be noted that the trap insulator must be made of a nonconductive material even when the bifilar coils themselves are insulated, since eddy current losses and transformer effects would reduce the effectiveness of the traps. 
     In FIG. 4 a hollow trap insulator is shown whereby the cross-connection wire 5 may pass through the center of the insulator and trap coils. A separate wire is not used for the cross-connection, but rather an extension of the black coil 4 at the start 8 of the winding passes through a hole in the insulator 14 and through the axis of the trap to connect to bifilar coil end 7 and antenna segment 2 at the opposite end of the trap. The trap shown in FIG. 4 uses the electrical connection of FIG. 1 in which the antenna segments are connected across only one of the bifilar coils, the white coil 3, at its ends 9 and 7. Antenna segments 1 and 2 are secured to the trap insulator by means of holes 15 drilled in the wall of the insulator. Electrical connections between coil and antenna are effected by joining coil ends directly to antenna wire and soldering. End 6 of black coil 4 is left unconnected in this arrangement. 
     The traps of FIGS. 3 and 4 may be tuned over a small frequency range, approximately 10% of center frequency, by adjusting the spacing between the bifilar turns. Both coils 3 and 4 have been shown wound with insulated wire, but since they must only be insulated from one another, one of the coils may be wound from bare wire, thereby reducing the separation of the bifilar turns and increasing the capacitance between the coils. A relatively thick insulation has been shown in the drawings but in some applications a relatively thin insulation such as an enamel or thin plastic coating will be more appropriate. 
     In FIG. 5(a) a cross-section of a pair of bifilar coils is shown in which the coils are wound one on top of the other. The bottom coil is wound with wire 16 having an insulated covering 17, and the second or outer coil is wound from uninsulated wire 18. In FIG. 5(b) both coils are made from insulated wire, the bottom coil having white insulation 17 and the outer coil having dark insulation 19. The essential bifilar relationship is preserved in this configuration, and exists even if the two coils are wound from a single continuous wire, doubling back on itself to achieve the necessary sense or direction of the winding. 
     In FIG. 6 two views of the winding of a bifilar antenna trap are shown in which a trap is introduced into an antenna with no breaks in the wire or electrical connections required. The winding begins with the electrical center of the bifilar coils, the cross-connection wire. The insulated wire is laid lengthwise against the trap insulator 20, preferrably into a longitudinal slot 21 in the insulator. In FIG. 6(a) the first of the bifilar coils has been completed. It was started by bending the wire out of the slot toward the left end of the insulator 8 and winding the wire away from the observer around the insulator and cross-connection wire. At the end of the first coil 6 the wire was secured to the insulator by folding it double, passing the doubled end through hole 22 in the insulator 20. This operation has just been done at the left end of the insulator in FIG. 6(b). The doubled end 23 is next opened into a loop and the loop passed around the end of the insulator. In FIG. 6(a) the loop 24 is shown tightened around the insulator, the excess wire having been pulled back to become part of antenna segment 2. The second bifilar coil is just being started in FIG. 6(a), with the wire coming out of the slot and bending toward the observer at 7, and starting the winding with the same direction as used in the first coil. However, since the windings start from the cross-connection or middle of the bifilar coils, the second coil is wound in a clockwise direction when viewed from the left while the first coil had been wound in a counterclockwise direction when viewed from the left. If the first coil has been wound with appropriate spaces between turns the second coil may be wound with its turns between those of the first coil, resulting in flat bifilar coils having the same diameter, as illustrated in FIGS. 3 and 4. If the first coil had been wound with its turns touching one another, the second coil may be wound on top of the first, as shown in FIG. 6(b). After finishing the second coil, the wire is again doubled, the doubled end 23 passed through a transverse hole 22 in insulator 20, the doubled wire opened to loop back around the end of the insulator as was done at the opposite end. The slack in the loop will be removed by pulling on antenna segment 1. This method of including a trap into an antenna results in the circuit of FIG. 2 for the connection of the antenna segments to the bifilar coils, with the numbering in FIGS. 2 and 6 corresponding to identical parts. 
     FIG. 7 shows a bifilar antenna trap included between antenna segments 25 and 26 formed from metal tubing. The trap insulator 27 is a rigid hollow cylinder. This cylinder may telescope inside the tubing 26 or telescope around smaller tubing 25. Bolts 28 are used to secure the insulator and tubing segments to one another and connect bifilar coil ends 9 and 6 to the antenna segments 25 and 26 through solder lugs 29. The electrical cross-connection 5 in this embodiment is an extension of end 7 of one the bifilar coils, crossing to connect to opposite end 8 of the other coil. This embodiment uses parallel conductor cable 30 to wind the bifilar coils of the trap. 
     In FIG. 8 a series of drawings shows how an alternate embodiment of this invention may be wound upon and secured to a hollow cylindrical trap insulator that is slotted to facilitate the trap construction. In FIG. 8(a) the hollow cylindrical trap insulator 31 has longitudinal slots 32 through opposite walls of the cylinder at each end of the insulator. There are also transverse slots 33 that intersect the ends of the longitudinal slots in one wall of the insulator. These transverse slots are approximately parallel to one another and are at an oblique angle to the insulator axis, with one end closer to the insulator end than the other end of the slot. 
     FIG. 8(b) shows the beginning steps in the construction of a bifilar trap that will be made from a wire that is continuous with the antenna wire segments next to the trap. The cross-connection wire has been laid against the trap insulator 31 longitudinally on the side away from the viewer in FIG. 8(b). The wire has been passed through the longitudinal slots 32 at the left end of the insulator and this wire 34 now is in the middle of an oblique slot 33. It has been bent upwards and will be slid along the slot tending toward the inner portion of the insulator, away from the insulator end. In FIG. 8(c) the winding of the first of the bifilar coils has started. Wire 34 has come against the inner end of oblique slot 33 and has been wrapped around both the insulator 31 and the cross-connection wire, following the direction established by its motion inward along oblique slot 33. FIG. 8(d) the winding of the first coil is complete and the wire has been passed through longitudinal slots 32 at the right end of the insulator and slid along the oblique slot 33 to its inner end where it passes through the insulator at 35. This winding wire 36 has been bent upward after emerging from the longitudinal slot on the side away from the viewer and will be wrapped an additional half turn to be slid down longitudinal slot 32, into oblique slot 33 and along it to the outer end of oblique slot 33. This has been accomplished in FIG. 8(e), where winding wire 36 is passing into the outer end of the oblique slot at 37. This wire is bent outward immediately after passing through the insulator wall at 37 and becomes antenna segment 2 adjacent the bifilar trap. 
     To begin winding the second of the bifilar coils, illustrated in FIG. 8(f), wire 38 was passed through the longitudinal slots 32 at the right end of insulator 31, down oblique slot 33 to its inner end against the finish of the first coil. This wire is bent downward at 39 to start winding the second coil in the same direction it received from oblique slot 33, over the first coil. In FIG. 8(g) the winding of the second coil is complete and the winding wire 40 has passed through the longitudinal slots 32 at the left end of the insulator and down to the inner end of oblique slot 33 where it lies at 41, over the start of the first coil 34. Wire 40 has been bent downward and will be wrapped around insulator 31 an additional half turn to pass through a longitudinal slot and into the oblique slot at the left of the insulator. It is slid along this oblique slot to its outer end. FIG. 8(h) shows this final half-turn in place at 42, with the winding wire bent outward after passing through the insulator wall to become the adjacent antenna segment 1. 
     The inner ends of the oblique slots serve to accurately define and position the starting and finishing turns of the bifilar coils, while the outer ends serve as a locking means or attaching means, absorbing the tensile force from the antenna on the insulator and trap. 
     Using a 5/8 inch diameter trap insulator, #18 stranded copper wire with a vinyl insulation known as &#34;hook-up wire,&#34; a first coil of 10 turns and a second coil of 9 turns produces a parallel resonant trap whose frequency is approximately 27 MHz. 22 plus 21 turns in a larger trap has a resonant frequency of approximately 10 MHz. Different wire sizes, insulation thicknesses, and trap insulator diameters result in different trap frequencies.