Broadband, Omnidirectional, Horizontally Polarized, Loop Antenna

Abstract

A broadband, horizontally polarized, omnidirectional antenna comprising a plurality of parallel-fed loop radiating elements is disclosed. This antenna has particular utility for reception of VHF entertainment broadcasts (Television channels 2-13 and the FM broadcast band).

Description

This invention relates to antennas. More particularly, it relates to antennas useful in the reception of horizontally polarized signal energy from a plurality of directions over a wide band of frequencies.

VHF entertainment transmissions are typically transmitted by the broadcasters with horizontal polarization which requires that the receiving antenna be horizontally polarized in order to couple the transmitted energy to the receiver with maximum efficiency. The most common type of receiving antenna now employed for these signals is the Yagi-type array which comprises a horizontally polarized parasitic linear array of dipole elements. This type of antenna is directive and can not efficiently receive energy over the entire two-octave range of the VHF entertainment bands. Because of the restricted bandwidth, it is common for users of such Yagi receiving antennas to use several of them to cover the desired frequency range; typically, three such antennas are used: one for television channels 2 through 6, one for the FM broadcast band, and one for television channels 7 through 13. In order to provide proper impedance matching, separate transmission lines should be used between each antenna and the receiver associated therewith. If the user is interested in receiving signals from one direction only, he need only aim his array in that direction. If, on the other hand, he desires to receive signals from transmitters located at differing azimuths from his location, he must either employ an antenna rotor, which is expensive and typically requires maintenance more frequently than the antenna does, or he must accept severely degraded performance of his receiving system. The problem occasioned by directivity of the antenna is especially severe when the receiving installation is mobile, for instance on a boat or the like.

In order to overcome the bandwidth limitations of Yagi antennas, the use of log periodic arrays has become increasingly popular for VHF entertainment reception in recent years. Log periodic antennas for VHF entertainment use are broadband, directional and horizontally polarized. The user of a log periodic antenna, therefore, may cover the desired frequency range with a single antenna and transmission line, but still requires a rotor just as the Yagi user does.

Other types of antenna which are known in the art but which have not achieved wide use in VHF entertainment reception include:

Marconi antennas which are end-fed quarter-wave vertical antennas. Marconi antennas are omnidirectional, but are not broadband or horizontally polarized.

Dipole antennas which are center-fed half-wave antennas which are oriented either vertically or horizontally. When horizontally oriented, a dipole antenna is horizontally polarized, but is not broadband or omnidirectional; when vertically oriented it is omnidirectional, but not broadband or horizontally polarized.

Discone antennas which are broadband and omnidirectional, but are not horizontally polarized.

Turnstile antennas which are horizontally polarized and omnidirectional, but are not broadband.

The principal object of the present invention is to provide a broadband, horizontally polarized, omnidirectional antenna.

Another object is to provide such an antenna which is light in weight, and simple and inexpensive to manufacture.

Briefly, the invention is embodied in an antenna comprising a plurality of closed loop radiating elements which are parallel-fed by a tapered pair feedline. Each loop element consists of a single turn of conductive material whose dimensions are uniform over the entire loop. This results in each loop being electrically uniform and continuous. The loops are fed in parallel by a pair of uniformly tapered feedlines comprising two congruent strips of conductive material which diverge at a small angle. A first loop of said plurality of loops has a circumference equal to one half wavelength at the lowest desired frequency. A second loop has a circumference equal to one half wavelength at the highest desired frequency. The remaining loops are of intermediate size between said first and second loops. Said first loop is connected to said feedlines at the point of maximum separation of said feedlines and the remaining loops are connected to said feedlines to form a conical structure whose height is equal to the diameter of said first loop. Nonconductive structural support members are employed to mechanically support the antenna structure. Means are provided at the end of the pair of feedlines near the apex of the cone whereby said feedlines are connected to a transmission line.

In the drawing the single FIGURE is a perspective view of the inventive antenna. The circumference of loop 11 is equal to one-half wavelength at the lowest desired frequency. The circumference of loop 12 is equal to one-half wavelength of the highest desired frequency. Loops 13 are of intermediate size between loops 11 and 12 and are disposed between them to define a conical surface. The loops are connected in parallel through a uniformly tapered feedline comprising metal strips 14 and 15. At the apex end of the feedline, strip 14 has terminal 16 and strip 15 has terminal 17 attached thereto. Terminals 16 and 17 are connected respectively to terminals 18 and 19 of connection box 20. Connection box 20 also contains coaxial connector 21. Terminals 18 and 19 are connected within connection box 20 to coaxial connector 21 either directly, or, for improved performance, through an appropriate balun transformer which will be more fully described below. Interconnection of the antenna is thus provided with a coaxial transmission line 23 by means of coaxial connector 21 and cable mounted coaxial connector 22. Structurally, the antenna also includes a base member 35 which supports nonconductive structural support members 31, 32 and 33. Feedline strips 14 and 15 and the ends of loops 11 through 13 are attached to support members 32 and 33 by means of a plurality of fasteners illustrated as a nut and bolt fastener at 37. Electrical connection of the loops to the feedline is accomplished by direct connection at the fasteners. The loops are also mechanically supported by support member 31 which is disposed on the opposite side of the conical surface from members 32 and 33. The loop members are attached to support member 31 by mechanical fasteners illustrated as nut and bolt fasteners at 36. Nonconductive spacer 34 is disposed between support members 32 and 33 at the upper ends thereof, and cooperatively with base member 35 maintains the desired taper of the feedline.

The antenna illustrated was designed for 50 ohm feedpoint impedance. The dimensions discussed below are particular to this design. Other impedances may be provided by design by changing the dimensions. The diameter of loop 11 is D and is equal to .lambda./2.pi. at the lowest desired operating frequency. The spacing between successive loop elements is uniform and is equal to D/ 5. This spacing is optimum to insure a continuous impedance characteristic with frequency. The spacing between facing edges of feedline members 14 and 15 is D/ 12 at the point of connection to loop 11 and is D/ 48 at the facing edges opposite terminals 16 and 17. This provides the taper of the feedline which determines the antenna characteristic impedance. Loops 11 through 13 are not less than D/ 24 nor more than D/ 12 wide. A width of at least D/ 24 is required to ensure a continuous impedance characteristic with frequency and a width of less than D/ 12 is required to maintain an omnidirectional antenna pattern.

Since the antenna illustrated was designed to provide a 50 ohm characteristic impedance at terminals 16 and 17, and since coaxial cable 23 has a 50 ohm characteristic impedance, the connection within connection box 20 between terminals 18 and 19 and coaxial connector 21 may be by way of a balun transformer of one to one impedance ratio. It has been found, however, that very acceptable results will also be obtained under these conditions with a direct connection within connection box 20 between terminals 18 and 19 and coaxial connector 21. If it is desired to use a coaxial cable 23 of impedance other than 50 ohms, the feedline taper may be changed or, alternatively, a balun transformer of appropriate nonunity ratio may be employed within connection box 20 between terminals 18 and 19 and coaxial connector 21.

Claims

The invention claimed is:

1. A broadband, horizontally polarized, omnidirectional antenna comprising:

a plurality of electrically and mechanically uniform and continuous loop radiating elements but having diverse diameters, said elements positioned in vertically spaced substantially horizontal planes to thereby define a right conical surface,

a pair of uniformly tapered feedlines, said feedlines interconnecting said radiating elements electrically in parallel, and

means for connecting said feedlines to a transmission line.

2. The antenna of claim 1 wherein a first loop of said plurality of loops has a circumference equal to one-half wavelength at the lowest desired antenna operating frequency, and a second loop of said plurality of loops has a circumference equal to one-half wavelength at the highest desired antenna operating frequency.

3. The antenna of claim 2 wherein said feedlines are disposed along said conical surface in conically parallel relationship whereby said conical surface determines said taper of said feedlines.