Miniaturized Transmission Line Top Loaded Monopole Antenna

Abstract

A miniaturized antenna having high radiation efficiency consists essentia of an open circuit transmission line top-loading a short monopole over a metal ground plane. The currents in the two sides of the transmission line, being oppositely directed, tend to cancel each other's effect in the distant field. Thus, only the short vertical monopole contributes to the distant field radiation. The omnidirectional monopole is a constant current radiating element operating at near RF ground potential (high current, low impedance).

Description

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without the payment to me of any royalty thereon.

FIELD OF THE INVENTION

The present invention relates to antennas, and more particularly to a miniaturized monopole antenna having high radiation efficiency and omnidirectional radiation.

BRIEF DESCRIPTION OF THE PRIOR ART

In the past, efforts have been made to reduce the size of monopole antennas to permit their use in applications where limited space was a problem. Successful radiating efficiencies have been obtained by employing antennas with quarter wavelength monopoles. In the VHF frequency range, the size becomes impractical where miniaturization is necessary. Further, prior art monopoles required the additional space for transmission line thus precluding their application to miniaturization at high radiating efficiencies. When prior art improvements were attempted by fabricating the transmission line portion in a single plane, above a ground plane of the antenna, capacitive loading resulted which seriously affected the radiating efficiency of the antenna.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

The present invention is directed to a miniaturized transmission line top-loaded monopole antenna, with a monopole length that is less than one-twentieth of a wavelength. This is due to the recognition that the spiral transmission line of the present invention is short circuited at the input - end thereof while the output end remains an open circuit. The point at which the transmission line is short circuited is adjustable and results in a variation of the input impedance thereby permitting one to obtain a desired characteristic impedance to excite the monopole. The short circuiting characteristic forces relatively high current to flow through the monopole which produces high efficiency radiation. The transmission line of the invention is formed on a printed circuit, in the form of a spiral to minimize the space requirement for the resulting antenna. The spiral has balanced inductance and capacitance which forces oppositely flowing currents in the transmission line to cancel their resulting effects in the distant field. Accordingly, this transmission line design does not suffer from the disadvantages of previously conceived capacitive loaded antennas.

The above-mentioned objects and advantages of the present invention will be more clearly understood when considered in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an electrical schematic diagram illustrating the electrical equivalent of the antenna presented by the invention.

FIG. 2 is a cross sectional view of the present invention illustrating the relationships of the antenna components.

FIG. 3 is a perspective view of the present invention, again illustrating the disposition of the antenna components, as shown in FIG. 2.

FIG. 4 is a disassembled view of an antenna in accordance with the present invention wherein multiple spiral transmission lines are employed with a single feed line to effect multiple frequency antenna operation.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the figures and more particularly FIG. 1 reference numeral 10 generally indicates a feed line from a coaxial cable 8 which is connected to a first terminal of an RF source 12, the opposite terminal of the source being connected to a ground plane 16 (conductor). A cylindrical conductor 14 serves as an external sleeve for the coaxial cable 8 and functions as a monopole radiator. Sleeve 14 is shorted to RF ground 16. A transmission line 17 is connected at its left end to the monopole 14, while the right end of the transmission line remains an open circuit. The right end of the feed line 10 is connected to a point 18, along the transmission line. Depending upon where this point is chosen, the input impedance will vary. Typically, the impedance may be equal to 50 ohms. Also, for purposes of illustration, the length of the transmission line is chosen to be less than one-quarter of a wave-length. The dimension between the transmission line and the ground plane is typically three-quarter inches. In order to achieve a true RF short-circuit at the monopole 14, the left-most end 19 of the transmission line is electrically shorted to the monopole 14. A variable capacitor may be connected as illustrated by 20, between the right open circuit end of the transmission line and the ground plane.

FIG. 2 shows a cross sectional view of a preferred embodiment of the invention which is shown in perspective, in FIG. 3. As illustrated in FIG. 2, a coaxial cable 8 extends upwardly in concentric relation with the hollow cylinder 14 which constitutes a radiating monopole. The feed line 10, extending through the cable, protrudes from the upper end of the cable and is connected to the transmission line. For purposes of consistency, the components denoted in FIG. 2 carry the same members ascribed in FIG. 1. Thus, the connection point between the feed line 10 and the transmission line 17 is shown at 18. The transmission line 17 is a spiral conductor formed on a printed circuit above the ground plane 16. The substrate of the printed circuit may be Teflon Fiberglass, and denoted by 26. The monopole 14 is electrically connected to the coaxial cable 8 as seen by the conductor ring 22. A similar ring connector electrically connects the monopole to the ground plane 16, as will be seen by connection 24. The transmission line is shorted at one end thereof at 19 by suitably connecting the upper end of the monopole 14 to a radially inward spiral section. The structure of the antenna, as shown in FIG. 2 will achieve omnidirectional (in horizontal plane), vertically polarized radiation as indicated by the E field vector 28.

FIG. 3 shows the components of the antenna in a perspective view. Similar components have been labeled alike in FIGS. 1, 2, and 3.

FFIG. 4 illustrates an alternate embodiment of the present invention which permits multiple frequency operation of the antenna. As will be observed, the transmission line printed circuit, ground plane, and monopole look fairly similar in both FIGS. 3 and 4. However, there are distinct differences in the structure of the two embodiments. The transmission line printed circuit 30, of FIG. 4 includes two central arcuate conductor segments 32 and 34 that respectively connect the spiral transmission lines 36 and 38 thereto. These spiral conductors, constituting two transmission lines, are electrically isolated from one another on the printed circuit board. Each spiral transmission line operates in a different frequency mode for the antenna. Thus, the feed line 10 can carry signals at two frequencies. This means that the antenna can operate at a first frequency for transmission while it receives at a second frequency. Alternately, it may transmit or receive at two respectively different frequencies. To complete the dual frequency operation, a T conductor having segments 40 and 42, is disposed at the top of the feed line 10 as it protrudes upwardly from the monopole. By connecting components 40-32 and 42-34, the feed line will communicate with the respective spiral transmission lines 36 and 38.

In order to tune an antenna operating at dual frequencies, it is first necessary to set the position of component 40 at a point along spiral 32, where the desired impedance is reached. Then, the same procedure is followed with respect to components 42 and 34. However, the connection of the latter components will cause a shift in the impedance between 40 and 32. Respective iterative tunings will finally result in the desired impedance settings for both transmissions lines.

In order to rigidify the components of the antenna, the space between the transmission line printed circuit board 30 and the ground plane 16 is filled with a suitable dielectric, such as polyurethane material.

Although the previous discussion included single and dual frequency modes, it is emphasized that a greater number of frequencies can be operated upon by the antenna. In certain applications, multiple feed lines, such as 10, can be used. However, this does not depart from the principals for the invention.

I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described for obvious modifications can be made by a person skilled in the art.

Claims

Wherefore I claim the following:

1. A miniaturized antenna structure comprising:

a metal ground plane member;

a hollowed cylindrical monopole perpendicularly disposed and electrically connected at one end thereof to the ground plane member for radiating electromagnetic energy therefrom;

a printed circuit board located at a second end of the monopole, and in parallel spaced relation to the ground plane member, the printed circuit board including an insulator substrate and a spiral conductor transmission line formed thereon, the transmission line having a first end short circuited to the monopole while an opposite second end is open circuited; and

a coaxial cable extending through the cylindrical monopole, the cable being connected to an r.f source at one end thereof, an outer cable conductor connected at an opposite end to the cylindrical monopole, an inner cable conductor connected between the source and a point along the transmission line for producing preselected transmission line impedance, whereby the monopole radiates energy in an onmidirectional pattern.

2. The subject matter of claim 1 wherein the printed circuit board includes a second spiral conductor, generally concentric with the first, and further means are provided to connect the inner cable conductor with a point along the transmission line for effecting multiple frequency operation of the antenna structure.

3. The subject matter as defined in claim 1 together with a variable capacitor connected across the open circuited end of the transmission line for tuning a preselected frequency of operation.

4. A small antenna structure comprising:

a ground plane member;

monopole radiating means perpendicular disposed and electrically connected at one end thereof to the ground plane member for radiating electromagnetic energy therefrom;

a spiral transmission line located at a second end of the radiating means, and in parallel spaced relation to the ground plane member, the transmission line having a first end short circuited to the radiating means while an opposite second end is open circuited; and

feed line means connected at a first end to an r.f. input source, a second end connected to a point along the transmission line for producing a preselected transmission line impedance;

a second spiral transmission line positioned concentric with the first transmission line;

means for connecting the feed line means to a point along the second spiral transmission line for effecting multiple frequency operation of the antenna structure.

5. The subject matter of claim 4 together with a second feed line means for connecting the source to a point along the second spiral transmission line.