Transmission Line Loaded Annular Slot Antenna

Abstract

A probe is connected to the bottom of a cavity, and a spiraled conductor is connected to the top of the probe, where the outer turns of the spiral are spaced from the cavity top edges to form an electrically small slot. This antenna is fed from the cavity bottom near the probe by the extension of the center conductor of a terminated coaxial line.

Description

The present invention relates to antennas, and more particularly, to a miniature transmission line loaded slot antenna.

Conventional annular slot antennas are approximately one-half wavelength in diameter. It is an object of the present invention to provide a similar antenna having a diameter of only approximately 0.05 wavelength.

The accompanying drawings illustrate one embodiment of our invention. In the drawings,

FIG. 1 is a top pr perspective view of the present slot antenna.

FIG. 2 is a cross section taken as indicated by broken line 2-2 in FIG. 1.

FIG. 3 is a plan view diagram of an alternate form of spiral conductor.

Referring first to FIGS. 1 and 2, a substantially square metal cavity 1 is provided, having a bottom surface 2, sides 4, and upper surfaces 5. A conductive probe 6 is electrically connected to the bottom surface 2 and projects upwardly to the plane of upper surfaces 5. The center of a spiral conductor 7 is connected to the upper end of probe 6. The spiral conductor 7 is wound substantially in the plane of the upper surfaces 5 of the cavity, and an annular space S acting as a slot is left between the outer spiral turns and the cavity sides 4. The diameter D of slot S is only approximately 0.05 wavelength. The diameter D is nominally either the outside diameter of the slot when the latter is round or the longest side of a rectangular slot when the slot has square corners. It is the same as the diameter of the cavity from sidewall to sidewall.

This antenna is shunt fed by a coaxial cable 9 having its outer sheath 10 connected to the bottom surface 2 and its center wire 11 brought up through an insulated aperture 12 to connect to the spiral conductor 7 near the probe 6. The feed connection wire 11 shown herein may alternatively be connected to the side of the probe 6 instead of to the spiral, as shown by dotted line 11a in FIG. 2. The choice as to location of this connection is dependent upon the antenna quality factor, Q, and the antenna input impedance which is desired at the terminals of the cable 9. The cavity 1 may have a ground connection 13.

The spiral conductor 7 loads the probe 6. This combination excites the radial mode of electromagnetic propagation inside the cavity 1. The radial mode across the top cavity surface 5 does not radiate substantially when the cavity is small whereas the vertical mode propagates along the upper surfaces 5 outward in the form of radiation. Along the plane of the spiral 7 and inside the cavity, these two modes are coupled to the transmission line and cavity conductors. The current flowing in the conductors transfers energy between the two modes as a function of position and time. At the frequency of resonance, the position of maximum current is in the proper phase-time lag to create standing waves in the structure. In this way, a real input impedance is achieved. The radiation pattern is similar to that of a quarter-wave monopole on a ground plane. In the position shown in FIGS. 1 and 2, the polarization is vertical. In other mounting positions of the assembly, the polarization is in the direction perpendicular to the surfaces 5 regardless of whether the probe 6 is perpendicular to the cavity bottom 2 or not.

A substantially square spiral is shown herein as an example. However, the slot S can be rectangular, circular, elliptical, or other shapes, while of course using a similarly shaped spiral.

In a conventional slot antenna as mentioned above, when the diameter of the slot is decreased below approximately one-half wavelength, the antenna becomes inadequate due to the high impedance which develops. The cavity top wall behaves as a small capacity, and insufficient current flows in the probe. However, the present invention circumvents this problem.

The image theory of a conductor above a conducting ground plane is well known in the art. The spiral conductor 7 of the present invention with its image below the ground plane can be treated as a two-wire transmission line which terminates the ends of the probe 6 and its image. Since current flow in the image conductor of the transmission line is opposite to that in the conductor itself, cancellation occurs and there is essentially no radiation from the transmission line (spiral conductor 7). However, current flow in the probe image (not shown) is in phase with that in the probe 6 itself, so that radiation from the probe image reinforces that from the probe 6.

In this case the spiral conductor 7 is made effectively one-quarter wavelength long, and the structure thus formed becomes resonant, resulting in high current flow in the probe 6. The probe thus efficiently excites the cavity in the radial mode.

This antenna can be tuned to various frequencies for a given configuration by means of one or more variable reactances as shown in FIG. 3. Here, two variable capacitors 14 are connected in series with the spiral conductor 7, for series tuning. Alternatively, a shunt capacitor 15 may be used, between the spiral and ground, or a combination of both series and shunt tuning elements may be used. These elements change the effective electrical length of the spiral, thus changing the resonant operating frequency.

Thus it is seen that the present antenna is only one-tenth the diameter of the convention annular slot antenna. This small diameter is a great advantage in aircraft, ground vehicles, and other applications, particularly in the HF and VHF ranges. The probe 6 and spiral 7 may conveniently be a single piece of metal tubing, supported if necessary by nonconductive supports. The antenna is preferably mounted with the upper cavity surfaces 5 flush with the outer skin of the object on which it is carried. A nominal preferred value for the depth of the cavity 1, i.e., the length of probe 6, is about 0.01 wavelength, but this value may vary considerably according to the antenna efficiency desired and other parameters.

While in order to comply with the statute, the invention has been described in language more or less specific as to structural features, it is to be understood that the invention is not limited to the specific features shown, but that the means and construction herein disclosed comprise the preferred form of putting the invention into effect, and the invention is therefore claimed in any of its forms or modifications within the legitimate and valid scope of the appended claims.

Claims

We claim:

1. An antenna comprising a conductive cavity, a spiral conductor lying substantially in the plane of the outer opening of said cavity with the outermost portion of said spiral conductor spaced from the sides of said cavity to form substantially an annular slot, and a conductive probe member electrically connecting the inner end of said spiral to the bottom of said cavity, the nominal diameter of said cavity being only approximately 0.05 wavelength at the opening frequency.

2. Apparatus in accordance with claim 1 including a first transmission line feed connection at the bottom of said cavity and a second such connection at said spiral conductor near said probe.

3. Apparatus in accordance with claim 1 including a first transmission line feed connection at the bottom of said cavity and a second such connection to the side of said probe.

4. Apparatus in accordance with claim 1 including tuning means connected to said spiral conductor.