Broadband High Efficiency Full Wave Open Coaxial Stub Loop Antenna

With the advent of tuned loop antennas as a convenient antenna structure for reduced operational space requirements, there is a need for a broadband loop antenna which does not require tuning and yields near 100% efficiency. With conventional tuned loop structures, one to three dB of antenna gain may be lost due to inherent losses of the LC networks operating at high current levels as a result of high Q. This results in narrow operational bandwidths.

The present invention relates to single wire full wavelength loop antennas which are usually fed at a high impedance and which exhibit a narrow bandwidth. A full wave open stub coaxial loop provides maximum efficiency and maximum bandwidth at a convenient inherent 50 ohm feed impedance. The electrical length of the coaxial loop is greater than 1.4 wavelengths.

Prior loop antennas yield gains of -4 dBil (.lambda./4 loop over a ground plane) at operating Q's of near 300. Efficient full wave wire loop antennas without frequency compensation yield Q's of 20 or greater.

The present invention was developed in order to provide the same high efficiency of prior loop antennas but with Q's of 3-4 (25-30% bandwidth). This compares favorably with the prior designs which have only about a 5% operational bandwidth.

Accordingly, it is a primary object of the invention to provide a broadband, high efficiency antenna radiator adapted for connection with an unbalanced feed line and source. The antenna comprises a section of coaxial cable having a length of one wavelength. The cable has a center conductor arranged in a continuous loop and an outer being arranged in a shorter loop to define an open section. A T-shaped connector connects the ends of the cable center conductor with the center conductor of the feed line and a bridging connector connects one end of the coaxial shield with the shield of the feed line. The radiator is thus configured as a coaxial open stub loop which provides maximum efficiency and bandwidth. The low Q or wide bandwidth of operation is due to the equivalent parallel tuned circuit achieved by use of the one wavelength open stub which comprises the loop itself.

According to another object of the invention, the loop configuration comprises a Yagi configuration for unidirectional operation with greater gain. Two loop and three loop Yagi configurations may be provided yielding high gain and extended operational bandwidths.

According to yet another object of the invention, the single loop may be narrowed physically for a wideband half-wave coaxial stub folded dipole having maximum gain and approximately 25% bandwidth.

Other objects and advantages of the invention will become apparent from a study of the following specification when viewed in the light of the accompanying drawing, in which:

FIG. 1a is a plan view of the open stub full wave coaxial loop antenna according to the invention;

FIG. 1b is a schematic representation of the dipole equivalent for the antenna of FIG. 1a;

FIGS. 2a and 2b are diagrammatic and schematic representations, respectively, of the circuit equivalent for the antenna of FIG. 1a;

FIG. 3a is a plan view of the current flow through the antenna of FIG. 1a and FIG. 3b is a diagrammatic equivalent of the antenna of FIG. 3a;

FIGS. 4a and 4b are graphical representations of the VSWR and impedance, respectively, of the loop antenna of FIG. 1a with a ground plane reflector;

FIGS. 4c and 4d are graphical representations of the VSWR and impedance, respectively, of the loop antenna of FIG. 1b without a ground plane reflector;

FIG. 5 is a graphical representation of the E-plane 3.2 dBi radiation pattern of the full wave loop antenna according to the invention;

FIG. 6 is a graphical representation of the E-plane radiation pattern of the full wave loop antenna according to the invention arranged one quarter wavelength above a ground plane;

FIG. 7 is a graphical representation of the E-plane radiation pattern of a two loop coaxial Yagi configuration with the longer reflector loop feed shorted and spaced at 0.2.lambda.;

FIG. 8 is a graphical representation of the E-plane radiation pattern of a three loop Yagi configuration with the loops spaced at 0.2.lambda.;

FIG. 9 is a graphical representation of the E-plane radiation pattern of a folded dipole coaxial loop antenna; and

FIG. 10 is a graph showing the gain and bandwidth for the open stub full wave coaxial loop antenna, and the two loop Yagi, and three loop Yagi antennas.

The full wave open stub coaxial loop antenna 2 is shown in detail in FIG. 1a. It comprises a section of coaxial cable 4, such as 50 ohm coaxial cable, having a length 1 of one wavelength .lambda.. The cable includes a center conductor 6 arranged in a continuous loop and an outer shield 8 arranged in a shorter loop to define an open section or stub 10.

The loop antenna 2 is connected with an unbalanced feed line 12 and source (not shown). More particularly, a T-shaped connector 14 connects the center conductor 6 of the loop with a center conductor 16 of the feed line and a bridging connector 18 connects the shield 8 of the loop with the shield 20 of the feed line. Due to the geometry of the antenna being in the form of a loop, the phase and voltage at the end of the center conductor 6 are the same as that of the feed line center conductor 16.

As will be developed in greater detail below, current flowing in the shield of the loop antenna generates two electric dipoles 22 shown in FIG. 1b.

The principle of the design of the full wave open stub coaxial loop antenna is that of an unterminated, unbalanced transmission line 24 which is near one and a half wavelengths long and provides an additional equivalent parallel tuned circuit in parallel with the series tuned circuit of the loop with the generator 25 feeding the loop as shown in FIG. 2a. The circuit equivalent is shown in FIG. 2b.

The open stub coaxial loop antenna operates at a lower Q or in a wider bandwidth due to the equivalent parallel tuned circuit accomplished by use of the one wavelength open stub which is actually the loop itself. The parallel tuned circuit appears across the equivalent series tuned circuit of the loop with the generator in parallel.

The current flow through the open stub coaxial loop antenna 2 is shown in FIG. 3a with the diagrammatic equivalent being shown in FIG. 3b. The current flow in the shield of the loop generates two electric dipoles, one centered at the feed point and the other .lambda./4 or one diameter away across from the feedpoint of the loop antenna.

Radiation occurs from the reflected standing waves from the unterminated end of the coaxial cable along the unterminated shield of the cable. The velocity factor of the coaxial cable does not slow the radiated wave since that radiation occurs on the outside of the shield as shown in FIG. 3a. This is verified by the physical length versus the electrical length of the coaxial cable determining the center of the operational frequency of the coaxial open stub loop antenna. No balun or impedance transformer is required.

The loop antenna according to the invention can be used with or without a ground plane reflector. FIGS. 4a and 4b illustrate the VSWR and impedance of the antenna with a ground plane reflector, while FIGS. 4c and 4d illustrate the VSWR and impedance of the antenna without a ground plane reflector.

In FIG. 5 is shown the radiation pattern generated by the two dipoles of the antenna. The dipoles are in phase .lambda./ apart. The radiation pattern has 3.2 dBil gain at bandwidths of near 30%. The antenna configuration according to the invention results in a structure that is not easily detuned by a high dielectric loading environment.

Referring now to FIG. 6, there is shown in the center a broadband 50 ohm stub coaxial loop 2 placed over a 1.lambda. by 1.lambda. ground plane metal plate 26 at a height of 0.2.lambda.. The E-plane radiation pattern is also shown. The positioning of the antenna relative to the ground plane metal plate yields an additional 6 dBi gain for a total gain of 9.2 dBil with front to back pattern ratios of 20 dB and cross polarization values of -10 to -15 dB. The impedance and operational bandwidth of the loop radiator vary less than 10% with the additional loading of the ground plane as compared to those shown in FIGS. 4b and 4d.

Loop arrays of up to four loops fed in phase and spaced 0.8.lambda. apart (center to center) at a height of 0.2.lambda. above a ground plane yield 15.2 dBil of forward gain, pattern front to back ratios of near 20 dB, with E and H half power beamwidth of 32.degree..

Other embodiments of the loop antenna and the E-plane radiation patterns therefor are shown in FIGS. 7-9. More particularly, in FIG. 7 there is shown a full-wave open stub coaxial loop in the form of a two loop Yagi antenna 28 and in FIG. 8 is shown a coaxial loop in the form of a three loop Yagi 30. The two loop Yagi yields a 6.5 dBil forward gain and the three loop Yagi yields a 7.2 dBil forward gain. Front to back pattern ratios of 10-20 dB are achieved. In FIG. 9 there is shown a coaxial loop in the form of a folded dipole 32 and its associated E-plane radiation pattern. The folded dipole comprises a single loop narrowed physically to define a wideband half-wave coaxial stub folded dipole with maximum gain and 25% bandwidth.

In FIG. 10 there is shown a comparison of the gain achieved by the open stub coaxial loop (FIG. 5), the two loop Yagi (FIG. 7), and the three loop Yagi (FIG. 8). As shown therein, the two and three loop Yagi arrays yield higher gains. The reflector and director elements of the Yagi arrays are typically 12% longer and shorter, respectively, and the arrays yield 20 to 30% operational bandwidths. The driven element maintains its 50 ohm match without adjustment with either or both a reflector or director added.

With the full wave open coaxial stub loop antenna according to the invention, there is provided a large gain bandwidth device due to the frequency compensation of the open stub and the elimination of the usual balun.