Terminated Microstrip Antenna

This invention relates to microstrip antennas and, more particularly, to an antenna design which permits a single antenna construction to operate over a range of frequencies with acceptable voltage-standing-wave-ratios (VSWR).

As is well known and understood, a microstrip antenna is a printed circuit device in which the radiating element is typically a rectangular patch of metal etched on one side of a dual-clad circuit board. As is also well known and understood, the microstrip antenna is a narrow band device which operates at a single resonant frequency. If different resonant frequencies are desired, then different circuit board constructions are needed, either changing the dielectric constant of the circuit board material for a given element size, or changing the size of the radiating element for the same dielectric constant. If it were desired to use such microstrip antennas in secured communications systems, Identification Friend or Foe systems or similar systems requiring operation in two or more discrete bands, it will be readily apparent that such arrangement would be fairly cumbersome and of increased manufacturing cost.

As will become clear hereinafter, the microstrip antenna design of the invention follows from a finding that the resonant frequency of a given size radiator can be changed by providing it with an output termination, and by open-circuiting or short-circuiting that termination. With the additional finding that the location of the open-circuit or short-circuit measured with respect to the output termination will determine the frequency at which the antenna resonates, it becomes possible to operate the antenna at different frequencies simply by positioning the open-circuit and short-circuit positions. When added to the further finding that the resonant frequency will change from an open-circuit termination to a short-circuit termination, it will thus be apparent that a tunable microstrip antenna can be easily fabricated, simply by varying the termination length and/or the output condition, either open-circuit or short-circuit. As will also be readily apparent to those skilled in the art, the open-circuit and short-circuit conditions and locations can be changed either locally or remotely, as by electronically controlling the effective length of a strip line set up to permit diverse frequency operations.

These and other features of the present invention will be more clearly understood from a consideration of the following description, taken in connection with the accompanying drawing in which:

FIG. 1 shows a microstrip antenna constructed in accordance with the prior art;

FIG. 2 shows a terminated microstrip antenna constructed in accordance with the invention;

FIGS. 3 and 4 are a series of curves showing resonant frequency characteristics as exemplified by microstrip antennas constructed in accordance with the present invention; and

FIGS. 5-10 show typical radiation patterns obtained using the terminated microstrip antenna technique of the invention.

In FIG. 1, the microstrip antenna 10 is shown as comprising a circuit board 12, the back side of which (not shown) is clad entirely of a metal material, typically copper. In conventional constructions, the front side of the circuit board is clad of like material, except in the areas 14 and 16, where the metal is etched away to reveal the dielectric material 17 underneath. (In the preferred embodiments of the invention described, dielectric materials available under the tradenames Polyguide and Duroid were employed.) A section of metal extends from the rectangular metal plate 20 so formed, to operate as a microstrip transformer in matching the impedance at the input to the patch 22 to the impedance at the signal imput jack 24, usually the output from a coaxial cable coupled through the back side of the circuit board 12. In one embodiment of the construction, a circuit board clad with copper 11/2 mils thick overlying a 1/8 inch thick Duroid dielectric was employed for radiating in the L-band of frequencies. When constructed 4.655 inches on a side, and with the etched areas 14, 16 extending approximately 0.988 inches each, the microstrip antenna of FIG. 1 exhibits a resonant frequency of some 1370 MHz, and exhibits a resonant frequency characteristic as shown by the curve A in FIG. 3. The dimensions of the microstrip transformer 18, illustrated by reference numerals 25-30 were as follows:

Length 25--0.772 inches

Length 26--0.872 inches

Arc 27--0.600 inch radius

Arc 28--0.400 inch radius

Width 29--0.200 inches

Distance 30--0.500 inches, measured with respect to the vertical center line of the circuit board 12.

In accordance with the present invention, however, I have found that the resonant frequency of this described radiator can be changed by providing it with an output termination, and by open-circuiting or short-circuiting that termination. For example, and referring to the microstrip antenna of FIG. 2 (wherein like reference numerals are employed to identify parts corresponding to those in FIG. 1), I have found that if an output termination corresponding in dimension to the impedance matching input transformer were provided at the opposite side of the rectangular metal plate 20, and the short-circuited, that the resultant resonant frequency would be reduced to approximately 1360 MHz, with the resonant frequency characteristic then being shown by the curve B in FIG. 3. I have further found that if this output termination were open-circuited instead, the resonant frequency of the radiator would be increased to approximately 1410 MHz, with the microstrip antenna then having a resonant frequency characteristic as depicted by the curve C in FIG. 3. In other words, with the dimensions of this output termination 32 being as follows:

Length 33--0.772 inches

Length 34--0.872 inches

Arc 35--0.600 inch radius

Arc 36--0.400 inch radius

Width 37--0.200 inches

Distance 38--0.500 inches, measured with respect to the vertical center line of the circuit board 12, the resonant frequency of the microstrip antenna can be lowered by some 10 MHz or raised some 40 MHz simply by short-circuiting or open-circuiting, respectively, the output terminal 40 of the termination 32.

In the foregoing description, it will be understood that a coaxial connector was employed at the output terminal 40 to perform the short-circuiting and open-circuiting conditions.

I have further found that different changes in the resonant frequency could be effected by adding different lengths of coaxial line onto the connector, and then short-circuiting or open-circuiting their ends. For example, I have found that the microstrip radiator will resonate at 1310 MHz if a 2.55 cm coaxial line were added to the output connector and its remote end short-circuited, whereas a resonant frequency of approximately 1385 MHz would be exhibited if the remote end of that 2.55 cm line were open-circuited instead. Curves D and E in FIG. 3 illustrate the resonant frequency characteristics, respectively, for these conditions. Other resonant frequencies have been implemented by the addition of different lengths of coaxial line to the output connector at terminal 40, and altering the termination condition from short-circuit to open-circuit at the end of the added line.

I have additionally found that it is possible to obtain double-resonance conditions from a microstrip radiator having a single length of short-circuited line added to the output connector. In particular, with a short-circuit condition providing a resonant frequency of 1312 MHz measured for a 2.55 cm line, the addition of an added length equal to .lambda./2 at this frequency, produces the original resonance at 1312 MHz, but a second resonance simultaneously at 1457 MHz, the overall length being then 13.8 cm from the output terminal 40. Such characteristics are illustrated by the curve A and B of FIG. 4. Experimentation has shown that it is possible to shift these two resonant frequencies by varying the length of the terminating line. Thus, in one experiment, one length of short-circuited line resulted in a resonant frequency of 1260 MHz and a VSWR of 1.58:1 at its low end, and a second, higher resonant frequency of 1400 MHz with a VSWR of 1.01:1. A second length of short-circuited line yielded a resonant frequency of 1330 MHz at a VSWR of 1.35:1, and a second resonance of 1450 MHz at a VSWR of 1.22:1. A further length experimented with, when short-circuited, yielded a resonant frequency of 1352 MHz at a VSWR of 1.14:1, and a higher resonance of 1500 MHz at a 1.48:1 VSWR. Analysis has shown that multiple resonances are also possible by further increases in the terminating line length as the impedance of the line repeats itself every one-half wavelength. Analysis has also indicated that similar double, triple, etc. resonances can be obtained by adding such lengths of line to the output terminal 40, and then open-circuiting their remote ends.

I have also found that the micorstrip antenna configuration of FIG. 2 can be made tunable by terminating the output port 40 with a variable short-circuit length. Results using such technique are tabulated below for tuning to a minimum VSWR at the indicated frequencies by varying the length of the short-circuited line.

As will be readily apparent to those skilled in the art, these results indicate that a microstrip antenna terminated in accordance with the present invention can operate over a range of frequencies more than 200 MHz, and with a VSWR of less than 1.50:1, simply by having a calibrated length of line which can be short-circuited at select locations. The experimental results also indicated that the instantaneous bandwidth obtained is comparable to those of the curves of FIGS. 3 and 4, and the single microstrip antenna still operate narrow band. As will be understood, prior art teachings of microstrip antennas indicated the need for a different antenna configuration for each frequency of operation desired. Analysis has shown that not only will the variable short-circuit mode of working with a terminated line provide this tunable embodiment, but that similar tuning could be obtained with appropriate design of a variable open-circuited line.

The radiation patterns of FIGS. 5-10 show typical E- and H- plane patterns for the terminated microstrip antenna of the invention, the "dashed" pattern being that for the E- plane and the "solid" pattern being that for the H- plane. In particular, FIG. 5 shows the pattern for the 1360 MHz operation with the output terminal 40 short-circuited, whereas FIG. 6 shows the pattern for the 1410 MHz operation with the output terminal 40 open-circuited; FIG. 7 similarly shows the radiation pattern for 1312 MHz with a short-circuit 2.55 cm from terminal 40, whereas the patterns of FIG. 8 show the operation at 1385 MHz for an open-circuit 2.55 cm from terminal 40; and the patterns of FIGS. 9 and 10 represent those obtained with a short-circuit condition 13.8 cm from the output terminal 40, the doubly resonant condition, in which FIG. 9 indicates the pattern at the lower resonance of 1312 MHz while FIG. 10 represents the pattern at the higher resonant frequency, 1457 MHz. As will be appreciated, any differences illustrated by these patterns obtained are very slight and insignificant.

In the simplest form of the invention, therefore, it will be seen that the microstrip antenna of the present invention provides an increase in usefulness by permitting a frequency diversity operation at two different frequencies, merely by changing the output termination from a short-circuit to an open-circuit condition. In transponder operation, for example, where transmissions are at one frequency and receptions are at another frequency, the usefulness of the terminated microstrip antenna will be evident, with the changes in termination being done manually, or made remotely through electromechanical or electronic means. For secured communications systems, or other arrangements requiring operation over two or more discrete bands, the tunable version of the invention will be seen to be the more attractive one, where different frequencies of operation can be had by terminating the line lengths at calibrated locations. Where the termination is to be by short-circuit, furthermore, it becomes a relatively easy matter to print a length of microstrip line on the microstrip circuit board itself, with apertures along the length thereof, for example, into which short-circuiting metallic pins could be inserted to provide the short-circuit termination at the desired location. With such a version, the simplicity of the approach of the invention, its associated low cost, and concomitant light weight will be apparent.

While there have been described what are considered to be preferred embodiments of the present invention it will be appreciated that changes may be made by those skilled in the art without departing from the scope of the teachings herein of changing the resonant frequency of a microstrip antenna by terminating the antenna with short-circuit or open-circuit conditions. For at least such reason, resort should be had to the claims appended hereto for a correct understanding of the scope of the invention.