This inventioll relates to microstrip antennas and, more particular-ly, to an antenna design whicll 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 whicll tlle radiating e]ement is typically a rectangular patch of metal etched on one side oE 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 diEferent resonant frequen-cies are desired, then different circuit board constructions are needed,either changing the dielectric c~nstant 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 werc desired to usc s~lch microstrip antennas in secured communications systems, Identification Fricnd or Foe systems or similar systems req~l;ring operation in two or more discrete bands, it will be readily apparcnt that sucll arrangement would be fairly cumbcrsome and of increased manu~acturing cost.
~ s will become clear hereinafter, the microstril~ 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 meas~1red with respect to the output termination will determine the frequency at which t~e antenna resonates, it becomes possible to operate the antenna at different frequencies simply by positioning the open-circuit and shor-t-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 contro]ling the - 1 - ~
effective length or a strip l;ne set ul, to permit diverse frequency opera-tions.
These and other features of the present invention ~ill be more clearly ~mderstood from a consideration of the following clescription, taken in connection with the accompanying drawing in which:
Figure 1 shows a microstrip antenna constructed in accordance with the prior art;
Figure 2 shows a terminated microstrip antenna constructed in accordance with the invention;
Figures 3 and 4 are a series of curves showi~g resonant frequency characteristics as exemplified by microstrip antennas constructed in accor-dance with the present invention; and Figures 5-lO show typical radiation patterns obtained using the terminated microstrip antenna technique oE the invention.
In Figure 1, ~he microstrip antenna 10 is shown as comprising a circuit board 12, the back side of whicll (not shown) is clad entirely of a metal material, typically copper. In conventional con,tructions, the Eront side of the circuit board is clad oE like material, except in the areas 14 and 16, where the metal is etched away to reveal the dielectric material 17 underneath. tIn the preferred embodiments of the invention described, dielectric materials available under the tradenames Polyguide and Duroid were employed.) A section of met:l 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 1-l/2 mils thick overlying a 1/8 inch thick Duroid dielec-tric was employed for radiating in the L-band of frequencies. When con-structed 4~655 inches on a side, and with the etched areas 14, 16 extending approximately 0.988 inches each, the microstrip antenna of Figure 1 exhibits a resonant frequency of some 1370 ~l~lz, and exhibits a resonant frequency : ~)97~Z~3 . .
characteristic as shown by the curve A in Figure 3. The dimensions of the microstrip transformer 18, illustrated by reference numerals 25~30 were as ollows:
Length 25 ........... 0.772 inches Length 26 ~ O~ 0.872 inches Arc 27 ~OO~O~ 0.600 inch radius Arc 28 ~O~ 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 120 In accordance with the present invention, however, I have fo~md 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 Figure 2 (wherein like re:Eerence numerals are employed to identify parts corresponding to those in Figure 1), I have found that if an output termination corresponding in dimension to the impedance matching input trans-former were provided at the opposite side of the rectangular metal plate 20, and then 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 Figure 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 micro-strip antenna then having a resonant frequency characteristic as depicted by tle curve C in Figure 3. In other words, with the dimensions of this output termination 32 being as follows:
Length 33 ~OOO~ 0.772 inches Length 34 ........... 0.872 inches Arc 35 ~OO~O~ OOO 0.600 inch radius Arc 36 ~ O~O~ 0.400 inch radius Width 37 ~O~OOO~ 0.200 inches `
1~397428 Distance 38 ~ O 0.500 inches, measured with respect to the vertical center line of the circuit board l2~ the resonant frequency of the microstrip antenna can be lowered by some 10 ~IIIz or raised some ~lllz 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-circuit-ing and open-circuiting conditions.
I have further found that different changes iTI the resonant fre-quency 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 thnt the microstrip racliator 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 Mllz would be exhibited if the remote en(l of that 2.55 cm line were open-circuited lnstead. Curves D and E in FLgure 3 illustrate the resonant Erequency charac-terlstics, respectively, Cor these conditions. Other resonallt frelluencies have been implemented by the addition of diff~erent lengths o 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 Mllz measured for a 2.55 cm line, the addition of an added length equal to ~/2 at this frequency, produces the original resonance at 1312 MHz, but a second resonance simultaneously at 1457 M~lz, the overall length being then 13.8 cm from the output terminal 40. Such characteristics are illustrated by the curve A and B of Figure 4. Experimentation has shown that it is possible to shaft these two resonant frequencies by varying the length of tlle terminating line. Thus, in one experiment, one length of short-circuited line resulted in a resonant frequency of 1260 M~lz an(l a VSWR oE 1.58:1 at its low end, and a second, ~L~)9~21!3 higher resonant frcquency of 1400 ~IHæ with a VSWR of l.01 1. A second length oL short-circuitcd line yielded a i-esonant frequency oE 1330 Mllz at a VSWR of 1.35:1, and a second resonance of 1450 Mllz at a VSWR o~` 1.22:1. A
further length experimented with, when sllort-circuited, yielded a resonant frequency of 1352 ~II-Iz at a VSWR of 1.14:1; and a highcr rcsonance of 1500 ~]llz at a 1.48:1 VSWR. Analysis has shown that multiple resonances are also possi-ble by further increases in the terminating line length as the impedance of the line repeats itself every one-half wavelength. Analysis has also indica-ted 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 microstrip antenna configurat;on oE
Figure 2 can be made t~mable by terminating the o~ltptlt port 40 Witll a variable short-circuit length. Results using such technique are ~abulate(l below for tuning to a minimum VSWR at the indicated frequencies by varying the length of the short-circuited iinc.
f ~IIIZ VSW~
1200 1.80:1 1250 1.70:1 1300 1.35:1 1350 1.14:1 1400 1.01:1 1450 1.22:1 1500 1.48:1 1550 2.20:1 As will be readily apparent to those skilled in the art, these results indi-cate that a microstrip antenna terminated in accordance with the present invention can operate over a range of frequencies more than 200 ~z, 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 bandwiclth obtained is comparable to those of the curves oE Figures 3 and 4, and the single m;crostrip antenna 742~3 still operate narrow band. As will be understood, prior art teachings of microstrip antennas indicated the need for a diffcrent antenna coniiguration for each frequency of operation desired. Analysis has shown thaL 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 Figures 5-10 show typical E- and }1- 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, Figure 5 shows the pattern for tlle 1360 MHz operation with the outp~lt terminal 40 short-circuited, whereas Figure 6 shows the pattern for the 1410 Mllz operation with the output terminal 40 open-cir-cuited; Figure 7 similarly shows the radiatic)n pnttcrn for 13]2 ~IIIz witll a short-circuit 2.55 cm from terminal 40, wllereas the patterns oE Figllre 8 show the operation at 1385 ~IIIz for an open-circuit 2.55 cm Erom terminal 40; and the patterns of Figures 9 and 10 represent those obtained with a short-circuit condition 13.8 cm rrom thc output terminnl 40, thc doul)ly rcsonanL condLLion, in which Figure 9 indicates the pattern at the lower resonance of 1312 MHz while Figure 10 represents the pattern at the higher resonant frequency, 1457 ~l~lz. As will be appreciated, àny differences illustrated by these patterns obtained sre 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 of 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 trans missions 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 electro-mechanical or electronic means. For secured communications systcms, or otherarrangements requiring operation over two or more discretc bands, the tunahle version of the invention will be seen to be the more attractive one, where ~1~97~28 different frequenc;es of operati.on can be had by terminati.ng the ].i.ne lengths at calibrated locations. Where the termination is to be by short-ci.rcuit, furthermore, it becomes a relatively easy matter to print a 3.engtll of micro-strip line on the microstrip circuit board itself, with apertures along the length thereof, for example, into which short-circuiting metallic p:ins could be insertecl ~o provi~e the short-circuit termination at Lhc clesired location.
With such a version, the simplicity of the approach oi~. the i.nventi.cn, its associated Icw cost, and concomitant light weight wi.ll be apparent.
While there have been described what are considered to be preferred embodiments oE 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 ehanging the resonant frequency of a microstri.p antenna by terminati.ng the antenna with short-circuit or open-ci.reui.t condi.tions. For at least such reason, resort should be had to the claims appencled llereto for a correct understanding of the scope of the invention.