BANDWIDTH LIMITATIONS OFLOG-PERIODIC MICROSTRIP PATCHANTENNA ARRAYS

Indexing terms: Antennas, Microstrip antennas, Log periodicarrays, Wide bandwidth

Results are presented for the frequency dependence of thepropagation constant of uniform microstrip patch arraysthat allow the bandwidth limitations of log-periodic patcharrays to be deduced. It is found that direct coupling of thepatch to the feed line limits the log-periodic bandwidth toabout a 2:1 frequency range. However, the introduction ofseries reactance into the equivalent circuit such as by the useof overlaid patches indicate that log-periodic action over avery wide bandwidth is possible which will then be limitedby the uniform substrate and production tolerances.

Introduction: The narrow operating bandwidth of microstrippatch antennas1 has recently prompted examination ofmethods for wideband operation using log-periodic tech-niques. Two examples are shown in Fig. 1 which use electro-magnetically coupled overlaid patches2 (Fig. la) and patchesdirectly coupled to the feed line through quarter-wavelengthmatching sections3 (Fig. \b). The arrays are analogous to thelog-periodic dipole array4 with the exception that in general abeam normal to the substrate is required in contrast to theend-fire dipole array and the dipole and patch radiators have,respectively, series and parallel connected equivalent circuitsnear resonance. These differences lead to significantly differentconclusions regarding how the radiating elements are con-nected to the feed line and the ultimate array bandwidth limi-tations. A k~fi analysis giving the frequency dependence of thepropagation constant of an infinite uniform series fed micro-strip patch array has been applied to the above configurations,and results are presented here that allow some conclusions onthis aspect to be drawn.

input*^-.

a

n f r fJ l c

feed"f~'=^~line

overlaidpatch

input-: •feed linequarter - wavelength

q_ coupling sectionpatch

Fig. 1 Log-periodic microstrip arrays

a Overlaid patch array; feed line on lower grounded substrate;patches on upper substrateb Quarter-wavelength line coupled patch array; patches and feedon single grounded substrate

Such analysis has been applied to dipole arrays5 andtogether with empirical deductions6 have allowed certain rec-ommendations to be evolved for good wideband array action.First, to prevent excitation of higher-order modes in the low-frequency elements, the array should be fed from the high-frequency end (Fig. 1) and should have high attenuationwithin and beyond the active region. Secondly, to ensure wide-band action, the propagation characteristic should have nostopbands below the frequency of the active region. Theserecommendations can be usefully applied in the design ofmicrostrip log-periodic arrays and are used here to interpretthe analytic results.

Analysis: The characteristic equation of an infinite period-ically loaded transmission line is given by5

cos fid =cos kd + jylx sin kd/2y0

(1)-jy 12sin kd/yo

ELECTRONICS LETTERS 24th May 1984 Vol. 20 No. 11

where /?' = /? + jot is the complex propagation constant, d isthe periodic length, k = 2nfyJ{ee)/c, ee and y0 are the micro-strip feed line effective dielectric constant and admittance,respectively, / is the frequency and c = 3 x 108 m/s. Mutualcoupling between adjacent elements only is considered; y n

and yl2 are the self and mutual admittances of the periodicloading elements—in this case the patches. The equivalent cir-cuits used are shown inset in Figs. 2 and 3. For the overlaidarray the transformer and coupling capacitor values wereempirically determined. In Figs. 2 and 3 kod = Infdjc.

-20 -10fid. rod

10 20 30otd. nepers

Fig. 2 Complex propagation constant /?' = /? + jtx of infinite uniformoverlaid patch array

w = 8 mm, / = 12 mm, d = 9-82 mm, p = 1-25 mm, patch and feedsubstrate thickness = 1-59 and 0-79 mm, respectively, er = 2-32;equivalent circuit of element shown inset

Results and deductions: Fig. 2 shows the propagation charac-teristic for the overlaid patch array. High attenuation occursaround kod = 1-42 where the patch is resonant, and byputting gr = 0 in the equivalent circuit this is identified asbeing largely due to radiation. It can also be seen that thereare no stopbands below the active region, indicating that it istheoretically possible to make a log-periodic array with aninfinite bandwidth using this element. In practice the band-width will be limited both by the uniform substrate thickness2

and, to a lesser extent, by the etching tolerances on the high-frequency patches.

25

10

0-5

-30 -10 0 10 20 30ad. nepers

Fig. 3 Complex propagation constant of infinite uniform quarter-wavelength line coupled patch array

w = 8 mm, / = 1 2 mm, d = 9-82 mm, substrate thickness =1 -59 mm, er = 2-32, wq = 0-5 mm; equivalent circuit of elementshown inset

Fig. 3 shows the propagation characteristic for the quarter-wavelength line coupled microstrip array. It can be seen thatthere is a stopband at kod = 0-4 below the active region atkod = 1-5, and this will result in a limited bandwidth capabil-ity. Although precise quantification is difficult, Fig. 3 suggeststhat this may well be between kod = 11 and 20, and henceless than a 2:1 frequency range. Such stopband action is dueto the high input admittance of the patch at the frequencywhen / ~ XJ4, where Xm is the microstrip wavelength.Although Pues3 has deduced that the array will have a limitedbandwidth due to the open-circuit feed line end and recom-mends the use of a matched load, the frequency limitation isidentified here ultimately with the patch parallel LCR equiva-lent circuit, and it is concluded that the directly coupled patchis unsuitable for wideband log-periodic action. However, theintroduction of a series reactance as in the overlaid array willovercome this; such reasoning is supported by propagation

437

characteristics for overlaid arrays with reduced series reac-tance xc which exhibit the onset of stopbands whose magni-tude when xc = 0 is similar to that in Fig. 3. This suggests thatit may therefore be possible to synthesise a patch couplingcircuit giving the k-p characteristics of Fig. 2 that can be madeon a single substrate. This would allow a wideband log-periodic array to be formed with a simpler construction thanthe overlaid array.

Conclusion: Results for k-fi analysis of two microstrip patcharrays has allowed deductions to be drawn on the design andbandwidth limitations of microstrip log-periodic arrays. If thepatch is directly connected to the feed line then low-frequencystopbands prevent wideband action and bandwidths of theorder of 2:1 are indicated. The introduction of series reactanceinto the equivalent circuit such as by the use of overlaidpatches does, however, allow wideband arrays to be formed.Furthermore, if suitable patch coupling circuits can be synthe-sised then construction of wideband arrays on a single sub-strate may well be possible.

Acknowledgments: The author would like to thank Capt. K.Barrett for measurements of the overlaid patch and Prof. J. R.James for helpful discussions.

P. S. HALL 11th April 1984

Department of Electrical & Electronic EngineeringRoyal Military College of ScienceShrivenham, Swindon, Wilts SN6 8 LA, England

References

1 JAMES, J. R., HENDERSON, A., and HALL, p. s.: 'Microstrip antenna

performance determined by substrate constraints', MicrowaveSystems News, Aug. 1982, pp. 73-84

2 HALL, P. s.: 'New wideband microstrip antenna using log-periodictechnique', Electron. Lett., 1980, 16, pp. 127-128

3 PUES, H., BOGAERS, J., PIECK, R., and VAN DE CAPELLE, A.: 'Wideband

quasi log-periodic microstrip antenna', IEE Proc. H, Microwaves,Opt. & Antennas, 1981, 128, pp. 159-163

4 ISBELL, D. E.: 'Log periodic dipole arrays', IRE Trans., 1960, AP-8,pp. 260-267

5 MITTRA, R., and JONES, K. E.: 'Theoretical Brillouin (K-B) diagramsfor monopole and dipole arrays and their application to logperiodic arrays', IEEE Trans., 1962, AP-12, pp. 533-540

6 RUMSEY, v. H.: 'Frequency independent antennas' (Academic Press,New York, 1966), pp. 87-110

7 JAMES, J. R., HALL, P. s., and WOOD, c.: 'Microstrip antenna theoryand design' (IEE Electromagnetic Wave Series no. 12, Peter Perig-rinus, London, 1981), pp. 42-66

LASER-DIODE OPTICAL SWITCH MODULE

Indexing terms: Lasers and laser applications, Switching

A pigtail structure laser-diode optical switch module wasfirst fabricated. A 6 dB gain in the 'on' state and 82 dB iso-lation were obtained.

Introduction: Low-loss and wideband optical switches areexpected to be key devices for evolving the optical fibre com-munication system of highly integrated service networksystems. Large insertion loss is inevitable in the case of a largematrix size or multistage passive optical switch networks.Laser-diode (LD) optical switches,1 having a gain in the 'on'state, are attractive for the aforementioned switch network.For the practical use of an optical switch, a pigtailed or inte-grated optical structure is necessary. First, the pigtail structureis suitable for constructing a laser-diode optical switchmodule.

This letter reports the manufacture of a pigtail structure LDswitch module. Measured switching characteristics are alsodescribed.

Module structure: Fig. la shows a diagram, and Fig. \b aphotograph, of the LD switch module. The laser diode, used

as a switch element, is a BH-type InGaAsP laser,2 whoseoscillation wavelength is near 1-3 [xm at room temperatureand whose cavity length is about 250 fim. The oscillationthreshold current for the laser diode is near 52 mA at roomtemperature. Antireflection (AR) coatings were sputtered onboth facets of the LD to stabilise the output signal level fortemperature deviation, injection current deviation, and spectradeviation in input light signals.3

current injectionterminal

input signalmonitor terminal

graded indexrod lens

1930/11

glasssphere lens

Fig. 1 Laser diode switch module structure

a Diagramb Photograph

For the LD to single mode fibre coupling, the two-lenscoupling method in the confocal condition* is adopted toincrease the alignment tolerance. The first lens is a glasssphere lens of 0 8 mm diameter. The second lens is a graded-index rod lens. They are fixed by bonding at the optimumpoint. The coupling loss was measured to be about 5 dB/facet.Two electrical terminals, a current injection terminal and aninput optical signal monitor terminal,4 are attached to themodule case. The fibre ends are finished to polished ferules toprevent breaking.

Switching characteristics: Switching speed and transmissionbandwidth for the LD switch were described in a previouspaper.5 Important switching characteristics for LD switchmodules are gain in the on state and at isolation, which is thepower level difference between the on and off states. The iso-lation characteristics were measured at 100 MHz with a spec-trum analyser.

The modulated input light is injected into the input port,where the input single mode fibre consists of two pieces con-nected by an optical connector to monitor the input power.The output power for the switch module is detected by aGe APD. The input power was measured by the sameGe APD. The Ge APD was biased at 20 V, where the multi-plication factor was M~l -2 , to prevent changing the APDsensitivity with the input light power. The power level wasmeasured with RF electric power.

Fig. 2 shows the relative output power level, which is nor-malised by the input power level, against the injection currentfor the LD switch, where the input power at the input opticalfibre was —10 dBm and modulation depth was about 60%.

* KAWANO, K., MITOMI, o., and SARUWATARI, M.: 'Efficient combinationlenses for a laser diode module' (unpublished)

438 ELECTRONICS LETTERS 24th May 1984 Vol.20 No. 11