Figure 3. Directivity10How to Specify Directional Couplers[DESIGNER’SCORNER] >>www.wirelessdesignmag.comurement errors, we should also deal withthe importance of Voltage Standing WaveRatio (VSWR) because reflections willadd and subtract to the incident signalcausing uncertainty in the coupling fac-tor. VSWR is defined as the ratio of inci-dent to reflected signals and is ideally1.00:1, meaning these signals are inphase and will not cancel. The better theVSWR, the less return loss is encoun-tered. Unsatisfactory coupler VSWR willdegrade measurement accuracy and isusually attributable to lesser quality con-nectors or inadequate design techniques.The frequency sensitivity or “flatness”of a coupler is a measure of how couplingvaries over a given frequency range.Optimum coupling frequency response isachieved by “centering” the design with-in the specified band of interest. Typicalcoupling flatness for a quarter-wave-length coupler operating over an octaveband is within ± 0.75 dB of nominal. Allthings being equal, stronger coupling fac-tors (3, 6 and 10 dB) exhibit greater flat-ness than weaker coupling factors (20through 50 dB). When operating over fre-quency bands greater than an octave, theflatness tolerance may need to be relaxeddue to the inherent characteristics of cou-pling roll-off.Another important consideration whenspecifying a coupler is to ensure the devicehas minimal mainline insertion loss.Through virtue of their design, coaxial air-line couplers offer the lowest possible losswhen inserted in a transmission path.Generally, the insertion loss of a coupler (orany microwave device for that matter)becomes more significant at higher fre-Over the years many different tech-niques have been utilized to bringabout the physical realization ofdirectional couplers. The scope of thisarticle is limited to discussion of quarter-wavelength, coaxial, directional couplers,as this type of coupler represents a bal-anced compromise between functionalrange, performance, and cost for mostbroadband applications covering up to anoctave bandwidth. The basic directional coupler is a fourport junction that is used in a wide varietyof microwave systems to satisfy almostany requirement for sampling incident andreflected microwave power convenientlyand accurately with minimal disturbance tothe transmission line. The basic configuration of a singledirectional coupler is shown in Figure 1,which illustrates two parallel transmissionlines over a length of one-quarter wave-length, corresponding with the center fre-quency of operation. The main and sec-ondary lines are separated by a calculatedphysical distance, which determines thecoupling factor of the device. The physi-cally closer the lines are to each other, themore power will be introduced on the sec-ondary line. The term coupling denoteshow much of the input power is sampledto the coupled port and is defined as 10times ratio of Incident Power and ForwardPower C = 10 log10(Pf/Pi). Typical cou-pling values found in practice are 3, 6, 10,20, 30, 40 and 50 dB; however, practical-ly any coupling value may be obtainedthrough proper design. A dual directional coupler (Figure 2) isessentially two single directional couplersconnected back-to-back sharing a commonmainline and providing two output portswith high isolation between those ports.This high isolation is critical for the accu-racy of reflectometer set-ups that simulta-neously sample input power to a device orload providing a ratio of signals for the pur-pose of determining return loss in decibels.Theory and DefinitionsWhen power is introduced at the inputport, all of the power appears at the outputport except for the portion intended to besampled. If power is reflected back from theoutput port, the ideal directional couplerdoes not allow any of the reflected power toappear on the secondary line. Regrettably,the ideal directional coupler does not existin our world. Consequently, a small amountof backward power will be coupled to thesecondary line 180° out of phase from theincident wave canceling power on the sec-ondary line and adding uncertainty to themeasurement. The term directivity (Figure3) denotes the ratio of forward to backwardcoupling and is defined as 10 times thecommon log of the ratio of forward andbackward power D = 10 log10(Pf/Pb). Thehigher the value of directivity, the lessbackward power is sampled and measure-ment uncertainty is significantly improved.Directivity is the qualitative benchmark bywhich couplers are compared.Since we are on the subject of meas-Write In 129 or Reply Online Write In 130 or Reply OnlineBy Eric Schiebler, MECA ElectronicsTable 1: Theoretical Mainline Insertion Loss Due to Coupling Factor (dB)Coupling Factor 3 dB 6 dB 10 dB 20 dB 30 dB 40 dB 50 dBSingle 3.0206 1.2560 0.4560 0.0436 0.0043 0.0004 0.00004Dual 6.0412 2.5120 0.9120 0.0872 0.0086 0.0008 0.00008PfPiSecondary LinePi = Incident PowerPf = Forward PowerMainlinePbPiSecondary LinePi = Incident PowerPb = Backward PowerMainlinePfPfPiSecondary LineSecondary LineMainlineFigure 1. Single Directional CouplerFigure 2. Dual Directional Coupler12[DESIGNER’SCORNER] >>www.wirelessdesignmag.comtional loss the device exhibits as a functionof the proximity of the two transmissionlines. It should also be noted that dualdirectional couplers exhibit twice the lossof single directional models because thereare two secondary lines drawing powerfrom the mainline.Coupler Solutions from MECAMECA designs and manufactures bothcoaxial air-line and stripline couplers.Through virtue of their design, air-linequency, namely because loss increases withfrequency and higher frequency powersources are considerably more expensive.Accordingly, the criteria of low insertionloss will prevent precious power from beingwasted on measurement components. When specifying a directional couplerwith a coupling factor stronger than 20 dB(3, 6 or 10 dB), consideration should alsobe given to the theoretical insertion losscaused by power coupling from the main-line. Table 1 illustrates the amount of addi-couplers are high power capable and offerthe lowest possible insertion loss. Theunique matching techniques used in theirconstruction also provide exceptionallyhigh directivity and the best possibleVSWR. Stripline couplers offer theadvantage of multi-octave frequency cov-erage in miniature package sizes forimproved packaging density.MECA offers a wide selection of direc-tional couplers optimized to the microwaveengineer’s critical