AIMarticle

[ Pobierz całość w formacie PDF ]
//-->An Antenna Impedance Meter forthe High Frequency BandsWhen SWR isn’t enough — here’s a tool that you can build.Bob Clunn, W5BIGSWR meter is a very usefulinstrument and in many situa-tions provides all the informa-tion needed to check an antenna. However,animpedancemeter provides a much moredetailed picture of the antenna parameters.There are several such instruments on themarket with prices in the range to appealto hams.1These typically have broadbandinputs and use diode detectors. The broad-band input is subject to incorrect reading dueto strong signals, such as broadcast radio sta-tions, even when the frequencies of these sig-nals are a long way from the test frequency.The diode detectors are subject to nonlinear-ity error at low signal levels, so their dynamicrange may be limited.AnDesign GoalMy goal was to design an instrumentfor accurately measuring impedance, withmagnitude and phase, so that all the desiredparameters of an antenna can be deter-mined and displayed in a graphical format.The resulting antenna impedance meter(AIM430) measures RF voltage and currentand uses these values to calculate compleximpedance and other parameters of inter-est. The AIM430 provides a detailed look atthe antenna system. Formulas in the designbooks become more meaningful when youcan quickly see how the real and imaginaryparts of the impedance vary with frequency.The AIM430 continuously covers thefrequency range of 500 kHz to 32 MHz andoperates in conjunction with a PC, whichallows easy control through a graphical userinterface. It can also be battery powered andconnected to a laptop computer for com-pletely portable operation.required frequencies are generated by twoAD9851 direct digital synthesizer (DDS) inte-grated circuits made by Analog Devices. OneDDS operates at the specified test frequencyand the other is programmed to operate 1 kHzabove it. These are both driven by a crystal-controlled oscillator running at 20 MHz. TheDDS chips internally multiply this clock by afactor of six, so the effective clock rate seen bythe DDS is 120 MHz. In general, the DDS canbe used to produce an output up to about one-third of its clock frequency.2A block diagramof the AIM430 is shown in Figure 1.The output of each DDS is followed by alow pass filter with a cutoff frequency of 45MHz. These filters remove the spurious highfrequency components that appear in the out-put. The DDS generates many frequency com-ponents in addition to the one that is desired.For example, if the DDS is programmed for 32MHz, there is a strong signal at the clock fre-quency minus 32 MHz, in this case 120 – 32 or88 MHz. Therefore, to get good attenuation at88 MHz and beyond, the DDS low pass filtercutoff is set at 45 MHz. The filter attenuationis greater than 60 dB above 88 MHz.After the DDS output is filtered, it is useddirectly to provide the stimulus signal for theimpedance measurement. There is no bufferamplifier. This eliminates the harmonic dis-tortion of an amplifier and keeps the outputsignal amplitude low to reduce the interfer-ence to nearby radio receivers. The maximumoutput power is less than 50 µW.The output amplitude of the DDS goesdown slightly as the frequency goes up. Thevariation over the entire operating range of theanalyzer is only about 3 dB. This is no problemsince we are using theratioof two RF signalsto calculate impedance and the amplitude ofthe stimulus cancels out in this ratio.To calculate impedance, we need twovalues, voltage and current. Both the magni-tude and the phase are measured. These twoparameters are sensed using 1% resistors.(There are no transformers in the AIM430.)The voltage across one resistor is proportionalto the voltage being applied to the circuitunder test and the voltage across anotherresistor is proportional to the current flowinginto the circuit connected to the analyzer’stest port. The ratio of these two voltages cor-responds to the impedance we want to mea-sure. Figure 2 shows the voltage and currentwaveforms.In Figure 3 there are two mixers, one forBasic OperationThe AIM430 uses two frequency sourcesthat are heterodyned to produce a low fre-quency signal in the audio range that can beeasily amplified, filtered and analyzed. The1Notesappear on page 32.Figure 1 — Block diagram of AIM430 antenna analyzer.Reprinted with permission; copyright ARRL28November 2006Figure 2 — Voltage and current waveformswith complex load.Figure 3 — Schematic of the voltage and current sensing circuits. Two mixers are usedto convert the load current and load voltage to the audio range (typically 1 kHz).Figure 4 — One of the two 1 kHzdifferential amplifiers and band-pass filters.sensing the current flowing into the load andthe other for sensing the voltage applied tothe load.FDRVis the programmed test signalfrom one of the DDSs. This is the stimulussignal for the load under test.FREFis the out-put of the other DDS, which is 1 kHz higherin frequency thanFDRV. This second DDSis the local oscillator. The SA612 has dif-ferential inputs, which make it very handy todirectly measure the voltage across a currentsensing resistor. Therefore, we don’t have touse transformer coupling.The output impedance of the SA612 isabout 1500Ω.A 0.01 µF capacitor to groundfilters out the high frequency component (thesum of the input and local oscillator), leavingthe 1 kHz difference signal. The differentialoutputs of the mixers are connected to op-amps through dc blocking capacitors. Thesecapacitors also provide attenuation at lowfrequencies.Figure 3 shows the input protection circuitof the AIM430. An isolation relay is openexcept when a measurement is in progress. Agas discharge tube (GDT) protects the inputagainst high voltage due to static charge onthe antenna.One of the op amp circuits is shown inFigure 4. There are two poles of high frequencyattenuation including the R-C filter at the out-put of the mixer. A third pole is provided by a ter because the program is computation-sample-hold circuit later in the analog signal ally intensive. I’ve run it successfully on aprocessing chain. The frequency response of 300 MHz laptop usingWindows 95.Thethe signal path peaks at 1 kHz and is 60 dB program doesn’t require an installation pro-down at 100 kHz. The op-amps provide filter- cedure; just click on the.exefile and it runs.ing and also convert the differential signal to a It can be copied to a hard drive or run directlysingle ended signal for input to the analog to from a floppy or a CD.digital converter (ADC). Since the desired sig-nal is always 1 kHz, we do not have to worryData Analysisabout variations in the amplitude and phaseThe two sets of digital data from the volt-response of the low pass filters.age and current sensors are analyzed using theIdentical mixer and amplifier circuits are discrete Fourier transform. This produces theused for both the voltage and current sensing amplitude and phase of the 1 kHz fundamentalpaths. Any small differences in the gain and signal and cancels out any dc component duephase shift of these two paths are taken care of to offsets in the operational amplifiers. Theby the calibration process, which will be dis- magnitude of the load impedance is the volt-cussed later. After the RF signals are converted age amplitude divided by the current ampli-to the audio range, it is much easier to measure tude. The phase angle of the impedance is thetheir amplitude and phase. This is done by difference in the phase angles of the voltagedigitizing the two signals with a 12-bit ADC and current. Knowing these two parameters,that is contained in the Texas Instruments we can calculate the equivalent resistance andMSP430F149 microprocessor. This micropro- reactance of the load impedance:cessor runs at 7 MHz and the ADC samplesR = Resistance = Impedance_Magnitudeare precisely timed by its internal clock. Both × cosine(phase_angle)the current and the voltage channels are sam-X = Reactance = Impedance_Magnitude ×pled with 16 samples per cycle.sine(phase_angle)The raw data is sent to a PC through theThe external load resistance is found byRS232 serial port (an RS232/USB converter subtracting the internal 100.6Ωseries resis-can also be used). The PC calculates the tance (R21 + R22 shown in Figure 3) from theimpedance and all the other desired param- calculated resistance. The equivalent serieseters. The PC then graphically displays a circuit is Z = R +jX,wherejis the square rootdetailed view of the parameters as the fre- of –1. The equivalent parallel circuit is alsoquency range is scanned.calculated and displayed in the data window asThe software has been used withWindowsthe cursor moves along the frequency axis.95, 98, 2000andXP.There is no definiteResistance is always a positive number.speed requirement, although faster is bet- Reactance can be positive or negative. PositiveNovember 200629at the antenna back toward the transmitter.(Its magnitude is also equal to the squareroot of the ratio of reflected power to inci-dent power.) If there is no reflection (i.e.,the reflection coefficient is zero) then all thepower from the transmitter is absorbed by theantenna, which is usually the desired case. Ifthe transmission line is open at the antenna(perhaps due to a broken wire), all the powerarriving at the break point is reflected backtoward the transmitter, none is radiated, sothe reflection coefficient has its maximumvalue of unity. If the transmission line is open,the reflection coefficient is plus one; if theStanding Wave Ratioline is shorted, the reflection coefficient isSWR is probably the antenna’s most minus one.interesting parameter. This is calculated byfirst determining a parameter calledreflec-Reflection_coefficient =ρ= (ZL– Z)/ (ZL+ Z)tion coefficient.When a signal travels down atransmission line with a characteristic imped- whereZL= Impedance of the loadance of Zand arrives at the antenna with aZ= Impedance of the transmission line.differentimpedance, some of the signal isreflected back toward the transmitter. ThisZLis a complex number; therefore,ρis, inreflection occurs even if the transmission line general, a complex number with a magnitudeis of the highest quality and the antenna is a between 0 and 1 and a phase angle in theperfect radiator. The reflection coefficient is range ±90°.the fraction of the voltage that is reflectedSince the reactive component of Zisreactance is associated with inductance andnegative reactance with capacitance. The truesign of the phase angle is determined by thedata processing routine, so capacitive reac-tance and inductive reactance can be distin-guished without ambiguity. As can be seenfrom the scan pictures, the phase changes rap-idly as it passes through zero. Critical pointsin the plot, such as maximum or minimumimpedances, can be located more accurately onthe frequency axis using phase rather than bylooking only at the impedance magnitude.usually very small, it is often ignored andZis considered to be a real number, such as“50Ω”or “75Ω.”The value of Zcan beentered from the program’s main menu, sothe SWR can be calculated for any value oftransmission line impedance.For the SWR calculation let U equal themagnitudeofρ.U will be in the range of 0 to 1.SWR = (1+U) / (1–U)Note the SWR only depends on the mag-nitude ofρ, so it isnota complex number. Ifρis zero (no reflection), the SWR is 1.0:1.Since a term 1–U appears in the denominator,the SWR can be very large when the trans-mission line is badly matched to the antennaand the magnitude of the reflection coeffi-cient, U, is almost equal to one.ApplicationsThe analyzer’s test conditions are speci-fied by entries on the PC. These include scanstart/stop frequencies, frequency incrementbetween data points and display scale factors.There is also a provision to enter the nominaltransmission line impedance so the SWR canbe calculated for any value. After the scan iscomplete, the mouse can be used to move acursor along the frequency scale to displaythe numeric values of several parametersincluding SWR, impedance magnitude andphase, equivalent series circuit and equivalentparallel circuit.The full-scale ranges for measurementsare:SWR up to 100:1Impedance magnitude 1Ωto 10 kΩ.Phase angle –90 to +90°.Frequency scan 500 kHz to 32 MHz.Figure 5 —Scan of 28 footunterminated coax.••••Figure 6 — Smith chart of 28 foot unterminated coax.Figure 5 shows the scan of a piece of RG-58 coax that is open at the far end. The coaxis 28 feet long. The frequencies at which thephase angle crosses the axis are called “reso-nant frequencies” and are listed across the topof the graph. In this case, the first frequencycorresponds to the1⁄4λof the coax. The sec-ond value is the1⁄2λfrequency. Because ofloss in the cable, the maximum impedanceat the1⁄2λfrequency (11.681 MHz) is onlyabout 1200Ωat the input end of the coax, notinfinity. At the frequency corresponding toa 1λ,23.467 MHz, the impedance is about800Ωbecause of increased loss at the higherfrequency. The 1λand1⁄2λfrequencies are notexactly in a 2:1 ratio because the velocity ofpropagation varies slightly with frequency.Notice the way in which the phase angle(violet trace) changes rapidly at 5.765 and17.532 MHz even though the magnitude ofthe impedance is changing slowly. Findingthe phase angle zero crossing makes the loca-tion of the1⁄4λfrequencies more accuratethan relying on the magnitude of the imped-ance. The cursor is the light colored verticalline at 11.697 MHz and the data displayed30November 2006Figure 7 — Scan of 28 feet of RG-58 coax with a 243Ωresistortermination.Figure 8 — Scan of Figure 7 configuration referred to antennaterminals.in the window on the right side of Figure 5corresponds to this frequency. Rsand Xsarethe series circuit values. Rpand Xpare theparallel circuit values.Figure 6 shows a Smith chart of the datafrom the scan in Figure 5. The small dot atabout the 1 o’clock position is a marker thatmoves along the Smith chart as the cursor cable loss increases with frequency.moves along the frequency axis. In this pic-ture the cursor is at 9.515 MHz. The equiva-Reference Transformationlent series and parallel circuit values areSometimes it is desirable to know theshown on the Smith chart along with the real impedance directly at the antenna terminals.and imaginary parts of the reflection coef- After a calibration phase during which theficient. The trace spirals inward because the properties (length and loss) of the cable aredetermined at each measurement frequency,measurements made at the transmitter endof the line can be transformed to the antennaterminals. This is done in real time during thescan and the displayed data is very close towhat would be measured if the analyzer wereFigure 9 — Twoactually mounted at the antenna.scans of a seriesThe calibration is done by disconnectingL–C tuned circuittermination. Thethe far end of the transmission line from thefirst is with theantenna and then scanning the cable inputcircuit connectedimpedance with two different resistive termi-directly to thenations. One terminating resistor is typicallyAIM430, the secondis referenced to thein the range of 20 to 100Ωand the other canend of the coax. Inbe in the range of 1 kΩ to 2 kΩ. The resis-the ideal case, theytor values are not critical, as long as they arewould be identical.accurately measured with a digital ohmme-ter. When the transmission line calibrationis performed, the exact resistor values areentered in the program via dialog boxes. Theterminating resistors can be low power filmdevices since they don’t have to handle thetransmitter power. After the cable calibrationis finished, the data are saved to disk so theycan be recalled anytime later.Using the ImpedanceTransformation FeatureFigure 7 shows a conventional scan withthe 243Ωresistor at the end of 28 feet of RG-58 coax. The green trace is the magnitudeof the measured impedance. As expected,the value varies over a wide range as a func-tion of frequency. At the1⁄2λfrequency,11.621 MHz, the indicated impedance isclose to 243Ωbecause the same impedanceis seen at both ends of a half-wave line.Now we clickSETUPandREF TOANTENNA. The legendREF TO ANTENNAisNovember 200631(A)(B)Figure 10 — Scans with and without interfering signal. At A, a scan without interference.The SWR reading (red trace) is 3:1 in this example. At B, a scan with a CW interferencelevel of +63 dB over S-9 injected directly into the input.Figure 12 — Thereare two PC boardssandwiched togetherwith 0.1×0.1 inchconnectors. The topboard contains allthe RF circuitry andthe bottom board hasthe microprocessorand electronic powerswitch. The 3.3 Vregulator is mountedon the rear panel thatacts as a heat sink.ers. The output into a 50Ωload is aboutAcknowledgments35 mVrms. The amplitude is not precisely cali-I would like to thank Dave Russell, W2DMR,brated but the variation over any of the ham Danny Richardson, K6MHE, and Paul Collins,displayed in red at the top of the graph while bands is less than 0.5 dB. The frequency can ZL3PTP, for evaluating the AIM and providingthis feature is enabled. The resistor (243Ω)be set in 1 Hz increments and it can be cali- suggestions that greatly enhanced the program.and the cable are the same as used in the brated against WWV.Thanks also to Bill Cantwell, WB5SLX, andprevious graph. The Zmagplot (shown inForest Cummings, W5LQU, for their proof-green) is relatively flat across the frequencyCalibrationreading and encouragement.range. The measured resistance now variesThe AIM430 is calibrated by measuringonly from 243 to 248Ω,a range of 2%. The the residual capacitance and inductance in itsNotesphase angle and the reactive component are output circuit. The phase shift and amplitude1J.Hallas, W1ZR, “Product Review — A Look atSome High-End Antenna Analyzers,”QST,nearly zero.differences in the voltage and current amplifi-May 2005, pp 65-69.Figure 9 shows that the transformation ers are also measured. This calibration data is2Direct digital synthesizers, theory ofoperation —www.analog.com/library/also works quite well with a complex load then used to compensate each reading. Straycircuit. A series L-C tuned circuit was used capacitance and inductance associated with3analogDialogue/archives/38-08/dds.pdf.AIM430 User Manual and demonstration pro-for the load. For the first scan, it was con- an external test fixture, if used, are also takengram —w5big.home.comcast.net/antenna_analyzer.htm.nected directly to the BNC connector on the into account by this procedure.4Data sheet for AD9851 DDS —w5big.home.AIM430. Then it was rescanned with the loadCalibration is performed by using a shortcomcast.net/AD9851.pdf.at the end of 28 feet of coax. The impedance circuit and an open circuit. First, a short cir-5Data sheet for SA612 mixer —w5big.home.and reactance curves almost coincide; it’s cuit is connected to the analyzer and severalcomcast.net/SA612.pdf.6Data sheet for MSP430F149 micropro-hard to see the difference between them on measurements are taken. Then the short iscessor —focus.ti.com/docs/prod/folders/the graph. There is only a small difference in removed and the open circuit properties areprint/msp430f149.html.the two phase-angle traces shown in violet.measured. This data is saved in a file that is7Schematic and printed circuit board designsoftware —www.expresspcb.com.automatically loaded each time the programInterference Rejectionis run. The whole calibration process takesThe band pass circuits in the AIM430 only a few seconds. Since the analyzer doesBob Clunn, W5BIG, received his Novicehelp to reject interfering signals that are more not have any internal adjustments (no pots orlicense in 1956 while in junior high andthan about 100 kHz from the desired test trim caps), the calibration is very stable. Ithis general license soon after. During highfrequency. Figure 10 shows the result with only needs to be done when the external testschool he was very active on 40 and 20 meterCW. During this time he made the decisionand without an interfering signal that has an fixture or cable adapter is changed.to study electrical engineering in college.amplitude of +63 dB over S-9. The distur-Bob received his BS degree in electrical engi-bance of the reading is confined to an intervalConstructionof about ±100 kHz.The microprocessor is initially pro-neering from Rice University in 1965 and hisgrammed through a 14-pin JTAG interface.MS from Southern Methodist University in 1969.Additional ApplicationsSubsequently, the program can be updatedHe was employed at Texas Instruments in Dallasfrom 1963 until 1991. His work there involvedIn addition to measuring antennas, the through the standard RS-232 interface.the design of computer controlled test equipmentAIM430 can be used to measure discretefor transistors and integrated circuits. From 1991components, such as resistors, capacitors andConclusionsto the present he has been working as a consul-inductors. It is particularly interesting to seeThe operation of an affordable vectortant for several companies in the fields of elec-how the component value varies as a function impedance meter for measuring antennas intronic circuit design and machine vision.of frequency. Inductors with metal cores are the high frequency range has been presented.In 2002, Bob renewed his interest in hamoften very frequency sensitive. It can also Using state-of-the-art components for signalradio and obtained his Amateur Extra classbe used for adjusting tuned circuits, such as generation and analysis, the AIM430 provideslicense. Soon afterward he got interested intraps, and for measuring the parameters of a high level of accuracy and wide dynamicequipment to evaluate antennas and beganquartz crystals and other resonator devices.range for complex impedance measurements.the design of this antenna analyzer. He can beThe output signal from the analyzer can The unit is also quite useful for measuring dis-reached at 509 Carleton Dr, Richardson, TXbe used as a test signal for checking receiv- crete components and tuned circuits.75081 or atw5big@comcast.net.32November 2006Figure 11 — The enclosure is 5×5×2inches, which leaves room inside for anoptional battery pack. The dc currentrequired is about 150 mA while takinga measurement and 30 mA if idle. After10 minutes of inactivity, the dc power isturned off automatically. Two LEDs on thefront panel indicatePOWER ON(green) andTEST IN PROGRESS(red). [ Pobierz całość w formacie PDF ]