MULTIMEDIA UNIVERSITY FACULTY OF ENGINEERING LAB SHEET

MULTIMEDIA UNIVERSITY FACULTY OF ENGINEERING LAB SHEET ELECTROMAGNETIC THEORY EMF016 MW1 MICROWAVE FREQUENCY AND SWR MEASUREMENTS

EM Theory Faculty of Engineering, Multimedia University 1 EXPERIMENT MW1: MICROWAVE FREQUENCY AND SWR MEASUREMENTS OBJECTIVES: 1. To trace out the standing wave pattern developed along a waveguide.. To make direct SWR measurements using an SWR meter. 3. To make indirect SWR measurements using the Double Minimum Method. 4. To make a direct frequency measurement using a cavity wavemeter. 5. To make an indirect frequency measurement via a waveguide wavelength measurement. APPARATUS: Klystron Source Klystron Power Supply Cavity Wavemeter Isolator Slotted-line probe SWR meter Variable attenuator Short-circuit plane BNC coax cable x 1 INTRODUCTION: At radio frequencies (RF) and microwave frequencies, signals must be viewed as waves instead of currents and voltages. One of the reasons is the wavelength becomes very short that the current at one point of a long conductor is not the same as that at another point. As a result, the ordinary Circuit Theory can no longer be used for the analysis of transmission lines that have a physical dimension greater than 1/10 of the signal wavelength. This kind of problems can be solved using Electromagnetic Theory. The electromagnetic field at any point on a transmission line may be considered as the sum of two traveling waves: the incident wave propagates from the generator and the reflected wave from the load. These give rise to a standing wave along the line due to constructive and destructive interference, as shown in Fig. 1. The electromagnetic field strength varies periodically with distance. The maximum field strength E max is found where the two waves add in phase, and the minimum E min where the two waves add in opposite phase. The Voltage Standing Wave Ratio (VSWR or just SWR) is defined as the ratio of the maximum to the minimum field strength along the line, as given in Eq. (1). It is a parameter used to describe the size of standing wave or the strength of reflected wave relative to that of incident wave. The SWR will be higher (i.e. larger peaks and nulls in the standing wave pattern) if there is more wave power reflected from the load end. The magnitude and phase of the

EM Theory Faculty of Engineering, Multimedia University reflected wave are related to the load impedance which is generally a complex value. Thus, the SWR and the positions of constructive and destructive interference depend upon the load impedance. E max Power E min g / Distance Fig. 1: Standing wave pattern For a waveguide system, the SWR can be measured using a slotted-line method. A short length of wire, acting as a probe, protrudes into the waveguide through a longitudinal slot to couple to the electric field confined within the waveguide. A small fraction of power is picked up and detected by a diode detector connected to the other end of the probe. By sliding the probe along the longitudinal slot, the maximum power P max and minimum power P min can be measured. The SWR is then given by or SWR P P max min E E max min 0 min log 10 SWR Pmax ( dbm) P ( dbm) (1) () In normal operation, the diode detector gives a voltage which is proportional to the electromagnetic wave power (or square of the electric field strength). This is the case if the power is small enough to allow the diode detector to work in its squarelaw region. The probe must not protrude too deep into the waveguide so that the power picked up by the probe is small. This is also necessary to ensure that the field distribution in the waveguide is not significantly disturbed. An SWR meter can be used to give direct readings of SWR. A simple one consists of a power meter with SWR scales on it. One way to establish an SWR scale on a power meter is to make P max a known constant using a variable gain amplifier. A convenient constant for this purpose is the full scale value of the power meter. With P max made constant, there is a one-to-one conversion between P min and SWR, and SWR values can be marked on the power meter in place of the power values. Fig. shows an example SWR meter. There is a power scale and an SWR scale on the meter. To use the SWR scale, one has to first set P max to the full scale value, 1 W in the example, with the aid of a variable gain amplifier. Then, P min is measured. The SWR will be

EM Theory Faculty of Engineering, Multimedia University 3 SWR P P max 1 min P min (3) Note that this is exactly the relationship between the power and SWR scales. In other words, the SWR can be read directly from the meter at the point of P min, no further division is necessary. 0. 0.4 0.6 0.8 0. 1.6 1.3 1.1 1.0 1.0 Power (W) SWR Fig. : An example SWR meter For the case where the SWR is high (for example, SWR > 3), the null points of the standing wave pattern along the waveguide may have very low field strength. The probe depth can be increased so that a sufficiently high meter deflection can be obtained at the minimum power point. However, this will deform the field when the probe is moved to the maximum power point. Also, the large electric field strength at the maximum point may cause the diode detector to operate out of the square-law region. The above problem can be overcome using the Double Minimum Method. In this method, one measures the distance between the points where the detector output is double the minimum field strength, as illustrated in Fig 3. It can be shown that the SWR is given by: SWR E max 1 E 1 min sin d / g (4) E max Power d E min E min 3 db Distance g / d 1 d Fig. 3: The Double Minimum Method

EM Theory Faculty of Engineering, Multimedia University 4 A wavelength is defined as the distance between two successive points with the same phase on a waveform. An electromagnetic wave is reflected from one waveguide s wall to another as it propagates along a waveguide As a result, the apparent longitudinal wavelength is different from the free-space wavelength, as shown in Fig. 4. The relationship between the apparent longitudinal wavelength or simply waveguide wavelength g and the free space wavelength o is given by: 1 1 1 o g c (5) where c is the cutoff wavelength for the particular mode of propagation. For an electromagnetic wave with freespace wavelength equals to c, g will be infinitely long, which means that no field variation occurs along the waveguide. A wave with freespace wavelength equals or larger than c would not be able to propagate through the waveguide (in the particular mode of propagation) and therefore c is called the cutoff wavelength (for the particular mode of propagation). For the usual TE 10 dominant mode operation of a rectangular waveguide, c = a with a being the broad-face dimension of the waveguide. waveguide s wall equiphase wavefront o plane wave propagation g the longitudinal direction Fig. 4: Waveguide wavelength Eq. (5) provides an indirect way of measuring the signal frequency. The waveguide wavelength g can be measured as twice the distance between two successive minimum points of the standing wave pattern. Using Eq. (5), the free space wavelength can be calculated. The frequency f can be determined using the relationship u = f o, where u is the speed of light in the waveguide medium. For an air-filled waveguide, u = 3 10 8 m/s. Frequency can also be measured using a variable size resonant cavity known as cavity wavemeter. A resonant cavity is made of a rectangular or circular waveguide with both ends short-circuited. A cavity wavemeter is coupled to a waveguide by a small hole. Because the hole is small, the cavity wavemeter can normally absorb only a tiny fraction of the energy propagating in the waveguide. At a certain cavity size, a standing wave can be set up and sustained between the cavity walls. When this happens, the cavity will absorb a larger fraction of power from the waveguide system, which results in a progressive building up of the the standing wave within the cavity.

EM Theory Faculty of Engineering, Multimedia University 5 This in turn causes a small dip in the power in the down-stream of the waveguide system. In this condition, the cavity is said to be in resonance with the signal frequency. The resonant frequency can be calculated by solving Maxwell s equations with the dimensions of the cavity defining the boundary conditions. Varying the cavity size will alter the resonant frequency. Some cavities are marked in frequency for direct measurement reading. Others come with calibration charts so that the frequency can be read off the charts. It is important to note that resonant frequency is not unique. There are numerous resonance modes, each gives rise to a different resonant frequency. A solution to this problem would be to use only the dominant resonant mode (lowest resonant frequency), to search for the resonance from the smallest cavity size and gradually increasing its size, and to read from the first resonance occurrence. In the following experiments, you will trace out the standing wave pattern developed along a waveguide. You will make direct and indirect SWR measurements using an SWR meter and the Double Minimum Method, respectively. Also, you will make direct and indirect measurements of the signal frequency using a cavity wavemeter and Eq. (5), respectively.

EM Theory Faculty of Engineering, Multimedia University 6 PREPARATION Conduct background study about the experiment, e.g. read the lab sheet. Bring log book for recording observations and results. Bring scientific calculator. PROCEDURE: CAUTION 1: The RF power levels in the following experiments are not harmful, but a human eye may be damaged by low level of radiation. Do not look into the waveguide at any time when the equipment is on. CAUTION : Klystron tube get extremely hot when it is operated and must not be handled by hand. A) Initial Setup (30 minutes) The experimental setup (see Fig. 5) is rather simple. It mainly consists of a source (Klystron source), a transmission line (slotted-line probe), and a variable load (variable attenuator terminated in a short-circuit). An isolator is used to protect the source from reflected power. The cavity wavemeter is for direct frequency measurement. Further details about the components are given in the Appendix. Note that the SWR meter used in the setup is a power meter with SWR scales on it and it has a built-in variable gain amplifier. Power increases from the left to the right side of the scales. A primary objective of this initial setup is to tune the components for maximum signal strength at the SWR meter. This includes tuning the Klystron source s repeller voltage for maximum output power, the tuning gear and the source s modulation frequency and amplitude for maximum reception at the SWR meter. A1. Set up the equipment as shown in Fig 5. (already done by the lab technician but do verify). Caution: Make sure the SWR meter input setting is XTAL LOW. Cavity Wavemeter SWR meter Variable attenuator Klystron Source Isolator Slotted-line Probe short-circuit plane Fig 5: Setup for slotted-line measurement.

EM Theory Faculty of Engineering, Multimedia University 7 A. Set the slotted-line probe s depth to 0.5 mm (preset by the lab technician). A3. Set the attenuator to >.8 mm (> 0 db return loss). A4. Make sure that the SWR meter is in the normal deflection mode (set by the smaller dial at and on top of the range-gain knob). A5. Switch on the Klystron power supply (with internal modulation active) and SWR meter. A6. Adjust the SWR meter s gain until a significant deflection appears on the meter. Note that the SWR meter has 3 gain knobs: range-gain, coarse vernier gain and fine vernier gain. A7. Adjust the repeller voltage knob for maximum reception at the SWR meter* A8. Adjust the tuning gear of the slotted-line probe for maximum reception at the SWR meter*. A9. Adjust the modulation frequency and amplitude for maximum reception at the SWR meter*. A10. At this stage, the SWR meter should give a good deflection at gain settings of 30 to 40 db. If necessary, increase the slotted-line probe s depth to achieve the stated condition. Make sure that a full deflection can be obtained by adjusting the vernier gain knob. A11. Make sure that the cavity wavemeter dial is set to maximum clockwise. * As one is tuning towards the maximum, the SWR meter will indicate that power is increasing. As one s tuning reaches the maximum and then moves away from the maximum, the SWR meter will indicate that power reaches a maximum and then decreases. Adjust the SWR meter s gain if necessary to have a decent deflection on the meter and to avoid overdriving the meter. Note: Do not disturb the repeller voltage, slotted-line probe s depth, and tuning gear beyond this point. Else, you might have to repeat the above. For slotted-line measurements below, make your measurements at the central part of the waveguide; avoid the two ends of the waveguide. All measurement readings have to be in the correct precision. Please make it a habit. Make your own tables of observations. Make it a habit too. Evaluation A: (cognitive knowledge, level 1) [10 marks] i. Please make sure you know all components of the setup and their functions. You may consult the lab instructor if necessary. ii. Request for evaluation when you are ready.

EM Theory Faculty of Engineering, Multimedia University 8 B) Trace Out The Standing Wave Pattern (40 minues) B1. Set the attenuator s micrometer setting to 1.1 mm (this corresponds to about 6 db return loss, i.e. reflected power is 6 db below the incident power). B. Move the probe along the slotted-line waveguide to locate the point of maximum power. Henceforth, for convenience, it will be referred to as the maximum point (or minimum point for minimum power). B3. Adjust the gain of the SWR meter so that its reading is 0 db on the bottom scale (i.e. full scale deflection). Do not overdrive the SWR meter. B4. Move the probe along the slotted-line waveguide to measure and record the relative power at numerous points over a 50 mm distance along the waveguide. Do record both the relative power and position. Do record enough points to capture variation in the relative power. Relative power can be read from the SWR meter as the negative of that given by the bottom scale, e.g. relative power is -10 db if the bottom scale reads 10 db. Obviously, the SWR meter gain should not be disturbed throughout the measurement. B5. Sketch a plot of relative powers versus positions. Yes, sketch now. Evaluation B: (psychomotor complex overt response, level 5) [0 marks] i. Tabulate your measurements and sketch the plot. ii. Be ready to demonstrate how the measurements were done. iii. Request for evaluation when you are ready. C) Measure SWR Using an SWR Meter (30 minutes) C1. Set the attenuator s micrometer setting to 1.5 mm (about 1 db return loss). C. Move the probe along the slotted-line waveguide to locate the maximum point. C3. Adjust the gain of the SWR meter so that it has full scale deflection at the maximum point (remember? 0 db on the bottom scale). C4. Move the probe to locate the minimum point. Read and record the SWR from the SWR meter**. [10 marks] C5. Set the attenuator to 1.1 mm (about 6 db return loss). Repeat steps C. to C4. [10 marks] C6. Set the attenuator to 0.5 mm (about db return loss). Repeat steps C. to C4. [10 marks] ** There are 5 scales on the SWR meter. From the top, the first two are meant for the expand mode (Not used. Remember? We use normal mode). Third and fourth are linear scales. The third scale is used when the maximum and minimum are observed using the same gain setting. The fourth scale is used when the minimum is observed using a range-gain 10 db higher than that of the maximum. The fifth or the bottom scale is a log scale (db). Difference in range-gain between the maximum and minimum should be added to the reading on the log scale to obtain the SWR in db.

EM Theory Faculty of Engineering, Multimedia University 9 Evaluation C: (psychomotor complex overt response, level 5) [0 marks] i. Tabulate your measurements. ii. Be ready to demonstrate how the measurements were done. iii. Ask for evaluation when you are ready. D) Measure SWR Using The Double Minimum Method (40 minutes) D1. Set the attenuator to 0 mm (about 0 db return loss). D. Move the probe along the waveguide to locate two successive minimum points. Adjust the SWR meter gain if necessary to have a decent deflection at the minimum points. Record the positions of these two points (x 1 and x ). Determine the waveguide wavelength g (see Introduction). [10 marks] D3. Set the attenuator to 1.1 mm (about 6 db return loss). D4. Move the probe along the waveguide to locate the minimum point. D5. Adjust the SWR meter gain to obtain a reading of 3 db on the bottom scale (the power is 3dB below that at 0 db). D6. Move the probe to the left of the minimum point until the detected power increases to 0 db. Record the probe position, d 1. You see, this point has power 3 db more than the minimum. [5 marks] D7. Move the probe to the right of the minimum point. Record the position d where the meter reading is 0 db. [5 marks] D8. Calculate the SWR using Eq. (4). [10 marks] D9. Set the attenuator to 0.5 mm (about db return loss). Repeat steps D4. to D8.. [0 marks] Evaluation D: (psychomotor complex overt response, level 5) [30 marks] i. Tabulate your measurements. ii. Be ready to demonstrate how the measurements were done. iii. Ask for evaluation when you are ready.

EM Theory Faculty of Engineering, Multimedia University 10 E) Frequency Measurement (0 minutes) E1. Using the waveguide wavelength g from the above experiment, calculate the free-space wavelength o using Eq. (5). The inner dimensions of the waveguide is given as a=.870cm and b=1.0160cm. Calculate also the frequency f o. E. Set the attenuator to >.8 mm (> 0 db return loss). E3. Adjust the SWR meter gain to have a decent deflection on the meter. E4. Starting with the cavity wavemeter dial set to maximum clockwise, unscrew the dial slowly. Initially there will be little effect on the SWR meter reading. At one point, cavity resonance causes a noticeable dip*** in signal power (so is the meter reading). Read the resonant frequency f o of the cavity wavemeter at this condition. *** In some cases, the power dip is small. Though small, the dip is noticeable if monitored closely. A quick approach to locate the dip is to rotate the wavemeter at moderate speed while closely monitoring the meter deflection. When a suspect dip is found, rotate the wavemeter forward and backward about the suspect dip to verify. Evaluation E: (psychomotor complex overt response, level 5) [0 marks] i. Tabulate your measurements. ii. Be ready to demonstrate how the measurements were done. iii. Ask for evaluation when you are ready. Evaluation F: (cognitive analysis, level 4) [100 marks] Discussions i. Write a summary of this lab experiment. The lab report should include a. An introduction section. [0 marks] 1. Compare b. A the formulation SWR measurements section to explain you obtained the theory from behind the SWR the experiment. meter and that [0 from the marks] Double Minimum Method.. Estimate c. An return experimental losses from setup the section SWR measurements to explain how (see the Appendix). experiment Discuss was whether conducted. the estimated [0 marks] return losses are consistent with the given attenuator settings. d. A results and discussions section to present and discuss the results. [30 3. Discuss marks] whether you can use the Double Minimum Method to measure the SWR at 1.5 e. mm A conclusion attenuator setting. section to conclude the experiment. [10 marks] 4. ii. Compare Do not copy the anything frequency from measurements this lab sheet. f o and Plagiarism f o. is strictly prohibited. 5. iii. Discuss Submit the lab sensitivity report to of the error lab in technician f o to error within in 14 slotted-line-probe days from the position day the measurements. experiment was conducted. Late submissions are subjected to penalty.

EM Theory Faculty of Engineering, Multimedia University 11 Isolator APPENDIX An isolator is a non-reciprocal device, which attenuates waves propagating in one direction and on the other hand causes negligible attenuation for waves propagating in the opposite direction. Such a device is, therefore useful in transferring power from a microwave generator to the load with negligible loss and at the same time ensuring all reflections from the load end are completely absorbed by the time they reach the generator. This is important to ensure that the frequency stability and output power level are not affected by the load variation. The isolator must be connected in the proper direction. An arrow, which indicates the forward (lossless) direction, usually comes with the label on the device. Variable Attenuator An attenuator can be constructed by placing a resistive card (made of lossy material) at the centre of a waveguide section at which the electric field strength is the highest for TE 10 mode. A fraction of the microwave will be absorbed, with the remaining power transmitted. The resistive card is tapered at both ends to maintain a low VSWR. A variable attenuation can be realised by varying the position of the resistive card across the waveguide broad dimension. If the device is terminated with a short-circuit plane, a variable load impendence can be emulated for which the return loss is equal to twice the attenuation level (in db). For example, if attenuation is 3 db, then Return loss 0log 6dB where is the reflection coefficient 0.50. 1 1 0.50 And, SWR 3. 01 1 1 0.50 wave propagation Tapered resistive card

EM Theory Faculty of Engineering, Multimedia University 1 Cavity Wavemeter A cavity resonator is a rectangular or circular waveguide with both ends shortcircuited. A given resonator can have an infinite number of resonant modes, and each mode corresponds to a definite resonant frequency. The mode having the lowest resonant frequency is known as the dominant mode. When the frequency of the wave signal is equal to a resonant frequency, a standing wave will be set up in the cavity. high-order mode dominant mode moving plunger The cavity resonator can be used as a frequency meter. A moving plunger in the cavity gives a variable resonant frequency. If the cavity is coupled to a waveguide system, a small amount of energy will be extracted through the coupling hole only if the resonant frequency is tuned to the frequency of the waves. At other frequencies, it will have no effect. An absorption-type cavity wavemeter acts by absorbing a small amount of the microwave power at its resonant frequency causing a small dip in the indicated power in the waveguide system. The transmission-type wavemeter acts as a high-q bandpass filter. The power extracted by the cavity is coupled to an output waveguide or detector. Calibration against some standard frequencies is usually performed and a calibration chart or a direct-reading scale can be produced to relate the cavity size to resonant frequency.

EM Theory Faculty of Engineering, Multimedia University 13 Slotted-line Probe The electromagnetic field at any point along a transmission line may be considered as the sum of two travelling waves; the incident wave generated by the source, and the returned wave reflected by the load. The interference between the two travelling waves gives rise to a standing wave. The maximum field strength is found where the two waves add in phase, and the minimum is encountered where the two waves add in opposite phase. The distance between two successive minimum field points corresponds to one half the guide wavelength g. The Voltage Standing Wave Ratio (VSWR) can be determined by taking the ratio of the maximum field strength E over the minimum E min. max Variable shortcircuit tuner slotted waveguide A SWR Meter The electric field strength along the waveguide can be probed using a slotted-line section. A short length of wire (electric probe) is inserted so as to couple to the electric field in the interior of the waveguide. The probe is mounted on a carriage, which slides along the slot. The field picked up by this probe may be detected by a diode detector. By moving the probe along the slot, a sampling of the E-field variation can be performed. Usually the probe protrusion into the waveguide is adjustable so that coupling is variable. The deeper the protrusion, the larger will be the detected output. However, the shortest length of the probe is preferred because the deeper the protrusion the greater will be the perturbation of the probe to the fields within the waveguide. In some designs, a tunable stub assembly is cascaded with the probe so as to maximise the detector output. In addition, this tuning also serves to minimise the small loading effect (i.e. shunt capacitance and resistance) of the probe on the transmission line circuit. Microwave Diode Detector When microwave voltage is applied across a diode (pint-contact diode or Schottkybarrier diode), a DC current is produced. If the microwave is AM modulated, the output current will be the modulating signal with a frequency span from DC up to a few MHz. The current is proportional to the microwave power. This is true if the diode is operating in the square law region of the non-linear I-V characteristic curve

EM Theory Faculty of Engineering, Multimedia University 14 at which the incremental current I is proportional to the square of the incremental voltage V across the diode. The applied signal level must be low enough to prevent saturation. Otherwise, the diode will operate in the linear region and the current will instead be proportional to the applied voltage. In order to obtain maximum output, the impedance of the detector should be matched to the characteristic impedance of the transmission line. In the case of a standing wave indicator, this is usually accomplished by using a movable coaxial short circuit known as the stub the matching tuner. SWR Meter The output from a diode detector is usually very low in amplitude. A high gain tuned amplifier is incorporated in the SWR meter to amplify this detected signal prior to the displaying meter. The bandpass response of the tuned amplifier shall be set at the modulating signal frequency range. Tuning of the centre frequency and the bandwidth may improve the signal selectivity. Square law characteristic ( Io KV s ) of the detector is usually assumed, and the SWR meter scales are then calibrated such that the needle deflection is proportional to the microwave power ( P Vs R K' Io ). The gain of the amplifier may be adjusted subject to the range of signal level being measured so that the reading is within the meter scales. Klystron Source There are basically two types of Klystron tubes: reflex-type and single-transit-type. The reflex Klystron is generally used in microwave oscillators. A highly focused beam of electrons passing through the cavity gap will be accelerated or decelerated by the induced current on the cavity. The alternate action of increasing and decreasing the electron velocity will modulate the electron beam into varying dense and sparse of the electrons referred to as electron bunches. This electron bunching mechanism is also called intensity modulation or velocity modulation. At the exit chimney of the cavity gap, a high negative electrostatic potential on the repeller will stop the movement of the electrons, turn them around and send them back through the cavity gap. As the repelled electrons re-enter the cavity, they give up their energy. Their volume charge due to bunching will be phased such that it reinforces the next wave of bunched electrons. As a result, this energy serves as a regenerative feedback to sustain the oscillation at the resonant frequency of the cavity. This is the case if the repeller voltage is set such that the electrons complete their travel through the gap, turn around, and back through the gap in three-quarter cycle of the RF waveform. In other words, the Klystron tube will oscillate only if the repeller voltage is properly set within certain voltage windows. Switching the repeller voltage in and out of the operating region will produce an over-modulated AM waves in the form of the square pulses. The oscillation frequency may be altered by turning the compression set screw that changes the cavity size.

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