LRL Model 550B-SS Microwave Training Kit

Transcription

1 MICROWAVES FOR EVERYONE LRL Model 550B-SS Microwave Training Kit Microwave Training Kit 5 Experiments I-95 Industrial Park 651 Winks Lane Bensalem, PA rd edition

2 INITIAL SET-UP PROCEDURES INVENTORY & SET-UP COMPLETE PARTS LISTING 1 #503A with K5 Klystron Tube Mount 1 #508 Thermistor Mount 1 #504 Frequency Meter 1 #518 Detector Mount 1 #505 Slotted Line 4 #53 Waveguide Stands #506 Variable Flap Attenuator #531 Horn Antennas 1 #507 Termination 1 #53 Shorting Plate Microwaves for Everyone LRL 550B Microwave Training Kit

3 CONTENTS Inventory & Set Up The LRL 550B Microwave Student Lab 4 Introduction A Brief Discussion of Transmission Line Theory 9 Experiment 1 Measurement of Power 15 Experiment Measurement of Standing Wave Ratio 3 Experiment 3 Measurement of Frequency and Wavelength 31 Experiment 4 Propagation of Microwaves 37 Experiment 5 Microwave Antennas 43 LRL Model 550B-SS X-Band Microwave Training Kit Frequency Range X-Band (GHZ) The System Klystron is tuned to (9.3 GHZ) Microwaves for Everyone LRL 550B Microwave Training Kit 3

4 INITIAL SET-UP PROCEDURES INVENTORY & SET-UP LRL 510A POWER SUPPLY (thermistor bridge & amplifier) 4 Microwaves for Everyone LRL 550B Microwave Training Kit

5 PLEASE READ CAUTIONS CAREFULLY BEFORE CONTINUING CAUTION 1: Klystrons get extremely hot when in use and must not be handled while hot! Serious burns can result. CAUTION : Klystron mount, power supplies, and Klystron tube plate caps have high voltages present when in use. Exercise extreme CAUTION! Shock or death can result. CAUTION 3: R-F power levels in this kit are not harmful, but a human eye may be damaged by low levels of radiation. Do not look into any waveguide at any time when units are on. Microwaves for Everyone LRL 550B Microwave Training Kit 5

6 INITIAL SET-UP PROCEDURES INVENTORY & SET-UP INITIAL SET-UP PROCEDURES The following steps will help you test your new 510A Solid State Power Supply to make sure it is fully operational and ready for use. It also will give you the opportunity to operate and become familiar with the entire system before attempting the experiments listed in this manual. Equipment & Components 510A Power Supply 503A Klystron Mount - with k5 Klystron installed. 506 Flap Attenuator 518 Crystal Detector 508 Thermistor Mount 515 4" BNC Cables () 53 Waveguide Stands () Part 1 Control Settings Set switches and controls on 510A as follows: 1. AC Power Switch OFF. Speaker Switch ON 3. RF Switch ON 4. Attenuator Switch 0dB 5. Meter Switch POWER 6. VSWR Output Control MAX. COUNTER CLOCKWISE 7. Power Balance Control 1 O CLOCK 8. Klystron Repeller Control 1 O CLOCK Part Waveguide Component Assembly Procedure 1. Connect 503A Klystron Tube Mount to left side of 506 Variable Flap Attenuator.. Connect right side of 506 Variable Flap Attenuator to left side of 518 Detector Mount with the BNC Connector in the UP position. 3. Connect right side of 518 Detector Mount to the 508 Thermistor Mount with the BNC Connector in the UP position. 4. Using BNC Cable, connect 518 Detector Mount to VSWR input connector located on right side of 510A Power Supply. 5. Using second BNC Cable, connect 508 Thermistor Mount to POWER input connector located at center of 510A Power Supply. 6 Microwaves for Everyone LRL 550B Microwave Training Kit

7 6. Insert 8-Prong Plug from 503A Klystron Mount into socket located on left side of 510A Power Supply. 7. Place a 53 Waveguide Stand under each side of Test Assembly, and adjust each stand until the assembly is elevated in a stable position. Part 3 Operation and Checkout 1. Turn AC POWER switch ON.. Turn 506 Flap Attenuator to 4dB. 3. Observe meter and adjust POWER BALANCE knob to obtain a zero meter reading. Turning knob clockwise increases meter reading; counter-clockwise deceases reading. 4. Allow 3-5 minutes warm-up at this time. 5. Readjust POWER BALANCE knob to compensate for drift if necessary. NOTE: The meter reading must be at zero to progress further. 6. Set Variable Flap Attenuator to 0dB. 7. Slowly turn KLYSTRON REPELLER knob clockwise and stop when you get a peak reading on the meter. (Normally about o clock). NOTE: Adjust Variable Flap Attenuator as needed to keep meter level between 70% and 100%. 8. Turn METER knob to VSWR position. 9. Fine tune KLYSTRON REPELLER knob as needed to obtain a stable, peak meter reading. NOTE: This reading should be between 50% and 100% and must be stable. If not, adjust VSWR OUTPUT knob on right side of 510A Power Supply. 10. Turn METER knob to KLYSTRON position. 11. Remove BNC Cable from VSWR Input Connector at right side of 510A Power Supply and reconnect cable to KLYSTRON connector on left side of 510A Power Supply. 1. Adjust Variable Flap Attenuator as needed to attain a peak meter reading. DO NOT exceed a 100% reading on the meter. 13. Record meter reading and Variable Flap Attenuator level as your Klystron life reading. NOTE: You can compare this reading to current readings anytime in the future to determine the condition of the Klystron tube. 14. You have now successfully operated and checked out your Microwave Trainer. It is important that you turn off the Trainer properly as described below. Part 4 Shut Down Procedure 1. Turn off AC POWER switch located on right side of 510A Power Supply.. Unplug 510A Power Supply from 115AC Power Source. 3. Wait for Klystron Tube to cool down (minimum of 10 minutes). 4. Disconnect all cables and disassemble waveguide components. Microwaves for Everyone LRL 550B Microwave Training Kit 7

8 INTRODUCTION LET S TALK ABOUT TRANSMISSION LINES INTRODUCTION Transmission lines come in a variety of physical types coaxial, parallel conductors, and waveguide are the most common. No matter which type we use, there are four elements that influence any physical transmission line: A. Series inductance: L B. Series resistance: R C. Shunt or parallel capacitance: C D. Shunt or parallel conductance: G First, let us consider how a unit length of line is represented by Figure 1.1. Figure 1.1 Unit Length of Transmission Line The four different elements are represented by the fractional distribution along the entire length of the line. If the line is to be satisfactory for practical use, it must remain fractional proportional for any length long or short. Once this principle is understood, we can now use the simplified circuit diagram in Figure 1.. Figure 1. Simplified Unit Length of Transmission Line 8 Microwaves for Everyone LRL 550B Microwave Training Kit

9 The most widely used unit of measurement for transmission lines is the meter. This should come as no surprise to you because wavelength and frequency have been expressed through this unit of measure since the very start of microwave technology. The expressions of L, R, C, and G are representative of one unit length of transmission line dx. When unit lengths appear as two or more lengths, they can be seen as depicted in Figure 1.3. Figure 1.3 Several Unit Lengths of Transmission Line However, the importance of fractional proportionality along any length of line may be described simply as shown in Figure 1.. If we apply a voltage e between the input conductors shown in Figure 1. and the voltage varies at the rate de/dt, the shunt current, di per unit length of dx equals the sum of the capacitive current and the conductive current: di s dx =EG+C de dt Consequently, the shunt current represents the way in which the input current i is able to change in a unit length dx Therefore: di dx di s di de + = 0 and = -( e ) dx dx G + C dt When the input voltage e is changing with time, the current will follow. This current change can be shown as di/dt. The current change causes the series voltage to change along dx and can be expressed as: de dx di dt ( ) = - i R + L These equations are usually most helpful when a general approach (Fig. 1.1) to transmission lines is desired. However, with most applications, a general approach is not necessary. In practical use, a short transmission line is utilized, and the line is said to be lossless there by simplifying our analysis even further. The application circuit is shown in Figure 1.4: Microwaves for Everyone LRL 550B Microwave Training Kit 9

10 INTRODUCTION LET S TALK ABOUT TRANSMISSION LINES Figure 1.4 Lossless Transmission Line In Figure 1.5, we will examine this line as a number of T sections connected together to represent the line. Figure 1.5 Sectioned Lossless Line When we close s1, a signal travels through the line. The energy Q taken from the signal source is: Q = it where t is the time of current flow. As the signal flows down the line, its energy will be stored in the capacitors. So any single section is Q = Ce. If capacity C and voltage e are shown, it follows that it = Ce. Once C 1 has fully charged, C is still at zero charge and the inductance L between them has a voltage equal to e. Therefore: e = L Simplifying this relationship produces: et = Li di dt If we divide: et it = Li Ce 10 Microwaves for Everyone LRL 550B Microwave Training Kit

11 We end up with: e i = L/C This last equation is most often referred to as impedance or Z O. Accordingly: e i =Z 0 = L/C Figure 1.6 Terminated Line Figure 1.6 shows a line terminated with an impedance load Z L. When a signal is applied to this line, <100% of the total signal reaches Z L because oa small amount of it is reflected back toward the input. For this reason, the incident current ii flows in the opposite direction of the reflected current i R. So the total current i T felt by the load Z L will be: i T =i I - i R, but because the direction of the signal does not affect its potential, the total voltage et felt across load Z L will be: e T = e I +e R. The impedance must equal the total voltage and the total current. It does not matter if we look at the end load or any other point along the line. So: e T i T Z L = = e + e I R i I + i R We can then restate the equation for Z O as: e I e R i I = - = Z 0 i R Microwaves for Everyone LRL 550B Microwave Training Kit 11

12 INTRODUCTION LET S TALK ABOUT TRANSMISSION LINES Now we can see a ratio between incident and reflected voltages: e R Z L - Z = 0 = G e I Z L + Z v 0 This ratio is called the voltage reflection coefficient. We can also look at the current reflection coefficient as: i R i I G i = = Z 0 - Z L Z 0 + Z L Let s look back at the equation: e T = e I + e R. Combining this equation with the relationships described by the previous equation, we can conclude: e R = G V e I so that e T = e I + G V e I. When a transmission line experiences a time when the incident and reflected voltages are in phase, this yields a maximum voltage and can be expressed as: e max = e I (1+ G V ). Equally important is the circumstance when the incident and reflected voltages are completely out of phase, which may be expressed as : e min = e I (1- G V ). The relationship that these two points in time share is known as VSWR, or voltage standing wave ratio, expressed as: e VSWR = max = e min 1+ G V or simply: VSWR - 1 G 1- G V = V VSWR + 1 VSWR is very useful in determining how well a load is matched to the signal source. This fact is very important because most signal sources suffer damage when not properly loaded. Since the maximums and minimums take place alternately, every half wavelength VSWR is helpful in matching loads to frequency. Shown by: ƒ = u l We can also use this ratio to give us the amount of reflected power. rr G P = = G ri V = G t Consequently, the relationship of the power standing wave ratio and the voltage standing wave ratio is: PSWR = ( VSWR ). Now we can consider how much power is really used by the load. More commonly referred to as the loads transmission coefficient voltage, current and power coefficients t are expressed as follows: t v + G v = 1 t i + G i = 1 t p + G p = 1 t p = t v = t i 1 Microwaves for Everyone LRL 550B Microwave Training Kit

13 Up to now, everything we have talked about applies to all types of transmission lines. In the experiments that follow, we will be using only waveguide lines. In wave guide velocity, surfaces and physical dimensions enter the picture; so lets talk about this by looking at Figure 1.7. Figure 1.7 Wave Travel along a Surface Looking only at the RF wave that is traveling near the surface, the velocity V g is expressed: sin Q = V 0 V g But when a wave front enters the waveguide at angle Q we see the wavelength of the RF wave and the physical dimension a of the waveguide are expressed: cos Q = l 0 a Because the frequency in free air is the same as the frequency inside the waveguide, frequency can be expressed: V ƒ = 0 = l 0 V g V and 0 l = 0 = sin Q l g V g l g when V 0 is free air and V g is waveguide velocity. The wavelengths are l 0 and l g. Because: sin Q = 1 - cos Q l 0 l 0 l g = 1 - a taking the square root of each side gives: l 0 l ( 0 ) = 1- or = l g a l 1- ( 0 ) l g l 0 a Microwaves for Everyone LRL 550B Microwave Training Kit 13

14 MEASUREMENT OF MICROWAVE POWER EXPERIMENT 1 EXPERIMENT 1 Introduction Microwave power is made up of the same things that make up power at any frequency. Voltage and current flowing through a load E x I =P. The problem in measuring microwaves results from the fact that most high frequencies cannot be measured with standard voltmeters or standard current meters. So in the experiment, we will explain the most popular form of measuring microwaves. Discussion When microwaves are absorbed by most materials, they tend to heat up; if they have a negative or positive temperature coefficient, their base resistance decreases or increases. This fact becomes very useful because these materials can be utilized to make a device called a bolometer. We can use a bolometer as one leg of a bridge network (Figure 1.1) to monitor changes in resistance. Bolometer Figure 1.1 Simple Power Bridge We see that the total current i T can be adjusted with the potentiometer labeled R s. When we use R s to adjust the current to balance the brige half of the TOTAL current will flow through R b, our bolometer. So we see: i B = ½ i T Power will be expressed: P T = i B R b = ¼ i T R b 14 Microwaves for Everyone LRL 550B Microwave Training Kit

15 Now let s apply RF power to R b. It gets hot, and its resistance changes; now the bridge is no longer balanced. When we adjust R s to balance the bridge again, the total current changes. This DC power change is equal to the RF power already applied. DC power is seen as: P DC = ½ i DC R b so RF power is the difference between p t and p DC. shown: P RF = P T P DC This is equal to: P RF = ¼ R b ( i T i DC ) Another way of expressing this relationship is: P RF = ¼ R b ( i T i DC ) ( i T + i DC ) Now we want to express: ( i T i DC ) by D i Therefore: P RF = ¼ R b ( i T + i DC ) D i The power bridge in Figure 1.1 is acceptable for measurements when RF power is 1mw or above, but when it goes below 1mw, Di becomes difficult to deal with. A better method of measurement is shown in Figure 1. where the voltages e 1 & e are read across the meter as: e m = e e 1 Figure 1. Directly Calibrated Bridge Microwaves for Everyone LRL 550B Microwave Training Kit 15

16 MEASUREMENT OF MICROWAVE POWER EXPERIMENT 1 Now that current is small enough to ignore, we see: e 1 = e DC R b R b + DR b R b + DR b and e R = e DC b ( ) Or: R e m = e DC ( b + DR b R b + DR b ) R b R b use the common denominator DR and: e m = e DC ( b ) If DR b is very small 4R b + DR b 4 R b + DR b 4 R b and DR e m e DC ( b 4R b ) using ohm s law on the meter: i m = e m / R m So: e i m DC ( DR ) 4R b R m Now the meter reading is directly proportional to the bolometer resistance. Equipment and Components 1 Microwave RF source (supplied- 510A). 1 Thermistor mount (supplied- 508). RF attenuators (supplied- 506) ohm 1% metal film; 1/ watt resistors 1 10K ohm potentiometer volt DC power supply Volt-ohm meters 16 Microwaves for Everyone LRL 550B Microwave Training Kit

17 PLEASE READ CAUTIONS CAREFULLY BEFORE CONTINUING CAUTION 1: Klystrons get extremely hot when in use and must not be handled while hot! Serious burns can result. CAUTION : Klystron mount, power supplies, and Klystron tube plate caps have high voltages present when in use. Exercise extreme CAUTION! Shock or death can result. CAUTION 3: RF power levels in this kit are not harmful, but a human eye may be damaged by low levels of radiation. Do not look into any waveguide at any time when units are on. Microwaves for Everyone LRL 550B Microwave Training Kit 17

18 MEASUREMENT OF MICROWAVE POWER EXPERIMENT 1 Objective SEE CAUTION ON PREVIOUS PAGE To show the relationship between a thermistor s resistance cold and when RF power is applied. Part 1 1. Hook up circuit in Figure Fill in Table 1-1 starting at zero volts and not exceeding 1 volt. Record the currents first. Figure 1.3 Set-up for Determining a Thermistor Volt-Ampere Characteristic Table 1.1 Data for Procedure, Part Thermistor Voltage Thermistor Current Thermistor Power Thermistor Resistance 18 Microwaves for Everyone LRL 550B Microwave Training Kit

19 The thermistor element as such cannot be removed from the waveguide mount. However, its two terminals are readily accessible through the (BNC) connector mounted on top of the guide. Under no condition should the student try to remove the thermistor element. Note: Show Thermistor Voltage from 0.0 up to 1.0 volt. DO NOT GO BEYOND 1.0 VOLT. 3. Then calculate the power readings. 4. Then calculate the resistance readings. 5. Using information from Table 1-1 plot a volt ampere curve. 6. Then plot a power verses resistance curve. Part 1. Hook up circuit in Fig A Figure 1.4 Simple RF Power Bridge Microwaves for Everyone LRL 550B Microwave Training Kit 19

20 MEASUREMENT OF MICROWAVE POWER EXPERIMENT 1. Balance the bridge with 10 K ohm potentiometer (This is zero current through meter i m ). 3. Enter reading from meter i T as total current i T on Table 1.. Table 1. Data for Procedure, Part i T i m i DC RF Power The thermistor element as such cannot be removed from the waveguide mount. However, its two terminals are really accessible through the (BNC) connector mounted on top of the guide. Under no condition should the student try to remove the thermistor element. 4. Apply RF power to bridge as in Figure 1.5 keeping attenuators with full flap insertion (maximum attenuation). Enter reading from i m in Table Microwaves for Everyone LRL 550B Microwave Training Kit

21 503A Enter reading of new total current i DC in Table Calculate and enter reading of RF power in Table Adjust RF attenuators for less attenuation; then repeat steps Take three more readings at new RF inputs (adjusting the attenuators) repeating steps - 6 each time. 9. Now you can plot a curve showing RF power and bridge current i m. Microwaves for Everyone LRL 550B Microwave Training Kit 1

22 MEASUREMENT OF VOLTAGE STANDING WAVE RATIO EXPERIMENT EXPERIMENT Introduction When RF is transmitted down a line into a load, even one with a good match, some of the signal reflects back toward the source. The amount of signal sent vs. the amount reflected back is compared and referred to as the standing wave ratio. When this pattern deals with measurements of voltage, it is called the voltage standing wave ratio (VSWR), it is simply expressed as: VSWR = e max e min Discussion The equation of VSWR = VSWR = e max e min would be very easy to use to calculate VSWR if the voltage measurements were as easy to take, but they are not. The most popular method of measuring VSWR employs the use of a slotted line. To help us understand this device, let us look at Figure.1a. Figure.1 Voltage Distribution along a Transmission Line This diagram shows us a sample length of transmission line, in our case X-band waveguide. In Figure.1b, we see an expanded view of the RF sine wave rising and falling along the length of the line. Placing a slotted line into the transmission line from point A to point B allows us to inspect a probe into the slot in the line. We can then move the probe along the line reading peaks and dips, e max and e min which are shown as X and Y. Because RF voltages are so difficult to measure, we will make the probe like the one in Figure.a. Microwaves for Everyone LRL 550B Microwave Training Kit

23 Figure. Slotted Transmission Line and Probe (a) Cutaway Sketch of a Slotted Section with Probe, Carriage, and Detector (b) Equivalent Detector Circuit The key addition here is the crystal detector which will change the RF voltage into a DC current and will be read by the meter in Figure.b. Because some detectors act differently from others, we will use a square law detector. These detectors have an output proportional to the square of the input. So if we place the probe at Y on the slotted line, we will get: i min = ke min Or: e min = i min / k Then if we move the probe to point X, we get: i max = ke max Or: e max = i max / k Together we get: e VSWR = max i max / k = = i max / k e min i max i min Now we can measure the X and the Y on the meter in Figure -b and we see: VSWR = maximum meter reading minimum meter reading Microwaves for Everyone LRL 550B Microwave Training Kit 3

24 MEASUREMENT OF VOLTAGE STANDING WAVE RATIO EXPERIMENT This way of measuring VSWR is by far the easiest, but if the minimum meter reading cannot be measured with a reliable degree of accuracy, the inherent flaws in the detector may create distortion increasing the chance of error. When standing wave ratios are larger than ten to one, the double minimum measurement system can be used to in crease the accuracy and reduce the percentage of error. Let us examine this method in Figure.3. Figure.3 Double Minimum Method The mathematical expression of this curve is: e = e min + (e max e min ) sin pl ( ) l starting at point A, we move to the right until the reading is twice what it was at point A. This gives us: pd e min = e min + e max sin e min sin pd ( ) ( ) l l This relationship can also be expressed as: e max e min = 1 + sin ( p d / l ) sin ( d / l ) Because in mathematics: sin Q = 1-cos Q 4 Microwaves for Everyone LRL 550B Microwave Training Kit

25 we can restate the formula as: e max e min = cos ( p d / l ) sin ( p d / l ) and also as: e VSWR = max cos e = ( p d l ) min sin ( p d / l ) The actual measurement of VSWR, however, is done by measuring the distance between two double minimum measurements. Then we use this information and a trigonometric table like the one in Table.. VSWR s with ten or greater angle pd/l will be small and so will sin (p d/ l) because of this: 1 + sin ( p d / l ) 1 So then: VSWR because of the small angle: sin p d l ( ) Now we can reduce this equation to: VSWR 1 sin ( p d / l ) l p d p d l This is only required when VSWR is greater than ten. In some cases, the measurements of e max and e min can be amplified to provide better accuracy. Equipment and Materials 1 Microwave source (supplied- 510A). Waveguide attenuators (supplied- 506). 1 Slotted line with probe and detector (supplied- 505). 1 DC microammeter (supplied). 1 Short circuit terminator (supplied- 503A). Waveguide stands (supplied- 53). Microwaves for Everyone LRL 550B Microwave Training Kit 5

26 MEASUREMENT OF VOLTAGE STANDING WAVE RATIO EXPERIMENT PLEASE READ CAUTIONS CAREFULLY BEFORE CONTINUING CAUTION 1: Klystrons get extremely hot when in use and must not be handled while hot! Serious burns can result. CAUTION : Klystron mount, power supplies, and Klystron tube plate caps have high voltages present when in use. Exercise extreme CAUTION! Shock or death can result. CAUTION 3: RF power levels in this kit are not harmful, but a human eye may be damaged by low levels of radiation. Do not look into any waveguide at any time when units are on. 6 Microwaves for Everyone LRL 550B Microwave Training Kit

27 SEE CAUTION ON PREVIOUS PAGE Objective In this experiment, we will look at some common ways to measure VSWR. Part 1 Medium VSWR 1. Set up test fixture in Figure A Connect one of the video cables from the detector on the slotted line to the input marked VSWR on the power supply. 3. Turn on power supply and apply RF power. 4. Move probe on slotted line and adjust for maximum deflection (try to keep this at the center of the slotted line if possible). 5. With the shorted attenuator set at maximum, adjust the RF feed attenuator for a close to full scale reading. 6. Move the probe for a minimum reading. 7. Adjust the short feed attenuator for a reading of about 1/4 scale. 8. Measure and record the readings at the points indicated on Table.1. Microwaves for Everyone LRL 550B Microwave Training Kit 7

28 MEASUREMENT OF VOLTAGE STANDING WAVE RATIO EXPERIMENT Table.1 Data Distance along the Line (in cm) Meter Readings in u amp Part 1 Part Part 3 9. Plot a curve of distance and readings. 10. Using Table. and equations from the discussion section, fill in the VSWR calculations on Table.1. 8 Microwaves for Everyone LRL 550B Microwave Training Kit

29 Part High VSWR 1. Use the same test fixture and adjust the RF feed attenuator to minimum attenuation.. Move the probe to minimum reading and then adjust the short attenuator for a reading of 1 /10 full scale. 3. Repeat steps 8, 9, and 10 of part 1 (record any off scale readings as off scale). Part 3 Low VSWR 1. Adjust short attenuator to maximum attenuation.. Adjust RF feed attenuator for a 50% of full scale with probe at a peak point. 3. Repeat steps 8, 9, and 10 of part 1. Table. πd λ sin ocs πd sin cos πd sin cos λ λ Microwaves for Everyone LRL 550B Microwave Training Kit 9

30 MEASUREMENT OF FREQUENCY AND WAVELENGTH EXPERIMENT 3 EXPERIMENT 3 Introduction Microwaves differ from standard RF because they consist of extremely high frequency waves. Consequently, in order to understand microwaves, we must understand microwave frequency measurements. In this experiment, we will look at some common ways to measure microwave frequency. Discussion The relationship of a RF wave could be seen as an equation between frequency, wavelength, and velocity: ƒ l = V Frequency remains fixed in our equation so that wavelength is directly affected by the velocity. The velocity of our wave is determined by its physical path of travel if the wave is traveling through air, velocity is equal to: V 0 = 3 x cm per sec When it travels along paths other than air, the velocity is affected by other factors, as expressed: V 0 e t m t Here, e t equals the permitivity of the path, and m t equals the permeability of the path e t and m t are different for different materials, and these factors can be found in microwave engineering guides or handbooks. Since we are considering velocity to be a constant through a given path, frequency and wavelength can be measured. We will look at two methods of measuring frequency and wavelength; any one of the two can be used to calculate the other. First, let us look at the slotted line to measure wavelength. Look at the wave in Figure Microwaves for Everyone LRL 550B Microwave Training Kit

31 Figure 3.1 Distance along the Line The distance d between c 1 and c is represented by the equation l L = d, l L is equal to the wavelength as measured along the slotted line. When using a coaxial line, velocity V L will be about equal to air velocity V 0. so we see that l L = l O. When using waveguide, velocity is determined by the larger dimension a (Table 3-3 in this chapter will assist you with the values for a ). We can compare the wavelength in waveguide to the wavelength of air by: l L = l 0 l 0 1 a Then we calculate: ƒ = V 0 l L + 4a a l L These equations work well in calculating frequency and wavelengths as long as d can be measured with little problem. So when we have a high VSWR on the line (five or more), there is no problem, but a low VSWR makes c 1 and c difficult to locate (refer to Figure 3.1). We can select a value for e any value larger than e min. X1 will be at the center of measurements of e shown as y and y 1. This is shown: c 1 = y' + y Microwaves for Everyone LRL 550B Microwave Training Kit 31

32 MEASUREMENT OF FREQUENCY AND WAVELENGTH EXPERIMENT 3 PLEASE READ CAUTIONS CAREFULLY BEFORE CONTINUING CAUTION 1: Klystrons get extremely hot when in use and must not be handled while hot! Serious burns can result. CAUTION : Klystron mount, power supplies, and Klystron tube plate caps have high voltages present when in use. Exercise extreme CAUTION! Shock or death can result. CAUTION 3: RF power levels in this kit are not harmful, but a human eye may be damaged by low levels of radiation. Do not look into any waveguide at any time when units are on. 3 Microwaves for Everyone LRL 550B Microwave Training Kit

33 We see a much improved accuracy if we take several readings on the slotted line for d, and then take the average of d and use it in place of d when using the equation to calculate wavelength. The next method of measuring frequency and wavelength is a resonant cavity. In this type of measurement, we monitor the power flowing through the cavity and adjust the internal plunger until a dip is seen on the power meter. The point of lowest power is the point of resonance. Some cavities come with calibration charts so the frequency can be read right off the chart. Others are marked in frequency and can be read direct. Another way to use this approach is to know the frequency f o and set the plunger to the setting R 0. When the cycles per dial division are known, we can use the different plunger settings shown Df to calculate the new frequency: ƒ = ƒ 0 + D ƒ ( R 0 R) As an example, we might establish that a setting of 550 is equal to 9.0 kmc and that every dial division equals a difference of 5.0 mc. if the plunger setting changes to 345. What is the new frequency? Let s see: ƒ = 9 x x 10 6 ( ) cps ƒ = 9 x x 10 9 cps ƒ = x 10 9 cps = kmc Our new frequency is now kmc. Equipment and Components 1 Microwave RF source (supplied- 510A). Adjustable waveguide attenuators (supplied- 506). 1 Cavity wave meter (supplied- 504). 1 Termination (supplied- 507). 1 RF power bridge (supplied). 1 Short circuit termination (supplied- 503A). Waveguide stands (supplied- 53). 1 Detector mount (supplied- 518). SEE CAUTION ON PREVIOUS PAGE Microwaves for Everyone LRL 550B Microwave Training Kit 33

34 MEASUREMENT OF FREQUENCY AND WAVELENGTH EXPERIMENT 3 Objective To demonstrate both cavity tuned and slotted line frequency and wavelength measurements. Part 1. The Cavity Wave Meter. 1. Set up Figure A Turn on power supply. 3, Adjust the attenuator for a power reading of at least 50 percent. 4. Starting at a cavity reading of 0.000, slowly tune the plunger until a dip is obtained (this will be the lowest power reading on the meter). 5. Record the reading on Table 3.1. Table 3.1 Data From Wavemeter At Dip Computed R o f o Δf R f λ L 6. Record the cavity calibrated point. 7. Record the rate of change per dial increment. 34 Microwaves for Everyone LRL 550B Microwave Training Kit

35 8. Calculate the frequency of the source. 9. Calculate the wavelength. Part. 1. Replace the cavity with the slotted line.. Replace the termination & detector mount with another attenuator (terminated with a short). 3. Adjust the VSWR for about Using the slotted line, complete readings for X 1 and X and record them in Table 3.. Table 3. Data VSWR Y 1 Y 1 1 Y Y 1 X 1 X d λ L f 5. Using Measurements for low VSWR, find Y and Y Now calculate for frequency and wavelength. Table 3.3 Data Frequency Range TE 10 mode (Kme) Jan. No. RG-()/U Inner Dimensions a (cm) b (cm) Outer Dimensions c (in.) d (in.) f (Kme) Approximate Attenuation (db, ft) Microwaves for Everyone LRL 550B Microwave Training Kit 35

36 PROPOGATION OF MICROWAVES EXPERIMENT 4 EXPERIMENT 4 Introduction Microwaves travelling in free space (that is out of the confines of a transmission line) can be altered and affected by many things. In this experiment, we will try to examine some of the factors affecting microwaves traveling in free space. Discussion If an RF source is radiating power equally in all directions, then the wavefront moving away from the starting point will be a sphere. This starting point is known as an isotropic point source, or point source to be short. Figure 4.1 shows such a wavefront. Figure 4.1 Spherical Wavefront Because the total power radiated at the point source is spread over the entire sphere, the power density per unit of the wavefront is the transmitted power P tr divided by the area of the sphere: r tr power density = 4p r where r is not the radius, but the range of the measured area from the point source. If we concentrate the radiation at the point source by a factor G T (the ratio of the area of the sphere to the actual area covered), we now get: r tr G T power density = 4p r If we pick up a portion of the transmission with an antenna, which has an area Ar the received power is: r rec = r tr G T A r 4p r This formula describes one of the basic properties of radiated power. When the transmitted power and the antenna are constant, the received power is proportional to 36 Microwaves for Everyone LRL 550B Microwave Training Kit

37 the inverse of the range squared. This relationship is referred to as the inverse square law. A problem appears when we try to make the source appear to be a point source. To do this, we must use a range that is very large in relationship to the source antenna s area. The minimum range to take this measurement is: d r min l 0 where d is the largest dimension of the source antenna, and l o is the free-space wavelength of the transmitted signal. Let us consider the reflection of a wave bouncing off a flat surface as shown in Figure 4.. Figure 4. Reflection from a Flat Surface The reflected wave has the same angle as the incidence wave with a perpendicular to the surface. The angle between the incidence wave and the perpendicular is called the angle of incidence. Likewise, the angle between the perpendicular and the reflected wave is called the angle of reflection. The fact that these two angles are the same always holds true for a flat surface. If the surface is not flat, but is concave or convex, this rule changes. Figure 4.3 shows us these two surfaces. Figure 4.3 Reflection from a Curved Surface (a) Concave (b) Convex Figure 4a shows how a concave surface focuses the reflected waves to a fixed point, while Figure 4.3b shows how a convex surface sends them off in many directions. Microwaves for Everyone LRL 550B Microwave Training Kit 37

38 PROPOGATION OF MICROWAVES EXPERIMENT 4 PLEASE READ CAUTIONS CAREFULLY BEFORE CONTINUING CAUTION 1: Klystrons get extremely hot when in use and must not be handled while hot! Serious burns can result. CAUTION : Klystron mount, power supplies, and Klystron tube plate caps have high voltages present when in use. Exercise extreme CAUTION! Shock or death can result. CAUTION 3: RF power levels in this kit are not harmful, but a human eye may be damaged by low levels of radiation. Do not look into any waveguide at any time when units are on. 38 Microwaves for Everyone LRL 550B Microwave Training Kit

39 Equipment and Components 1 RF microwave source (supplied- 510A). 1 Crystal detector (supplied- 518). 1 Waveguide termination (supplied- 507). 1 Variable attenuator (supplied- 506). Waveguide horns (supplied- 531). 1 Flat metal surface (not supplied). 1 Oscilloscope (optional). 4 Waveguide stands (supplied- 53). SEE CAUTION ON PREVIOUS PAGE Microwaves for Everyone LRL 550B Microwave Training Kit 39

40 PROPOGATION OF MICROWAVES EXPERIMENT 4 Objective To reflect and receive a microwave signal. 1. Look at the two waveguide horns. If they are not identical in size, use the smaller one as the source antenna.. Calculate the minimum range to be used. Use the dimensions of the source antenna and 3 cm as the approximate free-space wavelength. 3. Set up the source side of Figure A Set up the receive side of Figure 4.4. Turn on power and allow a warm-up period. 5. Set up receiver directly in line with the source at about 10 times the r minimum. Record this value in Table 4.1. Table 4.1 Data r min Initial Range Pulse Height Range with 3 db Range with 6 db Range with 9 db 6. Set the attenuator to minimum and record pulse height at the oscilloscope. Record this in Table 4.1. (if no oscilloscope is available, you can take measurements with the VSWR portion of your trainer.) 7. Set attenuator for 1/ pulse height. Then move the receiver closer to the source until the pulse height is the same as in step 6. Measure and record the new range in Table Microwaves for Everyone LRL 550B Microwave Training Kit

41 8. Repeat step 7 two more times recording the values in Table 4.1. NOTE: Every time your pulse height drops in half, you have added about 3 db of attenuation to the line. 9. Set up Figure 4.5 so that the source and the receiver are facing at right angles to one another and are about 1.5 times r min a part. 10. Find the spot (with the flat reflection) where the pulse height is maximum. Table 4. Data Trials θ i θ r 11. Record in Table 4. the angle θ i between the reflector surface and the source antenna. 1. Record in Table 4. the angle θ r between the reflector surface and the receiver antenna. 13. Repeat steps 10 through 1 at least five times. Microwaves for Everyone LRL 550B Microwave Training Kit 41

42 PRIMARY MICROWAVES ANTENNAS EXPERIMENT 5 EXPERIMENT 5 Introduction There are a number of types of primary antennas, such dipoles, horns, and polyrods, but we will be discussing the horn antenna in this equipment. Discussion The microwave horn is a very simple antenna. It physically just tapers out from the waveguide and has one job, to match the impedance of the waveguide to free space. Horn antennas may be made several different ways. They can be tapered as E plane horns, or H plane horns, but the antennas we will use is pyramidal or tapered in both planes as in Figure 5.1. Figure 5.1 Pyramidal Horn Because the H plane and the E plane are not the same size, the resulting radiation will not be the same width in both planes. They will be inversely proportional to the dimensions of that plane. The widths are shown: θ H 80l 0 a and θ E 53l 0 b 4 Microwaves for Everyone LRL 550B Microwave Training Kit

43 The gain of an antenna is the ratio of the surface area of a sphere with (radius) to the actual area radiated by the antenna at range. It is important that you understand that an antenna s gain has nothing to do with the power fed into it. The gain will be the same for a small signal or for a large one. or g = pab l 0 When two horns are used together (one to transmit and one to receive), we can see the transmitter gain and the receiver gain and separate them by a range. They give us: r rec = g rec g tr l 0 ( 4pr ) r tr But when the two horns are the same, we get: r rec = Or: g l 0 ( 4pr ) r tr g = 4p r l 0 r rec / r tr When taking measurements in the laboratory, there are two main problems that must be considered. One is the fact that measurements must be made with a minimum range is no less than: r min = d l 0 Where d is the largest dimension of the transmitting antenna, the second problem is that some of the transmitted power will be reflected back up from the surface of the work area. To prevent errors in this area, the minimum distance to surface should by no less than: D min = l 0 r d r is the range to the receiver and d the largest dimension of the transmitting horn. Microwaves for Everyone LRL 550B Microwave Training Kit 43

44 PRIMARY MICROWAVES ANTENNAS EXPERIMENT 5 PLEASE READ CAUTIONS CAREFULLY BEFORE CONTINUING CAUTION 1: Klystrons get extremely hot when in use and must not be handled while hot! Serious burns can result. CAUTION : Klystron mount, power supplies, and Klystron tube plate caps have high voltages present when in use. Exercise extreme CAUTION! Shock or death can result. CAUTION 3: RF power levels in this kit are not harmful, but a human eye may be damaged by low levels of radiation. Do not look into any waveguide at any time when units are on. 44 Microwaves for Everyone LRL 550B Microwave Training Kit

45 Equipment and Components 1 RF microwave source (supplied- 510A). 1 Crystal detector (supplied- 518). 1 Waveguide termination (supplied- 507). Waveguide horns (supplied- 531). 1 Cavity frequency meter (supplied- 504). 1 Variable attenuator (supplied- 506). 1 Oscilloscope (optional). 4 Waveguide stands (supplied- 53). SEE CAUTION ON PREVIOUS PAGE Objective 1. Measure and record the dimensions of one of the horns in Table 5.1. Table 5.1 Data a b r min D min Range P rec θ θ f λ 0 Attenuation P tr G 1. Calculate the value of r min using l 0 3cm. Microwaves for Everyone LRL 550B Microwave Training Kit 45

46 PRIMARY MICROWAVES ANTENNAS EXPERIMENT 5 3. Set up Figure A Pick a good value of range greater than rmin (refer to discussion for the equation). 5. Position the receiving horn at the chosen range from the transmitter horn. 6. Set the attenuator for minimum, and turn on the source and allow it to warm up. Record the pulse height Prec in Table Slowly rotate the receiver with respect to the transmitter until the pulse height drops in half. Record the angle the receiver was turned. 8. The value in step 7 is one half the beam width of the antennas. Complete the total beam width. 9. Return the test set up to the pulse height from step Measure the frequency with the cavity meter and calculate the transmitted wavelength Detune the cavity meter. 1. Remove the horns and connect the receiver directly to the attenuator. 13. Adjust the attenuator for a pulse height of step Record the attenuator setting. Calculate the pulse height that corresponds to the transmitted power using the equation: db = 10 log P tr P rec 15. Calculate the gain of the antennas using data from step 6 and 14 (use equations from the discussion). 46 Microwaves for Everyone LRL 550B Microwave Training Kit

47 Microwaves for Everyone LRL 550B Microwave Training Kit 47