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MIT 8 02 - Experiment 10: Microwaves

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8.02Experiment 10: MicrowavesINTRODUCTIONFigure 3: Spark Gap and Receiver.2. Circuit BoardFigure 4: Circuit Board Setup.GENERALIZED PROCEDURENext, you will measure the angular dependence of the radiation, determining if your position relative to the transmitter matters.END OF PRE-LAB READINGIN-LAB ACTIVITIESEXPERIMENTAL SETUPMEASUREMENTSPart 1: Polarization of the Emitted RadiationPart 2: Angular Dependence of the Emitted RadiationMASSACHUSETTS INSTITUTE OF TECHNOLOGYDepartment of Physics8.02Experiment 10: MicrowavesOBJECTIVES 1. To observe the polarization and angular dependence of radiation from amicrowave generator PRE-LAB READINGINTRODUCTIONHeinrich Hertz first generated electromagnetic waves in 1888, and we replicate Hertz’soriginal experiment here. The method he used was to charge and discharge a capacitorconnected to a spark gap and a quarter-wave antenna. When the spark “jumps” across thegap the antenna is excited by this discharge current, and charges oscillate back and forthin the antenna at the antenna’s natural resonance frequency. For a brief period around thebreakdown (“spark”), the antenna radiates electromagnetic waves at this high frequency.We will detect and measure the wavelength λ of these bursts of radiation. Using therelation 103 10 cm/sf cλ = = ×, we will then deduce the natural resonance frequency ofthe antenna, and show that this frequency is what we expect on the basis of the verysimple considerations given below. Figure 1: Spark-gap transmitter.The “33” is a 33 pF capacitor. It isresponsible for storing energy to berapidly discharged across a “sparkgap,” formed by two tungsten cylinderspictured directly above it (one with avertical axis, one horizontal). Two Mresistors limit current off of thecapacitor and back out the leads,protecting the user from shocks fromthe 800 V to which the capacitor willbe charged. They also limit radiationat incorrect frequencies.E09-1The 33-pF capacitor shown in fig. 1 is charged by a high-voltage power supply on thecircuit board provided. This HVPS voltage is typically 800 V, but this is safe because thecurrent from the supply is limited to a very small value. When the electric field that thisvoltage generates in the “spark gap” between the tungsten rods is high enough (when itexceeds the breakdown field of air of about 1000 V/mm) the capacitor discharges acrossthe gap (fig. 2a). The voltage on the capacitor then rebuilds, until high enough to causeanother spark, resulting in a continuous series of charges followed by rapid bursts ofdischarge (fig. 2b).Figure 2: Charging and Discharging the Capacitor. The capacitor is slowly charged(limited by the RC time constant, with R = 4.5 M) and then (a) rapidly discharges acrossthe spark gap, resulting in (b) a series of slow charge/rapid discharge bursts. This is anexample of a “relaxation oscillator.”The radiation we are seeking is generated in this discharge.Resonant Frequency of the AntennaThe frequency of the radiation is determined by the time it takes charge to flow along theantenna. Just before breakdown, the two halves of the antenna are charged positive andnegative ( , )+− forming an electric dipole. There is an electric field in the vicinity of thisdipole. During the short time during which the capacitor discharges, the electric fielddecays and large currents flow, producing magnetic fields. The currents flow through thespark gap and charge the antenna with the opposite polarity. This process continues onand on for many cycles at the resonance frequency of the antenna. The oscillations dampout as energy is dissipated and some of the energy is radiated away until the antenna isfinally discharged. How fast do these oscillations take place – that is, what is the resulting frequencyof the radiated energy? An estimate can be made by thinking about the charge flow in theantenna once a spark in the gap allows charge to flow from one side to the other. If l isthe length of one of the halves of the antenna (about 31 mml = in our case), then thedistance that the charge oscillation travels going from the ( , )+− polarity to the ( , )−+polarity and back again to the original ( , )+− polarity is 4l (from one tip of the antennato the other tip and back again). The time T it takes for this to happen, assuming thatinformation flows at the speed of light c, is 4T l c=, leading to electromagneticradiation at a frequency of 1 T.Detecting (Receiving) the RadiationE09-2(a) (b)In addition to generating EM radiation we will want to detect it. For this purpose we willuse a receiving antenna through which charge will be driven by the incoming EMradiation. This current is rectified and amplified, and you will read its average value on amultimeter (although the fields come in bursts, the multimeter will show a roughlyconstant amplitude because the time between bursts is very short).APPARATUS1. Spark Gap Transmitter & ReceiverThese have been described in detail above. Thespark gap of the transmitter (pictured left) canbe adjusted by turning the plastic wing nut(top). It is permanently wired in to the highvoltage power supply on the circuit board. Thereceiver (pictured right) must be plugged in tothe circuit board.Figure 3: Spark Gap and Receiver.2. Circuit BoardThis board contains a highvoltage power supply forcharging the transmitter, as wellas an amplifier for boosting thesignal from the receiver. It ispowered by a small DCtransformer that must beplugged in (AC in). Whenpower is on, the green LED (topcenter) will glow.Figure 4: Circuit Board Setup.3. Logger Pro Interface and Voltage ProbeWe read the signal strength from the receiver – proportional to the radiation intensity atthe receiver – by connecting the output (lower right of circuit board) to a voltage probeplugged in to channel 1 of the Lab Pro device.E09-3GENERALIZED PROCEDUREIn this lab you will turn on the transmitter, and then, using the receiver, measure theintensity of the radiation at various locations and orientations. It consists of three mainparts.Part 1: Polarization of the Emitted RadiationIn this part you will measure to see if the produced radiation is polarized, and if so, alongwhat axis.Part 2: Angular Dependence of the Emitted RadiationNext, you


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