MICROWAVE BOOT CAMP
Before you begin your individual microwave projects, there are a number of features of the microwave apparatus that all of you will need to become aware of, features that are useful for most of these projects. That is the reason for this brief “microwave boot camp”.
Make your choice of microwave experiment and confirm with your instructor that nobody else has already claimed it. Do your library research for the optical phenomenon you have selected.
The microwave transmitters, Pasco Scientific WA-9801, have a nominal frequency of 10.5GHz. You will notice this number printed on the unit. This of course implies a specific wavelength for the transmitters if you know that the waves have a speed equal to the speed of light.
· Calculate the wavelength from this value.
Divide into enough groups so that all of the transmitters can be characterized. Be sure to mark clearly in your notes the ID tag of your microwave transmitter. This will help you in case you need to study that receiver’s characteristics later.
· Measure the frequency of the transmitter, using the HP frequency counter. Remark in your notes as to whether or not the frequency changes if you change the distance between the transmitter and the detector. If so, write the frequency as a range: mean value plus or minus half the range.
· These transmitters are polarized. Rotate yours relative to the frequency counter’s detector. Describe in your notes what happens. Sketch in your notes the relative orientation of the transmitter and the detector which gives the largest signal.
The microwave receiver, Pasco WA-9800, is also polarized. It will record its maximum signal when its horn is aligned with the horn of the receiver.
· Place a transmitter and a receiver facing each other and locate the maximum signal by rotating first one, then the other. Sketch in your notes an orientation in which you get the maximum signal on the receiver.
· Turn the “variable sensitivity” dial all the way clockwise. Be aware that if you change the sensitivity partway through an experiment, the values displayed on the detector become meaningless. Connect the output (red and black ‘banana plug’ sockets) of the receiver to measure DC voltage on a portable digital multimeter [DMM]. Placing the transmitter facing the receiver at point-blank range, check to see what voltage you get when the signal overloads the receiver. Record this value. For future measurements, be aware that if you get this value, you need to alter the sensitivity multiplier (“Intensity”: 30x, 10x, 3x, 1x) until you are no longer overloading. Which meter overloads first, the analog dial on the receiver or the digital voltmeter?
In the future, when recording data, you need to record both the DMM voltage and the multiplier switch position. To compare signals measured using different multiplier positions, calculate the multiplier number times the voltage, e.g. switch=30x and voltage=1.2V means a signal of 30x1.2V=36V.
The signal that the receiver measures is not the intensity (power per area) of the microwaves, nor is it a measure of the electric field strength of the signal. For most of you, none of this matters, except that the detector is monotonic -- an increase in microwave intensity produces an increase in signal.
· Test this by placing the entrance of the horns of the transmitter and receiver about a handspan apart (9”) and rotating one of the horns until the signal goes to zero. (Use the digital meter, and adjust both the multiplier scale and the DMM scale.) If you get a negative signal, record the angles for which it is negative. Note whether the signal returns to zero when the two are turned 90º relative to each other. Repeat with the horns much closer and then approximately 1 meter apart. Notice that the angular scales on the back of your transmitter and receiver may not correspond to 0º when the two are aligned. When I say “angle” above, I mean the difference between the orientations of the two horns: 5º, for example, means that they are almost lined up exactly.
OTHER BASIC PROPERTIES OF MICROWAVES:
· Place a beaker of water between your transmitter and receiver. Observe its effect on the transmitted signal. Repeat with a moist sponge between the two. A moist paper towel.
· Place a metal sheet between your transmitter and receiver. Note the effect. Repeat with a piece of aluminum foil.
· Test whether microwaves reflect off the laboratory tables. If so, does the amount of reflection depend on the orientation of the polarization of the microwaves?
ON TO YOUR OWN PROJECT:
Now you may continue on to your own project. Be sure to always unplug the transmitters when done, and to dial the “Intensity” knob of the receiver to zero.