As you collect data, remember that Gunnplexers, stripboards, and other microwave components do not behave exactly as do their ideal, theoretical counterparts. For example, data from real Gunnplexers approximate, but do not exactly lie on, the ideal curve of Figure 13b. You should also have observed that stripboards are far from ideal polarizers, since they attenuate the microwave beam, even if they are perfectly oriented with respect to the polarization of the transmitter/receiver system. In addition, real-world polarizers also reflect a part of the beam, allowing “standing waves” to be formed at certain positions within the path between the Gunnplexers.
Conduct the experiment shown in Figure 16. Place the transmitter and receiver horns end-to-end, and then slowly move the units apart. Since the microwave horns are not perfect transmitters or receivers, they act as partial reflectors. As such, some of the radiation from the transmitter reflects back and forth between the transmitter and the reflector. However, if the distance between the transmitter and the receiver is an integer multiple of λ/2, then all the multiply-reflected waves entering the receiver horn will be in phase with the primary transmitted wave. Under this condition, the primary transmitted and reflected waves will be in phase, peaking the received signal.
Figure 16 The microwave horns act as partial reflectors, so that the radiation from the transmitter reflects back and forth between the transmitter and the receiver (a), (b) The transmitted and reflected waves will be in phase whenever the distance between the transmitter and the receiver is an integer multiple of λ/2, peaking the signal at the receiver.
Record the distance every time you find a maximum signal as you slide the units apart. How well are you able to calculate the signal’s wavelength and frequency using this method? Compare it to the specified operating frequency of your Gunnplexers (X-band Gunnplexers commonly found in microwave door openers and radar speed guns operate at a frequency of 10.525 GHz, which has a wavelength of 2.85 cm.)
Play with your Gunnplexer setup so that you uncover its idiosyncrasies. For example, see how placing your hand close, but not in the way of the beam, affects your readings. What about sweeping the receiver away from the direct line between the antennas facing each other? Go ahead—try it out! You will find out that the microwave beam is not a pencil-tight beam like that of a laser pointer, but rather fits a rather wide Gaussian distribution.
Understanding the equipment is always an important step in the design of an experiment, since real-world components rarely behave in the same way as their theoretical counterparts. The various glitches and artifacts in data due to real-world behaviors in your equipment may completely obscure the effect that you are trying to measure. Even worse, sometimes these peculiarities trick you into believing that they are the very signal that you are trying to measure.
Leave a Reply