Download Unit 7 Microwave Objective Study the characteristics of microwaves

Unit 7 Microwave
Objective
Study the characteristics of microwaves, such as wavelength, energy decay,
and polarization.
Apparatus
Microwave transmitter
Microwave receiver
Goniometer
Base ×3
L-type meter
Reflector
Polarizer
Meter
Set-up of Device
L-type meter
Reflector
Polarizer
Goniometer
Transmitter
Fig. 1.
Receiver
Principles
Fig. 2.
Fig. 3.
Microwave is a kind of electromagnetic waves whose wavelength is among
10 m ~10-4 m (the wavelength of the microwave we use in this experiment is
2.85 cm.), and the propagation direction can be determined by Maxwell’s
Equation. In vacuum, electric field E and magnetic field B of a microwave are
perpendicular to each other, and they are also perpendicular to the direction of
v v
propagation; propagation direction is determined by E × B , and it’s a transverse
wave. As shown in Fig. 3, E oscillates in y-direction and B in z-direction.
Therefore, the wave propagates in x-direction.
We determine whether a wave is polarized or not by the direction of its E
field. If the electric field E of a microwave oscillates in fixed direction, then we
call it as a linear-polarized wave. Examples of linear-polarized waves are those
transmitted from TV stations or broadcast stations. We can know the polarized
direction of a linear-polarized wave by a polarizer. As shown in Fig. 4,
microwave with the electric field parallel to the polarizer can pass through the
polarizer; on contrary, for a microwave whose electric filed is perpendicular to
the polarizer, it will be absorbed.
-2
Fig. 4.
Component (of electric filed) that can pass the polarizer:
E z = E 0 cos θ (θ is the included angle of polarizer and electric field), and
the intensity of a microwave is proportional to the square of electric field
magnitude, i.e. I ∝ E 2 . Hence, the intensity of microwave passing through the
polarizer can be expressed as I = I 0 cos 2 θ .
Instructions
I. Intensity and Distance
1. Set up the devices as Fig. 5, and the angles of transmitter and receiver are
both 0°. Switch off the intensity multiplier on the receiver. Plug in the
receiver and then switch the multiplier to 30X.
Fig. 5.
2. Adjust the distance R between transmitter and receiver at about 40 cm.
(NOTE: R is measured from the effective points of emission and
3.
4.
5.
transmission signals (as shown in Fig. 6)).
Adjust the position of the receiver back and forth to find a local maximum
of the intensity meter.
Tune the variable sensitivity switch and set the value to 1.0.
Move the receiver backward smoothly. Measure the value of R and recorded
value on the meter (Intensity) when you find a minimum value of intensity.
Then move the receiver backward again until you find another maximum
value, and measure the value of R and the value on the meter (Intensity).
Repeat this step until you find 15 maxima and 15 minima of intensity.
Q1: Why should the receiver be moved backward but not forward?
6. Do the plot of intensity I versus R, and I versus R-2.
7. Find the average value of wavelength and the standard deviation.
Q2: For a maximum or minimum value, a standing wave exists between the
transmitter and the receiver. Is the distance between a maximum and
an adjacent minimum λ ,
II.
λ
2
or
λ
4
?
Q3: According to the plots you made, what do you think the relation
between I and R is? Moreover, do you think it’s a planar wave or a
spherical wave?
Lloyd’s Mirror
1. Set up the devices as Fig. 7, and R is suggested to be greater than 0.8 m. Be
sure the Receiver and Transmitter are equidistant from the center of the
Goniometer degree plate and that the horns are directly facing each other.
Measure the distance d as Fig. 7 showed (better between 40 cm to 45 cm).
2.
Slide the mirror smoothly away from the Goniometer platform, and observe
the change in the intensity meter. Record the value of h when local maxima
and minima appears (6 maxima and 6 minima).
Q4: Why does the intensity changes with the position of the mirror?
3. Use d, h, and the concept of optical path difference to calculate the average
value of λ and the standard deviation. Compare the result with that in step I.
4. Change the value of d and repeat step 3 and 4 twice (Three different d).
Fig. 7.
Fig. 8.
Q5: Why should the distance R better be greater than 0.8 m?
Q6: Why should the receiver and transmitter be at the same distance from
the center of the Goniometer?
III. Polarization
Fig. 9.
1.
2.
3.
4.
5.
Fig. 10.
Set up the devices as Fig. 9, and R is about 60 cm. Note that the angle at the
back of transmitter and receiver are both 0°.
Adjust the position of receiver slightly to find the local maximum.
Tune the variable sensitivity switch and set the value to 1.0.
Unfasten the screw in the back of the transmitter and then adjust the
angle from -90° to 90°, 10° for each step, and record each value on the
receiver intensity meter.
Do the plot of intensity versus θ and cosnθ.(determine the value of n by
yourself. n=1, 2 ,4 are suggested)
Q7: What is the relation between the recorded value on the receiver and θ ?
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6.
As shown in Fig. 10, place a polarizer in the middle of the receiver and
transmitter. Note that both the angles at the back of transmitter and receiver
should be zero.
7.
8.
9.
10.
Adjust the fringes of the polarizer to horizontal.
Adjust the position of the receiver slightly to find a local maximum.
Tune the variable sensitivity switch and set the value to 1.0.
Unfasten both screws on the receiver and transmitter. Then adjust the
angles from -90° to 90°, 10° for each step, record the value on the intensity
meter. (NOTE: The receiver and transmitter are parallel ALL THE TIME
when you change the angles.)
11. Do the plot of intensity versus θ and cosnθ.(Determine the value of n by
yourself. n=1, 2, 4 are suggested.)
Q8: What is the relation between the recorded value on the receiver and θ ?
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12. Rotate the angle at the back of transmitter back to 0°, and the receiver’s to
90°.
13. Rotate the polarizer to horizontal, 45°, and vertical, and record each of the
value on the intensity meter.
Q9: Try to explain your data and compare it to the result in Q8.
Q10: From the above experiment, can you know the direction of
polarization?
Remarks
1. Do not look directly into the transmitter!
2. The distance of transmitter and receiver should not be shorter than 30 cm. Switch
the variable sensitivity to the left-end before you plug in the power. And note that
whether the value on the meter is greater than 1.0 to prevent damage of the
device.
3. Do not stand in front of the devices because your body may cause the reflection
of microwave. You had better stand in back of the transmitter or receiver, and do
not place any extra objects (especially metal items) on the table in case of
reflection of microwave.