322_Microwave_Measurement_2002.doc

>

The University of Hong Kong
Department of Physics
PHYS3431 Advanced Experimental Physics
Experiment number 322: Microwave Measurements

IMPORTANT:��
Never look directly into the waveguide when operating,

��
serious damage with your eyes could be resulted.

Everyone today
is affected by microwaves. You may not realize it, but each day your
life is made better by mans ability to utilize microwaves. When you
make a telephone call across the country or watch television from your
home, microwaves help you. Satellites circling the earth are exchanging
information millions of times every day through microwave transmission.
Communications, health care, national security, alarm systems, the police
officer clocking the speed of your car ---- all require microwave technology.
Try to imagine an air traffic system today without radar, or the weatherman
attempting to predict tomorrows forecast without this developing science.

As well as demonstrating
common microwave measurement techniques this experiment is intended
to familiarize students with various microwave components, giving an
understanding of the physics involved in their design.

Apparatus Arrangement

Power Supply (1 kHz A.M.)

C.R.O.

1 kHz Amplifier

Gunn Diode

Variable Attenuator

Frequency Meter

Diode Detector

Slotted Line

Short Circuit
Load

Display

Figure 1 (A)

Figure 1 (contd).

Microwave Components

Carefully
examine the various components before connecting them together in the
sequence shown in figure 1. Connect the components using screws and
nuts through two diagonally positioned holes. All the components should
be supported at the same height above the bench using the supplied standing
supports. Notes on the Gunn diode, variable attenuator, frequency meter
and diode detector are given in appendix 1. The Gunn diode oscillator
is amplitude is modulated at 1 kHz by its power supply.

The slotted line consists of a vertical probe which is movable along
the centre of the waveguide. The E.M. wave induces a voltage in the
probe and after passing through a diode (demodulation taking place)
a 1 kHz signal is left, the amplitude being called the standing wave
voltage and this is measured using the 1 kHz amplifier (being displayed
on a meter). This arrangement ensures detection of small signals, since
there is rejection of pickup interference and electrical noise at
frequencies other than 1 kHz improving the signal/noise ratio.

Note that probe in the slotted line has always to be tuned to the
microwave frequency by rotating its collar until a maximum signal level
is observed on the meter of the 1 kHz amplifier. Attenuator controls should be used keep the meter pointer within
maximum scale reading.

Frequency measurement

With slotted line probe near a position of maximum
signal (near full scale meter deflection)

Adjust the micrometer screw of the frequency meter
until a sharp drop occurs in the C.R.O.
Then the frequency meter cavity is in resonance
with the microwave and power is taken away from the main wave-guide.
Use the meter to determine the frequency directly.

Standing Wave Ratio (S. W. R.) Measurements

You should
now measure the S.W.R. for the waveguide terminated with various load.(See
appendix 2 for an explanation of S.W.R.) When the probe carriage is
moved along the slotted line maxima and minima voltage readings will
be observed on the meter. If the sensitivity of the amplifier is adjusted
so that maximum reading is 1.0 on the S.W.R. scale the corresponding
reading for the minima will directly give the value of the S.W.R.

For the case of the matched load S.W.R. ~ 1 indicating a good load
match with
very little power reflected. However if the matched load is removed
you will observe that a standing wave with very pronounced minima is
set up in the waveguide indicating a strong reflected wave.
Now connect a horn to the end of waveguide and
you should see that the match is greatly improved, through
some power is reflected back giving and S.W.R. > 1. Microwave horns are used as antennae, usually placed at the focus
of a microwave reflector dish.
For the short-circuit load (metal plunger shorts
the electric field of the E.M. wave) the S.W.R. ~ , since it acts as an almost perfect reflector.

Free Space and Waveguide Wavelength measurements

Section 1:

Connect the short circuit load to the end of waveguide
to determine the free space wavelength () and the waveguide wavelength () for a range of microwave frequencies 8500-9500
MHz.
The frequency
of the Gunn diode is adjusted by altering the size of its tuned cavity
using the microwave screw. N.B: searching for the resonant frequency using the frequency meter can be tedious
and if done too rapidly can be missed. Having measured an oscillation frequency rotate
the micrometer only � turn
Maximize the meter signal by adjusting probe collar,
and retune the frequency meter so as to measure the new frequency
Using the slotted
line measure the difference between the node minima positions of the
probe carriage. Near each position of minimum the sensitivity of the
amplifier should be increased to increase location accuracy. However, after location
the sensitivity of the amplifier should be reduced considerably so as
to prevent overloading when the probe is moved to an anti-node (maximum). The
distance between node is .
The free space wavelength () can, of course, be
calculated from the measured frequency (f) using the relation , where c is the free space velocity of E.M. waves.
Plot a graph
of against f. Waveguide theory gives
the relation , where a is the broad dimension of the waveguide and so you should be able
to check, whether this is correct from your results. The actual value of a should be measured using calipers.

Dielectric Constant Measurements

Section 2:

Download

Electricity-Tool

www.physics.hku.hk/~phys3431/322_Microwave_Measurement_2002.doc