Standing Wave Ratio (SWR) Measurement.
Summary: In this experiment you will measure three different values of SWR; very low, medium, and
high, using the LabVolt microwave set. You will do that by measuring, in each case, the maximum and
minimum voltage in a slotted line. A slotted line is a microwave transmission line - a waveguide - with a
slot in the top through which a probe is lowered to measure the voltage distribution. When voltages are
expressed in dB, the difference between the maximum and minimum level is the SWR expressed in dB, or
20 Log(SWR ratio).
Procedure:
1) Assemble the equipment as in the
figure:
2) Set the power supply voltage to 9
volts, with the usual 1 kHz mode.
3) Set the SWR meter to the -30 dB
range, with full gain (fully
clockwise), BW = 20Hz.
4) The three different SWR ratio will be obtained by setting three different values for the variable
attenuation: 0 dB (no att.), 3 dB, and 20 dB.
5) In each case you will do the following:
a) Move the slot line probe to get the maximum reading of meter, with probe as close to the variable
attenuator as possible.
b) Adjust frequency knob on meter (tune meter input filter) for maximum reading.
c) Use the gain control so that the needle is at 0 dB (SWR=1).
d) Move the probe away from var. att. till it reads minimum. Record value in dB on a dB scale and a
direct SWR reading on an SWR scale (see notes below).
e) Subtract the min. dB value from the reference value (reading at maximum), to get the SWR value in
dB. Compare to 20 Log (SWR ratio) to make sure your readings are correct.
6) In each experiment, use the calibration curve of the variable attenuate to obtain the proper value for
blade position such that you get the required attenuation from the variable attenuator.
7) Notes about reading the SWR meter:
a) Use normal scales to read SWR ratios greater than 1.3:1, and use the expanded scales to have a
more accurate readings for low SWR values (between 1:1 and 1.3:1).
b) When you have to change gain range (one of the seven buttons) in order to read a minimum, for
example from -30 dB range to -40 dB range, you will need to switch from the 1-4 range to the 3.210 range if you are reading SWR directly on the top scale. If the min. voltage is very low (case when
the variable attenuator is removed, and the short circuit is applied directly to the output of the slotted
line), and you have to switch to the -50 dB range, then you read the SWR on the top scale again (the
1 - 4 range, and multiply the reading by 10 (corresponds to the 20 dB range change)
c) Examples of readings (all assume maximum voltage corresponds to 0 dB, while range is -30 dB):
- Min voltage possible while still on the -30 dB range. Reading (top scale):2.5. Actual 2.5
- Min voltage possible with -30 DB range, reading = 1.03. Switch to expanded scale.
- Min voltage not possible with -30 dB range, but possible with -40 dB. Use second (from top) SWR
scale (range 3.2 - 10)
- Min voltage too low, requires switching to -50 dB range. Read on top (1-3.2) scale. If, for
example, you read 2.6, the actual value = 10*2.6 = 26.
8) Notes on the effect of attenuation on SWR:
Recall: SWR = (1+Γ)/(1-Γ), but Γ is related to the line attenuation: If the attenuation changes by
A (Nepers), as due to the insertion of the variable attenuator), then
Γ2 = Γ1 e-2A (also review lecture notes)