EE 442 Microwave Engineering Laboratory,
Dept. of EEE, IIT Guwahati
Experiment 2
Low, medium and high VSWR measurements
OBJECTIVES
To determine the low, medium and high voltage standing-wave ratio (VSWR)
EQUIPMENTS
Klystron tube, Klystron power supply, Isolator, Frequency meter, Variable attenuator, Klystron mount,
Waveguide stand, SWR meter, Slotted line, Tunable probe, Movable short/termination or any unknown
load and BNC cables, S-S Tuner and accessories.
THEORY
The electromagnetic field at any point of transmission line or waveguide may be considered as the sum of
two traveling waves: the ‘incident wave’ propagates from generator and the ‘reflected wave’ propagates
towards the generator. The reflected wave is set up by reflection of incident wave from a discontinuity on
the line or from the load impedance. The magnitude and phase of reflected wave depends upon
amplitude and phase of the reflecting impedance. The presence of two traveling waves gives rise to
standing wave along the line. The maximum field strength is found where two waves are in phase and
minimum where they are in opposite phase. The distance between two successive minimum (or
maximum) is half the guided wave-length on the line. The ratio of electrical field strength of reflected and
incident wave is called reflection coefficient. The VSWR is defined as ratio between maximum and
minimum field strength along the line. Hence, VSWR is
s=
EI + ER
E max
=
E min
EI − ER
where E I and E R are respectively the amplitudes of the incident and reflected electric field strengths.
Standing wave ratio is frequently expressed in decibels, VSWR (dB) = 20 log 10 (s).
Further, the ratio of the reflected to the incident electric field intensities is defined as reflection coefficient,
i.e.,
ΓR =
ER Z L − Z0
=
EI Z L + Z 0
EE 442 Microwave Engineering Laboratory,
Dept. of EEE, IIT Guwahati
where Z L is the load impedance and Z 0 is the characteristic impedance. Hence, VSWR can be expressed
in terms of reflection coefficient as follows:
s=
1 + ΓR
1 − ΓR
When E max = E min , i.e., there is no reflection [ E R = 0 ], the resulting VSWR=1.00, this is the requirement
of a ‘matched circuit’. In other words, under given ideal conditions, the VSWR of a matched load is 1.00.
Under extreme mismatch conditions, E min = 0, i.e., total reflection, [ E R = E I ], the resulting VSWR =∞.
However, in most of the cases, VSWR is greater than 1.00. It is not rare to obtain VSWR as low as 1.02 in
well-designed set-up having matched components.
PROCEDURE
1.
Equipments are set up as shown in Fig. 1.1.
2.
Variable attenuator is kept at maximum position.
3.
Control knobs of VSWR meter are kept as below:
4.
Range dB
-
40 dB/50 dB
Input switch
-
Impedance low
Meter switch
-
Normal
Gain (Coarse-Fine)
-
Mid position approx.
The control knobs of Klystron power supply are kept as below:
Meter switch
-
‘OFF’
Mode switch
-
‘AM’
Beam voltage knob
-
Fully anticlockwise
Reflector voltage knob AM frequency & amplitude knob
Fully clockwise
Mid position
5.
Klystron power supply, VSWR meter and cooling fan are switched ON.
6.
Meter switch of Klystron power supply is switched to Beam voltage position and beam voltage is
set at 300 V.
7.
Reflector voltage knob is rotated to get deflection in the VSWR meter.
8.
Output is tuned by tuning the reflector voltage, amplitude and frequency of AM modulation.
9.
The plunger of Klystron mount is tuned for maximum deflection. The probe of the slotted
waveguide can also be tuned for maximum deflection.
10. The dB range switch, gain control knob of the VSWR meter can also be adjusted along with the
variable attenuator of the set up to get deflection in the VSWR meter if found necessary.
11. If the probe is moved along the slotted line, the deflection will be changing.
EE 442 Microwave Engineering Laboratory,
Dept. of EEE, IIT Guwahati
A.
MEASUREMENT OF LOW AND MEDIUM VSWR (VSWR<10)
1.
The probe is moved along the slotted line for maximum deflection in the VSWR meter.
2.
The gain control knob of the VSWR meter or the variable attenuator is adjusted until the
meter indicates 1.00 on normal SWR scale (0-∞).
3.
Keeping all control knobs as it is, the probe is adjusted for minimum position on the SWR
reading. The VSWR meter reading is recorded (SWR meter reading at the minimum,
when the maximum is set for 1.00, gives directly the VSWR). Note that SWR NORMAL
scale is for 1.00≤s≤4.00 and EXPANDED scale is for 1.00≤s≤1.33.
4.
The above step is repeated by moving the probe along the slotted line in the same
direction for more number of minimum position and the corresponding VSWR readings
are recorded.
5.
If the VSWR is between 3.20 and 10.00, the range dB switch is changed to next higher
position and the VSWR reading is taken by the VSWR scale of 3.00 to 10.00 (second SWR
NORMAL scale).
6.
This way the Table 2.1 is completed for the unknown load at different
frequencies by repeating the above steps 1 to 5.
Table 2.1 Low and medium VSWR measurements
Sl. No.
VSWR (dB)
Frequency (GHz)
1.
2.
3.
4.
5.
.
.
B.
MEASUREMENT OF HIGH VSWR (VSWR>10)
(Double minima method)
1.
Setup is prepared same as in the first case and steps 1 and 2 repeated.
2.
The output is set such that the SWR meter reads 3.00 dB.
3.
Move the probe carriage by a distance less than
λg
4
towards generator and record its
position (x1) in Table 2.2 where meter reading is double of the minimum reading set in
EE 442 Microwave Engineering Laboratory,
Dept. of EEE, IIT Guwahati
step 2 (if SWR meter is used, these positions shall correspond to the meter reading 0.00
dB on either side).
4.
The probe carriage towards load (<
λg
2
) to a position when meter reading is again
doubled. Record the position in table 2.2, say x2 (if SWR meter is used, these positions
shall correspond to the meter reading 0.00 dB on either side).
5.
 λg 
 is found

 2 
By moving the probe between two successive minima a distance equal to 
( )
which will give us the guided wavelength λ g .
6.
Compute VSWR using the equation below:
VSWR =
E max 1 λ g
= =
E min θ π∆x
At various frequencies by repeating steps 2 to 4 and plot VSWR versus frequency.
Table 2.2 High VSWR measurements
Wavelength
Sl. No.
1.
2.
3.
4.
5.
.
.
Group No.:
Roll No.:
Name:
(cm)
Minimum
Probe position for double
Double
VSWR
meter reading
meter reading at minimum
minimum
(3dB in SWR)
(0.00dB in SWR)
width
 λg 


 π∆x 


Left-hand
Right-hand
side x1
side x2
∆x = x2 − x1