# Antenna and microwave theory: Lab Manual

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Lab 1
Phase Shift Oscillator

Objective: To demonstrate the design and operation of a
phase shift oscillator.

Theory:
By providing the three RC networks having a total phase shift of 180˚ as
positive feedback to the input of an inverting amplifier, oscillation results.
The total phase shift of the amplifier and that of the phase-shift network is
0˚ and the loop gain is unity. The sinusoidal output of the oscillator has a
peak to peak voltage is equal to the difference between the op-amp
positive and negative saturation voltages.

Components Required:
 RESISTOR (¼ W)
 Three 1KΏ
 27KΏ
 5-KΏ potentiometer
   Three 0.1µ F capacitor
   741 op-amp (8 pin mini=DIP)
   Two 0-15 V dc power supplies
   Dual trace oscilloscope

Useful Formulae:
Output Frequency:
1)         fO =1/(2πRC√6)

For Oscillation:
2)           Rf/R = 29
Procedure:
1) Wire the circuit shown in the schematic diagram of the given figure
and set your oscilloscope to the following approximate settings :
Channel 1: 5V/Division, ac coupling
Time base: 0.5 ms/Division.
2) After you have checked all connections apply the +- 15 V power
supply connections to the board .
3) Depending on the setting of the 5-KΏ potentiometer the circuit may
or may not be oscillating when power is supplied. If a sine wave is not
displayed on the oscilloscope, carefully adjust the 5-KΏ
potentiometer until a sine wave starts to appear on the
oscilloscope’s display. If you continue to increase the resistance of
the potentiometer, you should observe that the peaks of sine waves
have become clipped and that the output frequency becomes lower.
Adjust the potentiometer to the point at which the circuit sustains
the oscillation.
On the other hand, if sine wave is seen when the power is applied on
the breadboard, carefully decrease the resistance of the
potentiometer to obtain the best looking sine wave.

4) Using your oscilloscope’s time base set at approximately 0.2
ms/Division. Measuring the output frequency of the phase shift
oscillator, recording your result in the given table. Compare this
value with expected frequency found using Equation 1 given in the
formulas section of this experiment.

5) Disconnect the power from the breadboard and carefully remove
and measure the total resistance (Rf) of the 27-KΏ resistor and of the
setting of the series 5-KΏ potentiometer that produced oscillation.
Record the value in the table. At the oscillation frequency set by
three RC networks, 1/29 of the output signal is fed back to the input
of the op-amp. For the loop gain to be unity, the voltage gain of the
inverting amplifier must then be 29, which implies that the feedback
resistor Rf must be equal to 29R. How does the sum of 27-KΏ resistor
and setting of the 5-KΏ potentiometer compare with the 1-KΏ
resistor of the phase shift network?
Observation:

PARAMETER            MEASURED             EXPECTED            % ERROR
Output frequency.
f◦
Rf                                    29-KΏ

Analysis:

This experiment demonstrates the design and operation of a phase shift
oscillator using a 741 operational amplifier. This type of oscillator uses
three RC networks having a total phase shift of 180˚ at a specific frequency
as positive feedback to the input of an inverting amplifier. When the loop
gain is 1 by making Rf = 29R, the circuit then oscillates.

Results:
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Dated: __________________                Checked By: __________________
Lab 2
Active Band-Pass Filters
Objectives: To design the circuit of multiple feedback active
band-pass filters and to observe its operation and
characteristics.
Theory:
Band-pass filters pass all input signal frequencies within a given range,
called the bandwidth, while rejecting all frequencies outside this range.
The bandwidth encloses a single frequency at which the output voltage
is maximum, called the center frequency.

The multiple feedback band-pass filters are only one of a number of
band-pass filter circuits which unlike the “twin-T” band-pass enable one
to specify individually the center frequency (fo), gain (Av) and quality
factor (Q). Because of its simplicity, it is limited for Qs less than 10.

Components Required:
 Resistors (1/4 W): 1.5 k, 2.7 k, 68 k, 180 k

 Two 0.01F capacitors.

 741 op-amp (8-pin mini-DIP)

 Two 0-15 V DC supplies

 Dual trace oscilloscope

Useful Formulae:
Center frequency,

[1]

Where,

Center Frequency Voltage gain,

[2]

Where, Ao must be less than 2Q2.

Shifting the center frequency with constant center frequency gain and
bandwidth,

[3]

Center frequency from upper and lower 3-db frequencies,

[4]

Quality factor,

[5]
Procedure:
1. Wire the circuit as shown in the schematic diagram of fig. 1 and set the oscilloscope
for the following approximate settings:

    Channels 1 and 2: 0.2 V/DIV, AC coupling

    Time base: 0.2 ms/DIV

2. Apply power to the breadboard and adjust the output of signal generator at 1V
peak-to-peak at the frequency of 1 kHz.

3. Now vary the signal generator’s frequency to the point at which the output voltage
of the filter, as displayed on channel 2 of the oscilloscope, reaches its maximum
peak-to-peak amplitude. Measure this peak-to-peak output voltage, and then
determine the center frequency voltage gain Vout/Vin, recording your data in table 1.
How does measured voltage gain is compared to expected value (eq. 2).

The measured center frequency voltage gain, which is based on resistors R1 and R3,
should be about 1.32. If your value is 10% or more off from this value, either you are
not at the filter’s center frequency, as evidenced by a maximum output voltage, or
the resistors you are using are significantly different from their rated values. Such is
the case because the input signal will be inverted from, or 180o out of phase at this
center frequency. Such is the case because the input signal is eventually connected
to the op-amp’s inverting input so the signal will be in inverted form, or 180 o out of
phase with, the input signal.
4. Using your oscilloscope, determine the filter’s output frequency without disturbing
the frequency setting of the signal generator, recording your result in Table 1. How
this value is compared with the expected value (Eq. 1)?

The band-pass filter’s center frequency is based on the value of both capacitors and
all three resistors, and it should be near 737 Hz

5. Now determine the filter’s bandwidth by measuring both the upper and lower 3-dB
frequencies at which the peak-to-peak output voltage drops to 0.707 times the value
at the center frequency. To do this easily, you should set the signal generator first to
the filter’s center frequency. Then, without changing the output frequency, adjust
the signal generator’s output voltage so that the output voltage of the filter is 1.0 V.
Make this setting as accurate as possible.

Then decrease the signal generator’s frequency and stop at the point at which the
output voltage drops to 0.71 V peak-to-peak (1.0 V x 0.707 = 0.71). Determine the
frequency at this point, called the lower 3-dB frequency (fL), and record your result
in Table. 2.

6. Continue to decrease the input frequency. Does the output voltage increase or
decrease?

Notice that the peak-to-peak output voltage of the band-pass filter decreases as the
input frequency moves away from the filter’s center frequency.

7. Now increase the signal generator’s frequency beyond the center frequency and
stop at the point at which the filter’s peak-to-peak ouput voltage is again 0.71 V.
Determine the frequency at this point, called the upper 3-dB frequency (fU) and
record this result in Table. 2.

8. Subtract the lower 3dB frequency from the upper 3dB frequency, obtaining the 3dB
bandwidth of the filter. Record your result in Table 2. Using this bandwidth and
center frequency experimentally found in Step 4, calculate the filter’s Q or quality
factor, and record your result in Table 2.
Within 10%, you should determine the filter Q. If not repeat Step 3 through 7
carefully measuring the voltages and frequencies.

9. From the two measured 3dB frequencies, you can determine the filter’s center
frequency by taking geometric average.

How does your result obtained from this equation compare with the value you
determined in Step 4?

10. Disconnect both power and the signal leads from the breadboard and replace the
2.7 k resistor (R2) with the 1.5 k. Connect the power and signal generator to the

11. Repeat steps 3 through 9 to determine filter’s center frequency, voltage gain (A v),
center frequency (fo) bandwidth, and Q. Record your results in Table 3.

12. When the value of R2 is changed, how does the new center frequency that you
determined in Table 3 compare with the expected value obtained from Eq. 3 in the
“Useful Formulas” section of this experiment?

If you have performed this experiment correctly, you should find that when resistor
R2 is changed, the bandwidth and the center frequency gain remain the same, while
the filter’s center frequency is inversely proportional to the value of R 2. For example,
if R2 changes from 2.7 k to 1.5 k, the new center frequency should be: 988 Hz

Since the center frequency changes, Q also changes.

13. Set the input voltage to the filter at 1 V peak-to-peak and vary the signal generator’s
frequency according to Table 4. Then plot dB gain response for all measured
frequencies. From this graph you should be able to estimate the filter’s center
frequency, bandwidth and Q. Compare these values with Table 3.
Observations:

Analysis:
This experiment demonstrated the operation and characteristics of a
multiple feedback band-pass active filter. This filter passed all signals
within given range about the filter’s center frequency while rejecting
those frequencies outside this range. In this experiment, the following
parameters were measured: center frequency gain, center frequency,
bandwidth and Q. In addition, it was shown how to vary filter’s center
frequency with single resistor while keeping the bandwidth and center
frequency gain constant. The filter’s frequency was graphed from
measured data.

Results:
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Dated: __________________            Checked By: __________________
Lab 3
555 Timer
Objectives: To design the circuit and observe 555 timer as an
astable multivibrator.
Theory:
The 555 timer is an IC device that allows the formation of an astable multivibrator whose
output frequency and percent duty cycle can be controlled by only two resistors and a single
capacitor. As a practical matter, the output frequency should be kept less than 200 kHz, while
the duty cycle can range from approximately 50% to 99%.

Components Required:
    Resistors (1/4 W): 1 k, 3.3 k, 15 k

    Capacitors: 0.01F, 1F

    555 timer (8-pin mini-DIP)

    0-15 V DC supply

    Dual trace oscilloscope

Useful Formulae:
Center frequency,

[1]

Percent duty cycle,

[2]
Procedure:

1. Wire the circuit as shown in the schematic diagram of fig. 1 using a 5 V
supply. Set the oscilloscope for the following approximate settings:

 Channels 1: 22 V/DIV, AC coupling

 Time base: 0.1 ms/DIV

2. Apply power to the breadboard. You should see a waveform that
switches back and forth between ground and the +5 V supply voltage,
similar to that shown in fig. 3. Measure the output frequency and
compare it to the value that you would expect based on the values of R1,
R2 and C (Eq. 1). Record your results in Table 1.

3. Determine the percent duty cycle of the output waveform of the 555
timer astable multivibrator by taking the ratio of time that the
waveform is at the positive supply voltage to the total time for one
cycle. Then multiply this result by 100%. Compare your result with the
expected value (Eq. 2) and record both in Table 1.

4. Disconnect the power from the power from the breadboard and reverse
the 3.3 k and 15 ktiming resistors so that the 15 kresistor is now
R1. Again connect the power to the breadboard and compare the
measured output frequency with the expected value (Eq. 1) and record
both in Table 1.

5. As in Step 3, measure the percent duty cycle, comparing it to the
expected value (Eq.2) and record your results in Table 1.

Observe that if resistor R2 is much greater than R1, the percent duty
cycle will approach 50%. On the other hand, if R1 is much larger that R2,
the percent duty cycle approaches 99%. However, note that when either
R1 or R2 is changed to adjust the duty cycle, the output frequency of the
555 timer also changes. Consequently, the output frequency and the
duty cycle, once set cannot be adjusted independently.
Observations:

Analysis:
This experiment demonstrated the operation of the 555 timer as
an astable multivibrator and determined what components
controlled its output frequency and duty cycle.

Results:
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Dated: __________________              Checked By: _________________
Lab 4
Introduction to the Microwave VCO source, Detector
and action of a 3-Port Circulator
Objectives:
 To be able to use the VCO oscillator and set its frequency to a
given value within its tuning voltage,
 To be able to use the crystal detector for the measurement of
microwave power,
 To know the basic properties of a circulator and its
applications in microwave systems.
Components Required:
   Voltage Controlled Oscillator
   3 port Circulator
   Crystal Detector
   50W Coaxial Termination
   2 SMA plugs to plug Coaxial Connectors
   Coaxial Short Circuit Termination
   Power Supply for VCO source
   Digital voltmeter for diode detector
Theory:
Introduction:
The microwave generator supplied in the MS3000 and used as the microwave source is
voltage tunable allowing the oscillator frequency to be set to any desired value within its
range. It incorporates a VCO (voltage-controlled oscillator).
The approximate specification of the voltage-controlled oscillator, the VCO, is as follows:
 Frequency range: 2.4 to 3.7GHz
 Power output into 50W: 10 mW, minimum
 Power variation: ±1.5dB, maximum
Tuning voltage limits:
 Low frequency (2.05GHz), 2V approx.
 High frequency (3.55GHz), 20V approx.
Fig. 1 External Connections to VCO Microwave Source

Fig. 1 gives the external connections to the VCO unit. Three 2mm sockets mounted on the side
casing are available for the connection of the DC supply and the VCO tuning voltages. The
connections are:
 Pin 1: Tuning voltage +2.0 V to 20 V
 Pin 3: Ground
 Pin 5: Power Supply +15V
The output from this oscillator unit may be either constant wave (CW) or switched-keyed (on-
off type) modulation at 1 kHz.
A light-emitting diode, LED, indicator is used to indicate in which mode (CW or modulated) the
oscillator is operating. In the CW mode the LED indicator remains on. When switched to 1kHz
modulation the LED flashes at a rate of approximately once every 2 seconds.
The diode crystal detector, see Fig. 2, is used in the MS3000 Microstrip Trainer to detect (rectify)
microwave signals and measure microwave power. The crystal detector is designed to effect an
excellent match to 50-ohm lines and for CW inputs produces a DC voltage output which may be
accurately measured by a digital voltmeter and converted to power using the calibration curves
provided. The detector sensitivity is better than 0.5mV per microwatt at low power levels and is used to
measure power levels over a wide dynamic range, typically 1 W to 30mW plus.

Fig. 2, the crystal detector used in the MS-3000 Microstrip Trainer to measure microwave power
A circulator is an important microwave component and is extensively used in microwave systems. It
depends on its operation the non-reciprocal properties of the ferrites non- conducting magnetic
materials with high permeability and permittivity. Fig 3shows a simplified diagram of a 3- port circulator.
The ferrite, placed at the center of the junction produces non-reciprocal effects on the transmission of
energy between junctions when correctly magnetized. Magnetization is usually produced by a
permanent magnet, not shown in the diagram; however, for switching applications current carrying
magnetization coils are also used. The effect of the magnetized ferrite on the transmission is as follows:
Microwave energy entering at Port 1 leaves at Port 2 with ideally zero energy reaching Port 3. Energy
entering at Port 2 leaves at Port 3 and energy at Port 3 emerges at Port 1.

Fig 3 Action of a 3-port circulator

The transmission loss between coupled parts 1 and 2 can be calculated as:

   Transmission loss 1 to 2 = 10log 10(P2/P1)>=1dB
   The isolation at the decoupled parts 1 and 3 =10log 10(P3/P1)<= -20 dB

Fig 4 Simplified diagram of test system
Procedure:
PART- I Initial setting-up and measurement of the transmission power
1) Connect up the microwave source (VCO), circulator (CIR) and crystal detector (D) as
shown in Fig 4. The VCO and circulator at its port 1 are interconnected using an SMA
plug-to-plug connector, 8 of which are supplied. The crystal detector input is an SMA
plug connector and mates directly with the circulator at its output port 2. These
connectors can be easily tightened by hand and finally by the spanner. Do not over-
tighten. Terminate port 3 of the circulator in a 50 ohm coaxial termination
(distinguished by a red spot on its outer casing)

2) The DC power supply (unit AX322C) has a dual 0 to 30 volt output. Using one set of
these terminals and the leads provided connect the positive (+) terminal of the supply to
the red 2mm socket terminal of the VCO unit, connect negative (-) terminal of the VCO.
Strap the negative terminals of the dual supply together. Connect the positive terminal of
the second pair of the power supply to the white terminal, the tuning voltage terminal, of
the VCO.

3) Connect the crystal detector output by means of the coaxial cable provided to the digital
voltmeter (unit MX545). The BNC female connector on the cable connects directly to the
BNC jack output of the detector. At the voltmeter end connect the black lead to the
terminal marked COM and the red lead to the terminal marked V ohm. Set the rotary
switch to DC volts, the position denoted by V …
When switched on, the voltmeter is in the auto-raging mode and this should not be
changed. The DC output voltage from the crystal detector is displayed directly. The display
also shows measurement units millivolts (mV) or volts (V) and the polarity, in this case
negative, since the detector produces a negative output.

4) If not already done so, switch on the digital voltmeter and switch on the DC power
supply. Set the VCO supply voltage to +15 volts. Set the VCO tuning voltage to 10 volts
or so. The digital voltmeter will display the crystal detector voltage corresponding to the
power output at port 2 of the circulator. Ensure the VCO is in its CW mode, i.e. LED
indicator is on continuously.

5) Now complete the following.
Using the tuning voltage – frequency data supplied for the VCO set the oscillator frequency
at 2.5GHz. Measure the transmission power P2 at port 2, i.e. record the digital voltmeter
reading and use the detector voltage-power calibration curves to convert the reading to
microwave power.
Use this procedure to obtain similar measurements for the frequencies given in Table 1.1
Record the results in Table 1
PART-II       Measurement of power transmitted to decoupled/isolated Port3
Interchange the crystal detector D and 50 ohm coaxial termination as shown in Fig 1.6 and
using the VCO voltage settings found in Part-I, measure the detector voltage and hence the
power P3 transmitted to port3 at the five frequencies, 2.5, 2.75, 3.0, 3.25 and 3.5GHz. Record
the results in Table 2. Calculate the ratios P 3/P2 and 10logP3/P2, where P2 is the power
transmitted to port2, measured in Part-I

Figure 5 Set-up for measuring power P3 at decoupled port3.

PART-III      Measurement at port 3 reflected at port2
Replace the 50-ohm coaxial termination at port2 with the coaxial short circuit termination
(distinguished by a white spot on its casing). Make measurements of P 3 at the five frequencies
recording the results in Table 3. Calculate the ratio P 3/P2

Fig 6 set-up with short-circuit at port2
PART IV       Measurement of transmission power P1 with circulator reversed
Finally set up the components as shown in Figure 1.8 with the circulator reversed so that port2
is now connected to the VCO output and port 1 to the detector. Terminate port3 in the 50- ohm
coaxial termination. Measure P1 and record the results in Table 4.
Calculate the ratios P1/P2 and 10log 10 P1/P2 is the reference power measured in Part-I.

Figure 7 Set-up with circulator reversed in direction

Observations:

Table 1
Table 2

Table 3

Table 4
Table 5

The results obtained in Parts I-IV and the calculations performed enable a summary of the basic
characteristics of the 3-port circulator to be made and its performance specified over the range
.5 to 3.5GHz.Use Table 1.5 to record its performance.
Comment on the results recorded in the first and third columns as regards the
directional/isolation properties of the circulator. Explain also the significance of the results in
the second column.
Analysis:
A basic microwave test system has been set up and the non-reciprocal transmission properties
of a 3-port ferrite circulator have been investigated.
It is observed that with power, incident at a given port, the circulator directs the power with
low loss in a given direction but not in the reverse sense, e.g. with power incident at port 1 low
loss transmission occurs to port 2 but little power reaches port 3 when both these ports are
terminated in matched (50 ohm) impedances; if power were to be reflected at port 2 it will be
directed to port 3.
An important application of the circulator is as an isolator –a one-way transmission device
which presents low-loss transmission in one direction but high loss (isolation) in the reverse. In
microwave measurements it is standard practice to use an isolator to protect the source. Any
reflection produced in the system will be effectively absorbed in the isolator (see Fig 1.4) thus
preventing these affecting the source output. The application of the 3-port circulator as an
isolator for the VCO microwave source is used in the next experiment for this reason.
Results:
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Dated: __________________   Checked By: __________________
Lab 5
Measurement of Insertion Loss: Characteristics of A Low
Pass Filter (LPF) and Band Pass Filter (BPF)
Objectives:
 To introduce the basic means of measuring the insertion loss of a
microwave component,
 To measure the insertion loss of a micro-strip low pass-filter.

Components Required:
   Voltage Controlled Oscillator
   3 port Circulator
   Crystal Detector
   Low Pass Filter
   Band Pass Filter
   2 SMA plugs to plug Coaxial Connectors
   Power Supply for VCO source
   Digital voltmeter for diode detector
Theory:
Introduction:
Filters are frequency selective networks designed so as to pass certain bands of frequencies and reject
others. Filters are normally classified as low-pass, band-pass, high–pass or band stop according to the
bands they pass with low loss or, in the case of the band stop filter, the band they reject. Typical insertion
loss characteristics for the four classes are sketched in Fig. 1.
Fig. 1
Low-pass, band-pass and band stop filters find important applications in signal selection in microwave
systems and may be fabricated in micro-strip. In this experiment the insertion loss characteristic of a
low pass filter designed to attenuate frequencies in the upper part of S-band whilst providing low
loss up to 2.7 GHz will be investigated.

The filter, LPF, and its equivalent circuit is shown in Fig. 2. The filter is the micro-strip equivalent of a 5-
element L-C ladder network. The L and C values are designed to produce the required cut-off
frequency, the frequency determining the limit of the pass band range, and the rate at which the loss
rises in the attenuation band.

Fig. 2
The series inductance L elements are synthesized using short lengths of high impedance line. Such
lengths behave predominantly inductively when terminated in low impedance lines as indicated in Fig.
4.3.(a). The characteristic impedance of a micro-strip line is a function of its width w; the smaller
w the higher the impedance. The shunt capacitance C elements are synthesized again by short
lengths but of relatively large w to give the line low characteristic impedance. Short lengths of low
impedance line terminated in high impedance lines simulate closely a shunt capacitance as indicated in
Fig. 3. (b). In the complete filter, see Fig. 4.3.(c), corrections must be made for the abrupt junction
discontinuities between adjacent elements and the effects of the terminations on the L and C elements.
In practice, micro-strip filters are designed using computer aided design packages which accurately
model all these effects and also contain facilities for simulated testing and optimization. In this way the L
and C line lengths are respective widths can be obtained with a good degree of confidence that they
satisfy the filter specification before fabrication is commenced.
Fig. 3
Fig. 4 Circuits for measurement of Insertion Loss of Low Pass Filter

The measurement of insertion loss of a component at a given frequency involves two
measurements: the determination of the power P1 in the direct connection with the microwave source
directly connected to a matched power sensor and the determination of the power the component
output with the device inserted between the source and power sensor:
Procedure:

The procedure described below measures first P1 over the band and then with
the filter inserted P2 over the same band, thus avoiding the rather tedious need to
break the measurement set up at each point-to-point frequency.
1) Set up the equipment as shown in the diagram 4. (a) with the filter out
of circuit, i.e. in the direct connection. Note the circulator with the 50 Ω
terminations at port 3 acts as an isolator for the microwave source. It
provides low loss transmission from port 1 to port 2 and any reflected
power from the test system is absorbed in the 50 Ω terminations at port 3.
1) Connect the power supply to the VCO microwave source and
using the frequency-tuning voltage calibration data set the source
frequency at 2.2 GHz.
2) Measure the crystal detector voltage and determine the
corresponding direct power the detector voltage-power calibration
curves.
3) Repeat at 50 MHz intervals up to 3.5 GHz, i.e. at 2.5, 2.55 ... 3.6, 3.65
GHz to cover the band.
2) Now insert the low pass filter LPF in circuit as shown in the diagram of Fig.4.
(b). Measure the power P2 from 2.45 to 3.65GHz in 50 MHz steps
3) Repeat the steps for Band Pass Filter. (Replace LPF with BPF).

Observations:
Using the results obtained in the direct connection plot a curve of the power
over the range 2.45 to 3.65 GHz.
At each of the set frequencies calculate the insertion loss ratio in dB:

Plot the insertion loss characteristic of the low pass filter, i.e. insertion loss
magnitude in dB versus frequency. From this curve, determine the 3dB critical
frequency, i.e. the frequency at which the insertion loss magnitude equals 3dB.
Determine also the gradient of the characteristic over the range 3.0 to 3.6
GHz.
IL
f     Tuning                                       ILR
Direct        Device Inserted           10log(P2/P1)
(GHz)   Voltage                                      P2/P1
V1      P1        V2       P2                (dB)
(V)
(mV)    (mW)      (mV)     (mW)
2.45
2.50
2.55
2.60
2.65
2.70
2.75
2.80
2.85
2.90
2.95
3.00
3.05
3.10
3.15
3.20
3.25
3.30
3.35
3.40
3.45
3.50
3.55
3.60
3.65
Analysis:
A simple 50 Ω t e s t system comprising a VCO microwave source isolator and crystal
detector acting as a power meter has been set up          and     the       insertion      loss
characteristic of a low pass micro-strip filter has been measured. The filter investigated was
the micro-strip equivalent of an L-C ladder network with the L and C elements synthesized
by short lengths of high impedance (relatively broad width) line.
Results:
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Dated: __________________                      Checked By: _________________
Lab 6
Properties of A Directional Coupler
Objectives:
 Know the definitions of and to have measured the coupling,
directivity, isolation and insertion loss of a directional coupler.
 Have Measured the coupling co-efficient and directivity of the micro-
strip directional coupler
 Appreciate the bandwidth limits over which the
micro-strip directional coupler operates
 To know applications of the directional coupler in
microwave measurements and systems.
Components Required:
   Voltage Controlled Oscillator
   3 port Circulator
   Crystal Detector
   50W Coaxial Termination
   2 SMA plugs to plug Coaxial Connectors
   Coaxial Short Circuit Termination
   Power Supply for VCO source
   Digital voltmeter for diode detector
Theory:
Introduction:
A directional coupler is a device which allows a wave traveling in one direction along a
transmission path to feed part of its energy to a secondary output port, but (ideally) prevents a
wave traveling in the opposite direction to feed energy to the secondary port. Such a coupler may
be used for various purposes, but probably the most important is that of separating incident from
reflected waves in a transmission line.

Microstrip Directional Coupler:

Fig. 1 Directional Coupler, unit DC

The directional coupler to be investigated, unit DC shown in Fig 1, is an edge coupled microstrip
coupler designed for S-band operation centered on 3GHz and to work in the standard 50Ω system.
Edge coupled types are suitable for relatively weak coupling applications, typically in the coupling
coefficient range 33 (15dB) to 1000 (30dB)

The important design parameters are coupling length L and the separation s between the coupled
lines. At the mid-band design frequency L should be approximately one quarter of a guide
wavelength, L=λg / 4 ( λg=λ0 /ε r,eff). The separation s determines the coupling coefficient. Smaller the
value of s, tighter the coupling.

(a) Forward Transmission                            (b) Reverse Transmission

Fig. 2 Transmission Properties of a Directional Coupler

A directional coupler consists essentially of a pair of coupled transmission lines designed so as a
specific fraction of the power flowing in one line in a given direction is coupled to the other line; to
propagate only in one direction but not in the other. The directional coupling properties and
definitions of coupling coefficient and directivity may be explained with reference to Fig. 2. In (a)
microwave power incident at port 1 is transmitted to port 2 with a certain fraction coupled to the
second line to emerge at port 3. Ideally zero power emerges at port 4. The coupling is directional.

The coupling coefficient of the directional coupler is defined as:

Coupling, C=P1 / P3 or 10log10 (P1 / P3) dB

Assuming all the ports are matched,

In the reverse direction, see (b), power incident at port 2 is transmitted to port 1 with
directional coupling this time to port 4. Ideally no power is coupled to port 3.

As a measure of the directional coupling properties, the term directivity is used. It is defined as the
ratio of the power at the coupled port to the power at decoupled port

Directivity, D= P3 / P4 or 10log10 (P3 / P4) dB in Fig 2 (a)

Good quality directional couplers have directivities ranging from 100 (20dB) to 10000 (40dB).

Insertion Loss, IL= P2 / P1 or 10 log10 (P2 / P1) dB Isolation,

I=P1 / P4 =C*D or 10log10 (P1 / P4) dB=CdB+DdB

P1: incident power at port 1
P2: through-put transmission power to port 2
P3: power to decoupled port 4

With ports 2, 3 and 4 are matched, i.e. terminated in 50Ω.

Directional Coupler Applications:

Fig. 3 Applications of Directional Couplers
Directional couplers find important application in microwave measurements, power monitoring and
leveling, signal combiners … etc. Fig. 3 shows three typical uses.
In, (a) a small amount of the source power is coupled to the power meter terminating the forward
coupled branch of the coupler. The power meter reading thus gives a direct measure of the source
power suitably scaled down by the coupling coefficient of directional coupler (typically 20dB or 30dB
down the main power)(b) shows a simple measurement of reflection coefficient / return loss. The
power reflected at the input to the device under test is measured by the power meter at port 4. (c)
shows a power leveling application. A small fraction of the sweeper’s power output is coupled to
port 3, detected by the crystal detector and the resulting voltage fed back to control, via a voltage
controlled attenuator network, the power output of the sweeper, so it remains levelled (constant)
with frequency.
Procedure:
To Investigate the directional coupler properties and determine the Coupling
Directivity and insertion loss of a directional coupler.

To investigate experimentally the directional properties and determine the coupling,
directivity and insertion loss of directional coupler we need to measure the power at
its 4 ports, as shown in Fig.4:

Fig. 4 Powers at directional coupler

P1=Incident power at port 1
P2=Through-put transmission power to port 2
P3=Power coupled to Port 3
P4=Power decoupled Port 4
With ports 2,3 and 4 matched, i.e. terminated at 50W,
Coupling, C=P1 / P3 or 10log10 (P1 / P3) dB
Directivity, D= P3 / P4 or 10log10 (P3 / P4) dB
Insertion Loss, IL= P2 / P1 or 10 log10 (P2 / P1) dB

1) P1 is measured using the circuit of (a). Note the PAD is acting as isolator for the
VCO microwave source with low loss transmission from port 1 to port 2
2) The coupled power P3 is measured using circuit (b): Note ports 2 and 4 of the
directional coupler are terminated in matched loads. The crystal detector also
presents a very good match at port 3.
3) The power P4 to the decoupled port is measured using circuit (c) with ports 2
and 3 terminated in 50 ohm matched loads.
4) Finally the transmission through-put power, P2 is measured using (d) with
ports 3 and 4 each terminated in 50 Ω.
5) So as to investigate the directional coupler performance over a band of
frequencies take measurements at 2.5, 2.75, 3, 3.25 and 3.5 GHz and tabulate
results in Table 2.1
6) From the results obtained, use Table 2 to record the values of coupling over
the range 2.5 to 3.5 GHz. Summarize these in a brief statement of specification
of the directional coupler’s performance over this band.
Fig. 2.8 Directional Coupler Measurement Investigations
Observations:

Analysis:
The performance of tan S-band microstrip directional coupler for low-level coupling applications of the
order of -20dB has been investigated experimentally. From the measurements, values of coupling,
directivity and insertion loss over the range centered on the mid-band frequency have been
calculated and a specification for the direction coupler drawn up.
Results:
_______________________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________

Dated: __________________                                   Checked By: ________________
Lab 7
Study of Dipole Antenna & its Radiation Pattern

Objectives:
 Know about the construction and feeding details of a dipole antenna
when fed from a coaxial cable.
 Describe the wave shape and polarization of the beam produced by a
dipole type microwave antenna.
 Describe the difference between the E and H plane radiation patterns
of dipole antenna.
Theory:
Dipole is the most commonly used antenna which consists of a thin wire of length equal
to 'lambda/2 and excited by a voltage source from the center as shown in Fig 2.1 above.
The current on the dipole is approximately sinusoidal distribution of current with nodes
at the open ends and current maxima at the center. The input impedance of Al2 dipole
is about 73 + j43 ohms. The dipole can be made resonant (with input - impedance pure
real and reactance equal to zero) by slightly reducing its length from a.5A. The
reduction in the length depends upon the diameter of the dipole wire, however. The
dipole is to be fed fr0111a balanced line for sinusoidal current distribution. At
microwave frequencies we prefer to use coaxial line which is an unbalanced line. A
BALUN is used to convert an unbalanced line to a balanced line. In this experiment the
dipole is fed from a coaxial port using a BALUN.
Polarization:

Consider the electric field in free space, in the vicinity of a \12- wave dipole in Fig 2.2. It can be
established that at the dipole ends, electric field strengths are produced (CW and CCW), which leave
the antenna, anti - phase to each other. Thus the fields cancel each other and .for this reason, no wave
propagation can take place from the ends of a dipole.
However, radial to the generator, in an imaginary equatorial plane, all E-ficld strengths are of the same
phase and open to the surrounding area. The fast changes in the E-fields produce magnetic lines of
flux as shown below and the main source of energy is propagated radially outwards from the dipole.

Further away from the centre of the antenna, the electric field E (as shown in Fig 2.3 and Fig 2.4) is
vertical, with the magnetic "field H horizontal, in phase with the E-field. These components move
away radially, from the location of the transmitter.
Procedure:
1) Mount the dipole antenna with E field in vertical plane on the turn table and
connect the dipole antenna port with the receiver through a coaxial cable.

2) Mount horn antenna with E field in ver1ical plane and connect it to the
transmitter through a coaxial cable. The Horn antenna will act as transmitting
antenna.

3) Set distance between the transmitting and dipole antenna such that both are in
far field zone. This can be done by calculating the far field range which is equal to
where D is the maximum dimension of the antenna (in the case of the
dipole antenna it is equal to the dipole length).

4) Align both the antennas both facing towards each other. Switch on transmitter
and receiver. Readjust alignment of the antenna till you get maximum signal in
the receiver. Note down signal level in millivolts (mY) from the digital display and
convel1it into power using the calibration graph. Record the result below:

5) Next rotate the base of turntable till it reads 0 deg without disturbing the
position of the dipole antenna. Turn dipole in' steps of "5" degrees, In both
clockwise and counter clockwise directions. For each setting note the
corresponding level of the received signal and record the results in first table of
next page:
6) Plot the radiation pattern on polar. The radiation plot will be E plane plot of
dipole. Measure -3dB beam with from the plot and record the results in table 1.

7) Plot the radiation pattern on polar. The radiation plot will be H plane plot of
dipole. Measure -3dB beamwidth from the plot and record the results in table 2:
Observations:
Table 1

Table 2

Results:
_______________________________________________________
_______________________________________________________
_______________________________________________________

Dated: __________________             Checked By: __________________
Lab 8
Study of Pyramidal Horn Antenna
Objectives:
 Describe the type of beam produced by a pyramidal horn antenna
 Determine how the design of horn antenna determines its gain
 Describe the wave shape and polarization of the beam produced by
flare type microwave antenna
 Describe the difference between the E and H plane radiation patterns
of dipole antenna.

Theory:
The pyramidal horn, like other antenna horns, has directional gain and beam
width depending upon the dimension of width and height of horn opening.
The greater the “a” dimension and smaller the “b” dimension, the more fan
like is the radiated lobe. The fan will be greater than height then the width. If
the horn is rotated so that the “b” dimension is parallel to horizon, then the
fan will be wider in the horizontal plane and narrower in vertical plane. The
direction of fan depends on the application of horn.

Fig. 1 Pyramidal horn antenna

It must be kept in mind that if the beam is to be increased in both the
vertical and horizontal directions then the power of beam must be
increased. In Lab experimentation, the beam of antenna is determined from
the physical measurement mad on the horn.
Procedure:

1) Physically measure the aperture of the pyramidal horn and record the
a1= _____________cm                        b1= _____________cm
2)   The gain of horn is given by:
G=a1 x b1 x6.4/           G dB=10 log (g)
G=_______________dB
3)   Next mount the horn antenna with E field in the vertical plane on the
turn table and connect the horn antenna port with the receiver
through coaxial cable.
4)   Align the transmitting antenna with the horn and set the distance
between them such both are in far field zone. For this repeat the
calculation given in “Lab 7”. The D dimension will now equal to
diagonal dimension of the horn aperture.
D= _____________cm                         = _____________cm
2
Min. distance between Tx and Rx=2D / cm
5)    Switch on the transmitter and the receiver. Set receiver gain till you
get of the meter deflection within its scale. Readjust alignment of the
antenna till you get maximum signal in the receiver.
6)   Next rotate the base of turntable till it reads 0 deg with disturbing the
position of the horn antenna. Turn horn in steps of 5 degrees in both
clockwise and counter clockwise directions. For each setting note the
corresponding level of the received signal and record the results in
table 1.
7)   Plot the radiation pattern on polar. The radiation plot will be E plane
plot of horn antenna. Measure -3dB beam width from the plot and
record the results below:
-3dB Beam width of the horn antenna in E plane=___________deg
8)   Rotate the transmitting antenna and receiving horn antenna by 90 and
repeat above step, record the results in table 2.
9)   Plot the radiation pattern on polar. The radiation plot will be H plane
plot of horn antenna. Measure -3dB beam width from the plot and
record the results below.
-3dB Beam width of the horn antenna in E plane=___________deg
Observations:
Table 1

Table 2

Results:
_______________________________________________________
_______________________________________________________
_______________________________________________________

Dated: __________________             Checked By: __________________
Lab 9
Measurement of the gain of Horn antenna-using method of
two antennas
Objectives:
 Measure gain of an antenna if the two similar antennas are available.
 Determine how the design of the Yagi-Uda antenna determines its
gain.
 Describe the wave shape and polarization of the beam produced by a
Yagi-Uda type microwave antenna.
 Describe the difference between the E and H plane radiation patterns
of Yagi-Uda antenna.

Theory:
Consider two similar antennas separated by a distance "R". Antenna #1 is connected to
a transmitter with, while antenna # 2 is connected to a receiver. If Pt available from the
transmitter, then, using Friss Transmission equation [1] the power received by the
antenna # 2 is given by:

Where ecd1 and ecd2 are radiation efficiencies, (1-||2), (1-||2) are reflection
efficiencies, D1 D2 directivities of antenna #1& #2 where PLF is their polarization loss
factor. Assuming that two antennas are exactly similar and both are polarization
matched (D1=D2=D & PLF=1) then above equation reduces to:

Rewriting above in terms of "G,

Taking log of both sides and multiplying by 10

G dB = 0.5(Pr dBm - Pt dBm) + 1Olog (4R/)-1Olog (1-||2)
If return loss of each antenna is < -10dB, then ||<316 or 10log (1-||2)>-5dB.
Assuming -10dB return loss the G in dBs of the antenna becomes as under:
G dB = 0.5(Pr dBm - Pt dBm) + 1Olog (4R/) + 0.5 dB ------ (1)
Procedure:

1) Connect coaxial cables to the transmitter and the receiver. Now interconnect Tx
and Rx through a coaxial attenuator of 20dB. Switch on the Tx and Rx.
2) Using the calibration curve, translate reading of the Rx display to received power
and record the result below:
Pr= ___________________ dBm
The transmitter RF power is given by:
Pt dBm = Pr dBm + 20dB
Pt= ___________________ dBm

3) Remove the attenuator. Connect transmitter with the Horn antenna and mount it
on the stand. Connect receiver with the Horn antenna and mounted it in the
4) Align the transmitting antenna with E field in vertical plane and set the distance
between the transmitting horn and receiving Horn antenna such that both are in
far field zone.
5) Vary distance between two horn antennas and note down the reading of the
receiver and translate received power in dBm. On each distance setting align the
antenna for maximum received power. Using equation 1, calculate antenna gain
G. Take at least three reading and record the results:
Observations:

Mean Gain of the horn antenna = _____________________dB
Results:
_______________________________________________________
_______________________________________________________
_______________________________________________________

Dated: __________________                          Checked By: __________________
Lab 10
Study of Yagi-Uda Antenna

Objectives:
 Describe the type of beam produced by a Yagi-Uda antenna. Determine
how the design of the Yagi-Uda antenna determines its gain.
 Determine how the design of the Yagi-Uda antenna determines its
gain.
 Describe the wave shape and polarization of the beam produced by a
Yagi-Uda type microwave antenna.
 Describe the difference between the E and H plane radiation patterns
of Yagi-Uda antenna.

Theory:
Yagi-Uda or simply Yagi antennas is the high gain antennas. It consists of
driven element, a reflector and one or more directors i.e. Yagi-Uda
antenna is an array of driven elements (or active element where the power
from the Tx is fed or which feeds receiver power to Rx) and one or more
parasitic elements (i.e. passive elements which are not connected directly
to the transmission line but electrically coupled). The driven element is a
resonant half-wave dipole usually of metallic rod at the frequency of
operation. The parasitic elements of continuous metallic rods are arranged
parallel to driven element and at the same line of sight level.
The parasitic elements receive their excitation from the voltages induced in
them by the current flow in the driven element. The phase and currents
flowing due to the induced voltage depend on the spacing between the
elements and upon the reactance of the elements (i.e., length) the
reactance may be varied by the dimensioning the length of parasitic
elements. The spacing between driven and parasitic elements that are
usually used, in practice, are of the order of) 10 i.e. 0.10 to 0.15 The
parasitic element in front of driven element is known as director and its
number may be more than one, whereas the element in back of it known
as reflector. The reflector is 5% more and director is 5% less than the
driven element which is  2 at resonant frequency.
Procedure:

1)   Mount the Yagi antenna with E field in vertical plane on the
turn table and connect its antenna port with the receiver
through a coaxial cable.
2)   Align the transmitting antenna with E field in vertical plane and
set the distance between the transmitting horn and Yagi
antenna such that both are in far field zone.
get of the meter deflection within its scale. Readjust
alignment of the antenna till you get maximum signal in the
4)   Next rotate the base of turntable till it reads 0 deg with
disturbing the position of the dipole antenna. Turn dipole in
steps of 5 degrees in both clockwise and counter clockwise
directions. For each setting note the corresponding level of
the received signal and record the results in table 1.
5)   Plot the radiation pattern on polar. The radiation plot will be E
plane plot of horn antenna. Measure -3dB beam width from
the plot and record the results below:
-3dB Beam width of the horn antenna in E
plane=___________deg
6)   Rotate the Yagi and transmitting horn antenna by 90 and
repeat above step, record the results in table 2.
7)   Plot the radiation pattern on polar. The radiation plot will be H
plane plot of horn antenna. Measure -3dB beam width from
the plot and record the results below.
-3dB Beam width of the horn antenna in E
plane=___________deg
Observations:
Table 1

Table 2

Results:
_______________________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________

Dated: __________________             Checked By: __________________

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