3 CHAPTER DESIGN AND IMPLEMENTATION
3.5 YAGI ANTENNA DESIGN
The antenna parameters element lengths and spacing are given in terms of wavelength, so an antenna for a given frequency can be easily designed. The lengths of various antenna
elements are related to the frequency (f=106 MHz) is as follows:
Planned frequency of transmission f = 100MHz
The following equations will be used to derive the appropriate length of the elements that will make up the yagi antenna and the spacing between them. Fig 3.7 will be used as the
reference.
The equations for length of the elements are: [24]
First Director Length =
4(BCD) --- (I)
22 Second Director Length =
4(BCD) --- (II) Third Director Length = &
4(BCD) --- (III) Fourth Director Length = &6
4(BCD) --- (IV)
Dipole Length = $&
4(BCD) --- (V)
Reflector Length =
4(BCD) --- (VI)
The Spacing between the elements can be found from the following equations: [24]
A = " Fourth Director Length = &6
= 1.38 meters
23 C = &
= 0.3 meters D = &
= 0.3 meters E = $6
= 0.48 meters
Figure 3.6 YAGI ANTENNA STRUCTURE [24]
24
4 CHAPTER 4 TEST AND RESULTS
4.1 INTRODUCTION
This section will discuss tests carried out on the final circuit and the results obtained. Measured waveforms from the oscilloscope will be used to illustrate the performance at each stage of the circuit and the method used to evaluate the obtained result will be described.
4.2 TEST EQUIPMENT
At various stages of the circuit different test were required to confirm the performance of the stages. The following test tools were used:
a) Digital Multimeter: This is an electronic device used to measure continuity, voltage and current. The multimeter was particularly useful for measuring the base-emitter voltage of each transistor in order to verify if it was within the voltage range (i.e 0.6V to 0.7V) of the transistor active region.
b) Oscilloscope: This is a type of electronic test instrument that allows observation of constantly varying signal voltages with respect to time. It allows the observation of signal amplitude and the period of the signal. The oscilloscope was used to check if the oscillator part of the circuit was oscillating as desired. Also the performance of the audio amplifier and the output of the electret microphone was evaluated with the oscilloscope.
c) Analogue FM Radio Receiver: An analog FM receiver was required to tune to the transmitting frequency of the transmitter. The FM receiver will intercept the transmitted FM signal and demodulate it to reproduce the original sound input. With the FM radio receiver it was possible to determine the range of the FM transmitter and also its sound quality.
4.3 CONSTRUCTION AND ASSEMBLY TOOLS a) Cutting Plier
b) Flat Nose Plier c) Digital Multimeter
25 d) Soldering Iron and Lead
e) Small flat screw driver f) Drill Bit
4.4 CONSTRUCTION AND ASSEMBLY
The FM transmitter was built using discrete electronic components (such as resistors, capacitors, transistors) soldered on a vero board. The vero board was made up several vertical conducting strips, on which components were soldered. A drill bit was used to etch out sections of the strips where an electrical bridge was not wanted. The inductor was fabricated by winding 4 turns of a 2mm gauge copper wire on a threaded bolt; while the yagi antenna was constructed by cutting the elements of a ready-made yagi antenna to fit the design specification.
The circuit assembled on the vero board is placed into a handheld instrumentation case 90 × 65 × 25 cm in dimension. A hole is drilled at the top to accommodate the electret microphone, another hole is drilled by its side with an audio jack fitted for the purpose of accepting an external audio signal source. An output for the yagi antenna connection is made on the right side of the case while the power switch is mounted on the reverse side.
4.5 COMPONENT LIST a) Electret Microphone b) Resistor
Table 4.1 RESISTORS
Component Type Quantity Use
68 KΩ Carbon Film 1 Bias for electret
microphone
4.7 KΩ Carbon Film 4 Voltage divider DC-Bias
for carrier Oscillator
26
10 KΩ Carbon Film 1 Provide Modulating
voltage
c) Capacitors
Table 4.2 CAPACITORS
Component Value Type Quantity Use
47 nF Ceramic 2 For stabilising D-C input
voltage
22 pF Ceramic 2 Feedback Capacitor to
enhance voltage swing of Oscillator
22 µF Ceramic 1 Audio Coupling Capacitor
2 – 10 pF Variable
capacitor
1 Capacitance for tank
circuit
1 Inductance for tank circuit
e) Transistor
Table 4.4 TRANSISTORS
Component Value Type Quantity Use
27
Component Value Type Quantity Use
BB204 Variable Capacitance
The following tests were carried out to evaluate the performance of the circuit.
I. Waveform Measurement
II. Voltage and current measurement III. Transmission Range
4.6.1 WAVEFORM MEASUREMENT
Fig 4.1 shows the combined waveform of the audio signal before amplification and after amplification. The upper waveform is the waveform measured at the collector of the first stage transistor, which is the output of the audio amplifier circuit. The bottom waveform is the waveform measured at the output of the electret microphone. The time per division setting was 1 milli-second; while the volts per division was 50 milli-volts.
28
Figure 4.1 PRE-AMPLIFIED VS AMPLIFIED AUDIO WAVEFORM
Volts / Div
Time / Div 50
mV
1 ms
A comparison of the waveform shows a significant amplification of the audio signal, which is very important to achieve a better modulation index.
4.6.2 VOLTAGE AND CURRENT MEASUREMENT
The voltage and current at key parts of the circuit was measured in order to derive the actual power consumption of the circuit and also the amount of power generated in the tank circuit.
29
Table 4.6 VOLTAGE AND CURRENT MEASUREMENT Operational
From the measurements in table 4.6, we can calculate the following:
Power Consumption = Vbattery × Icurrent = 9 × 165mA = 1485 mW Power in Tank circuit = 2 × Ic2 × Rinductor
Power in Tank circuit = 2 × (97 × 10-3)2 × 1 = 18.8 mW
4.6.3 TRANSMISSION RANGE MEASUREMENT
A FM receiver was used to demodulate the transmitted FM signal; a good quality audible message was received within a 30 meters radius of the FM transmitter. However the transistor’s performance degraded significantly as the collector’s current rises; this
significantly limited the transmission power and consequently the distance covered was also limited.
30
5 CHAPTER 5 CONCLUSION AND RECOMMENDATION
5.1 CONCLUSION
A direct FM transmitter with a range up to 10 meters can be built using the varactor diode modulator approach to generate frequency modulated signal. Within the 10 meters range the quality of the sound produced was very good and the bandwidth of the generated FM signal appeared to be within the ±75KHz. This was crucial in producing a good quality sound output.
The addition of a Yagi antenna to boost the transmitting distance did not yield a significantly better result; it is suspected that the power generated by the circuit was insufficient to drive a yagi antenna, as the transistor became excessively hot with the addition of a yagi antenna and the FM signal produced degenerated.
5.2 LIMITATION
It was difficult to evaluate the generated frequency modulated signal, which is about 80MHz.
Measurement of the modulated waveform was not possible due to non-availability of an oscilloscope capable of measuring up to the 80 MHz frequency range.
5.3 RECOMMENDATION
The FM transmitter is highly susceptible to frequency drift when touched or moved from one place to another. It is recommended that the components on the circuit are closely put together, as it was discovered that frequency drifting was reduced in this way.
The performance of the circuit can also be improved by building it on a Printed Circuit Board (PCB) or a well etched out vero board. It was found that the audio sound produced was clearer when the unused conducting rails on the vero board were etched out or cut out. Vero boards have relatively high parasitic capacitance between their conducting rails; these parasitic capacitance do affect the general performance of the circuit.
It is believed that the performance of this circuit can also be improved, if a D-C power source was used instead of a battery to power the circuit; however this would increase the power
31
dissipated by the transistors and a cooling fan will be required to prevent the transistors from getting damaged.
32
REFERENCES
[1] Ke-Lin Du, M.N.S., Wireless Communication Systems: From RF Subsystems to 4G enabling Technologies. Swamy Cambridge University Press, 2010.
[2] Jerry C. Whitaker, The electronics Handbook, 2nd ed. CRC Press Taylor & Francis group, 2005.
[3] A.P Godse and U.A. Bakshi, Analog Communications. Technical Publications Pune, 2009.
[4] V.K Mehta, Rohit Mehta, Principles of Electronics. S. CHAND & COMPANY, 2008.
[5] Jeffrey Wheat, Randy Hiser, Jackie Tucker, Alicia Neely, and Andy McCullough, Designing a Wireless Network. syngress.
[6] Prof. Ali M. Niknejad, “Negative Resistance Oscillator, Differential Oscillator and VCOs.”
[Online]. Available: http://rfic.eecs.berkeley.edu/~niknejad/ee142_fa05lects/pdf/lect23.pdf.
[Accessed: 27-Feb-2016].
[7] Paul McLane, “Radio World: FM Signal Count Grew 38% in Ten Years.” [Online]. Available:
http://www.radioworld.com/article/fm-signal-count-grew--in-ten-years/277873. [Accessed: 21-Feb-2016].
[8] Dale R. Patrick and Stephen W. Fardo, Electricity and Electronics Fundamentals, 2nd ed. The Fairmont Press Inc.
[9] Kellejian, Robert, Applied electronic communication: Circuits, systems, transmission. Science Research Associates, 1980.
[10] H. Ward Silver, The ARRL Extra Class License Manual for Ham Radio. .
[11] Dr. J.S Chitode, Communication Theory, 5th ed. Technical Publications Pune, 2010.
[12] “Miniaturised FM transmitter.” [Online]. Available:
http://pe2bz.philpem.me.uk/Comm/-%20Transmitters/-%20FMx/FMx-902-PortableMiniSterio/Index. [Accessed: 24-Jan-2016].
[13] “Single Transistor FM Transmitter Design | electronics hobby.” [Online]. Available:
https://dmohankumar.wordpress.com/2011/04/23/single-transistor-fm-transmitter-design/.
[Accessed: 24-Jan-2016].
[14] “2 Watt FM Transmitter.” [Online]. Available: http://electronics-diy.com/2-watt-fm-transmitter.php. [Accessed: 07-Mar-2016].
[15] Ali Hajimiri And Thomas H. Lee, “Design Issues In CMOS Differential LC Oscillators, IEEE Journal Of Solid - State Circuits,” vol. 34, May 1999.
[16] W. Storr, “Sound Transducer for Sensing and Generating Sounds,” Basic Electronics Tutorials, 18-Aug-2013. [Online]. Available: http://www.electronics-tutorials.ws/io/io_8.html. [Accessed:
18-Jan-2016].
[17] Eric Coates MA BSc. (Hons), “Amplifier Class A Biasing.” [Online]. Available:
http://www.learnabout-electronics.org/Amplifiers/amplifiers12.php#stabilisation. [Accessed:
18-Jan-2016].
[18] “How to calculate coil inductance (single-layer, cylindrical air core inductors) - Online Javascript.” [Online]. Available: http://zpostbox.ru/how_to_calculate_inductors.html.
[Accessed: 18-Jan-2016].
[19] “Varactor Diode | Varicap Variable Capacitance Diode | Tutorial.” [Online]. Available:
http://www.radio-electronics.com/info/data/semicond/varactor-varicap-diodes/basics-tutorial.php. [Accessed: 05-Mar-2016].
[20] “Community Radio Frequently Asked Questions and Answers.” [Online]. Available:
http://radio.xtreamlab.net/faqa.html. [Accessed: 27-Feb-2016].
[21] Andy Collinson, “Estimating Transmitter Distance.” [Online]. Available:
http://www.zen22142.zen.co.uk/Analysis/efftxd.htm. [Accessed: 27-Feb-2016].
[22] “2N3553 Datasheet.” [Online]. Available: http://www.futurlec.com/Transistors/2N3553.shtml.
[Accessed: 27-Feb-2016].
[23] “2 Watt FM Transmitter.” [Online]. Available: http://electronics-diy.com/2-watt-fm-transmitter.php. [Accessed: 27-Feb-2016].
[24] “BUILD YOUR YAGI ANTENNA.” [Online]. Available:
http://radio.meteor.free.fr/us/antenna.html. [Accessed: 30-Jan-2016].
33