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March 30, 2009

Build Yourself a DIY AM Radio Transmitter

diagram of a very simple, miniature medium wave amplitude modulation radio transmitterIntroduction: Understanding the fundamental principles of radio waves, their generation, propagation and reception is essential not only for the electronic enthusiasts, but also in other disciplines of science which employ radio-frequency (RF) in scientific investigation and even in medical imaging and treatment. Here, the construction of a cheap and simple DIY (do-it-yourself) transmitter (Tx) circuit is described and its operations explained in simple terms.

Materials you'll need: A microphone, a transistor (ask your friends to help with the emitter, base and collector terminals. Alternatively, look up the web), a radio frequency coil (choke, RFC), a variable capacitor (365 or 500 pF; pF = pico farad), 2 resistors, 3 condensers , an aerial (antenna), a soldering iron, solder wire, a stripboard (also known as veroboard) and a power supply (9V).

Assembly: Clean the leads (wire terminals) free of oxides/ enamel coatings by rubbing with fine grained sandpaper. Next, in accordance with the accompanying diagram, wire the components together in one of the following ways:
1. Connect the components directly and cover these joints by insulating tapes such as black tapes, cello tapes or scotch tapes (it is said that the scotch tape emits X-rays when pulled off, well it's unimportant here) . This will prevent accidental shorting with similar joints.
2. Or use a breadboard and press-fit the leads in the slots. The breadboard has prewired connections within it, which will take care of the rest. Make sure that these prefixed connection layouts are followed. You may need to make some extra connections by using external hook-up wires/jumpers.
3. Or use a stripboard/veroboard: Put the components into the drilled holes according to the circuit layout. Now, solder the leads using a 20 watt soldering iron. It is the best option among the three mentioned.

When the iron is sufficiently hot, apply the hot tip to the component lead (not to the solder wire!) to be soldered, and bring the solder wire near it. Within a few seconds, solder will melt and flow evenly around the joint. Withdraw the iron (take as little time as possible).
Check with the circuit again. Cut any extra strip of copper track which might act as a potential short. You may need to make some extra connections by using external hook-up wires/jumpers.

Turn on your radio: Set your radio receiver to any empty slot on the medium wave frequency band. Switch on your newly assembled wireless Tx. Turn the spindle of the variable condenser of 'your set'. At some point you'll hear a 'smooth hissing carrier tone' on the receiver. Keep the variable capacitor there. You're almost done! Speak on the mike, you'll hear the same on the radio receiver. If you press the mike on your left chest, you will hear your heart sounds on the receiver set. You can make a 'phonocardiogram' too this way.

How it works: The Tank oscillator produces the carrier frequency (freq = 1/{2*pi*sqrt.[LC]}; L= inductance of the coil in Henry, C = capacitance in Farad, as dictated by the variable capacitor). To see an animation of how a 'tank circuit' works visit this page. The amplitude of the freq. of the carrier wave is modulated by the input from the microphone. As we speak on to the microphone, the diaphragm vibrates. This makes a coil of copper, wound around a powerful magnet, vibrate too, making induced current in the coil, the magnitude and frequency of which depends on the loudness and frequency of the speech/audio signal. The mike is connected to the base of the transistor via a capacitor. This capacitor allows AC (alternating currents) to pass, but blocks any DC component. The second feature thus, does not let the microphone change the bias voltage conditions of the transistor. The biasing of the NPN type transistor is done by one or two fixed resistor networks (one in this case).
When some audio signal is present, collector current will increase. Since it is the tank circuit at the collector load, the amplitude of the 'carrier frequency' will thus be modified. Amplitude Modulation has been achieved! You will notice that one end of the output capacitor has been connected to the antenna, while the other end is connected to the ground (earth, ground of your wall power outlet, or safer still the traditional lead water pipe). This arrangement makes the capacitor plates look like, as if, they are placed far apart; earth and sky! This causes dissipation of the energy in the form of electromagnetic waves.
Frequency Modulation (FM) transmission is better, since the static (electrical noise from man made appliances, lightnings etc) is almost absent and stereo signal can be transmitted, to name a few.

Experiments and lessons on it:
1. Move a magnet at the back of the radio receiver (where the RFC choke is located) with the receiver tuned to the short wave band. You'll notice that the frequency changes: many different stations come in (without even having to turn the tuner knob!). This happens as the magnet alters the local oscillator of the receiver.
2. Put the radio inside a metal container and try to catch a station. You won't succeed. This is 'shielding'; the metal does not allow electromagnetic wave to pass in. Co-axial cables exploit this phenomenon to avoid noisy interferences. Sensitive instruments used in medicine (e.g. EEG) or radioastronomy were often built underground. This phenomenon is exploited in a Faraday cage.
3. Just touch two different metals in front of the radio receiver: the radio will make noise. This is due to passage of free electrons between the metals.
4. Short two ends of a battery (cathode and anode), the radio hisses and makes noise. Obviously, electromagnetic fields are created.
5. The telltale undulations heard in shortwave, is due to the reflection (actually total internal reflection, a type of refraction) of radio waves from the Kennely-Heaviside layer of the ionosphere, a charged layer in the higher atmosphere, that frequently changes its altitude. If you put a laser beam or any light on the ground and put a mirror higher up, you would get a reflection. Moving the mirror up and down will put the reflected beam closer or further away (light is also an electromagnetic wave). This layer is absent in night time and Appleton layer still higher up reflects then. This is why SW broadcasting stations are best heard at different frequencies during different parts of the daytime.
6. You can listen to cosmic noise, falling meteors (by their 'heated' ionization trail) etc.
7. Take a radio and tune it to a station. Now take a second radio and turn the tuner knob till that same radio station is obtained. You'll notice a drop in volume of the former radio (and vice versa). This illustrates energy transfer by resonance.

Uses of Radio waves: Telecommunications, radar, radioastronomy, medical investigations such as MRI (NMR), medical therapies (radio frequency ablation in ectopic pacemakers of the heart (electrophysiology), short-wave diathermy, research procedures such as thought broadcasting etc.

N.B. : 1. This is a very simple, low cost transmitter, but many countries might have legal restrictions on it. A crystal controlled version of the set is more likely to be approved, as the bandwidth is less.
2. Other forms of electromagnetic waves may be broadcast in much the same way e.g. our good old TV remote uses infrared waves.

Last modified: Mar10, 2014
Reference: hyper-links, unless specifically mentioned

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