If you ever worked with electronics before then you will not get any
difficulties to get the success of the project. It is a Direct Coupled AM radio.
Meter band radio is a simple AM (Amplitude Modulated) demodulator. It works in
the frequency range of 536kHz to 1600kHz in the radio band. Now we will briefly
explain what is AM and how AM radio works.
In AM communication system, audio signal is the message which we transmit via
radio carrier. Audio signals are our voice collected by a microphone and
converted to electrical signals. Then amplified to get considerable amplitude.
It is modulated with a high frequency which is a radio frequency often called a
carrier. There are three reasons why we use a radio frequency carrier.
If we directly transmit audio signals via antenna then all radio centers will
transmit in same frequency band as human being talks in same frequency band.
Thus we will receive all the channels simultaneously and hardly distinguish any
Again the antenna size will be so long because the frequency of audio signals is
Again Audio signals never propagate through the medium. It will be attenuated by
the surface of the earth.
Thus a high frequency carrier is an ideal for communication because
The antenna size will be reduced because of the higher frequency.
We get a wide frequency range to transmit different channels in different bans.
Again radio frequency propagates much higher distance ideal for long distance
Now modulation part is there. The instantaneous amplitude of the carrier is
modulated by the message signal that is why it is called Amplitude Modulation.
Then it is transmitted via an antenna.
The radio set receives the radio frequency from the medium via antenna. Then
after amplification the signal is envelop detected by diode and capacitor
network. Again the detected signal is weak and needs next stage power
amplification. After amplification it is fed to Headphone or speaker.
In this project we are going to build a pocket size AM radio. In an AM radio the
most important thing is a tuned ferrite rod antenna, which is available in
market. It is a coil wound on a ferrite rod and a gang tuner in parallel with
the coil. This parallel tank circuit has a resonant frequency in radio range and
can be changed by the gang tuner knob.
A shirt-pocket project, this direct-coupled radio uses transistor Q1 as a diode
detector and first audio amplifier. Detection is across the bass-emitter
junction which operates as a diode. Normal bass-emitter capacitance provides RF
filtering. L1 can be a tapped (transistor type) ferrite antenna coil. Tuning
capacitor C1 is a miniature poly-type variable.
Earphone E can be magnetic or crystal as long as its impedance is in the 2500-
to 5000-ohm range.
Control R1 is adjusted for best earphone sound - or least distortion consistent
with maximum volume.
During construction, carefully note that npn and pnp transistors are used. Don't
intermix them reverse polarity voltage can destroy a transistor.
Batteries B1 and B2 are the penlight (AAA) type - good for many hours of
List for DIRECT-COUPLED RADIO
B1, B2 - 1.5-V battery
C1 - 365-pFvariable capacitor
E1 - 2500-5000 ohm earphone
L1 - Tapped ferrite antenna coil
Q1, Q3 - Motorola HEP-641 npn transistor
Q2 - Motorola HEP-253 pnp transistor
R1 - 5000-ohm potentiometer
R2 - 100-ohm, 1/2-watt resistor
Testing a radio circuit is really a
tuff job. At first prepare the audio power amplification section. Tune R1 and
touch the base 'b' of Q2. If you hear a hiss noise, means the amplifier is
working in an ideal operating point. Tune R1 so that you get optimum sound
output at the Earphone. Then attach the output of Q1 to the input of Q2. Tune
the gang capacitor so that you can get any signal from the radio station. If you
don't get anything even hiss noise the attach another stage of cascading
amplifier after Q3. You can attach a long rod of metal or cupper wire as an
antenna with the ferrite rod for better reception.
Pack the circuit inside a plastic case. You get your personal radio made by you.
Now get connected to your favorite music station and relax. Don't forget to show
your radio to your friend.