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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 station.

  • Again the antenna size will be so long because the frequency of audio signals is too low.

  • 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 communication.
     

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.

Circuit Operation:-

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 service.
 

Circuit Diagram:-

DIRECT-COUPLED RADIO Circuit Diagram

Component Required:-

Parts 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:- 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.
 

Application:- 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.