Friday, June 20, 2014

Half-watt AM transmitter



"The Poppet" is a half-watt AM transmitter designed by Mr. Doug Gibson of England. The original design was published in issue 84 of SPRAT, newsletter of the GQRP Club. The version shown here incorporates changes suggested by Steve Hartley and others.

Although it was designed to work with a microphone and to be used in the 160 meter band (1800-2000 kHz), the Poppet can easily be modified to work in the 1600-1720 kHz end of the AM broadcast band and work with a line level input from a mixer instead of a microphone.

Construction details are not critical. The LM386 and the output transistor will need heatsinks. The circuit can be built "dead bug style" with the modulator chip stuck upside-down to the copper circuit board for heatsinking.

[schematic diagram]

parts list

C1, C2, C13: 0.5 uF
C3: 1 nF (1000 pF)
C4, C5: 10 uF electrolytic
C6: 10 nF (.01 uF)
C7, C15: 100 nF (.1 uF)
C8, C16: 330 pF
C9: 50 pF variable
C10: 200 pF
C11, C12: 1n8 (1800 pF)
C14: 68 pF
C17: 220 pF
D1: 1N4148
D2: 9 volt zener
J1: microphone jack
J2: RF output jack
L1: 60 turns, 38 SWG wire, T37-2 core
L2: 50 turns, 38 SWG wire, T37-2 core
R1: 560 K
R2: 4700 ohms
R3: 1K trimpot (mic. gain adjust)
R4: 270 K
R5: 100 K
R6: 560 ohms
R7: 33 K
R8: 5600 ohms
R9: 100 ohms

Q1, Q3: BC109 (possible equivalents: NTE123A, 2N2222A)
Q2: 2N3819 or similar
Q4: BFY51 (possible equivalents: NTE128, 2N3053)
RFC1: 10 turns of small enameled wire on ferrite bead
RFC2: 1 mH, rated for 500 milliamps
T1: 12 turns primary, 2 turns sec. on half-inch binocular ferrite core

To modify the Poppet for neighborhood broadcasting in the 1600-1720 kHz frequency range, either increase the capacitance of C8 to bring the VFO's frequency down into the broadcast band, or replace the VFO with a simple crystal oscillator or PLL synthesizer. The microphone pre-amp stage can be omitted if the unit is used with a line-level audio source such as a mixer or tape player.

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