This low-cost AM transmitter is tunable from 10 to 15 MHz with the help of ½J gang condenser VC1, which determines the carrier frequency of the amplitude modulation transmitter in conjunction with inductor L1.
The frequency trimming can be done with VC2. The carrier is amplified by transistor T4 and coupled to RF amplifier transistor T1 BD677 through transformer X1.
The AM radio transmitter does not use any modulator transformer. The audio output from condenser MIC is preamplified by transistor T3 (BC548). The audio output from T3 is further amplified by transistor T2 (BD139), which modulates the RF amplifier built around transistor T1 by varying the current through it in accordance with the audio signal’s amplitude. RFC1 is used to block the carrier RF signal from transistor T2 and the power supply.
For antenna, one can use a 0.5m long telescopic aerial.
Details of RF choke, inductor L1 and coupling transformer:
AM transmitter circuit diagram
The frequency trimming can be done with VC2. The carrier is amplified by transistor T4 and coupled to RF amplifier transistor T1 BD677 through transformer X1.
The AM radio transmitter does not use any modulator transformer. The audio output from condenser MIC is preamplified by transistor T3 (BC548). The audio output from T3 is further amplified by transistor T2 (BD139), which modulates the RF amplifier built around transistor T1 by varying the current through it in accordance with the audio signal’s amplitude. RFC1 is used to block the carrier RF signal from transistor T2 and the power supply.
For antenna, one can use a 0.5m long telescopic aerial.
Details of RF choke, inductor L1 and coupling transformer:
- RFC1 = 30 turns of 24 SWG copper wire on 6 mm diam, air core
- X1 (transformer) = 15 turns primary, 3 turns secondary using an IFT core with tuning slug
- L1 = 4 turns of 22 SWG copper wire on 15 mm diam air core
AM transmitter circuit diagram