The 27MHz frequency band has long been a favorite among electronics enthusiasts and amateur radio operators. This citizen’s band frequency offers excellent propagation characteristics and serves as an ideal starting point for understanding amplitude modulation (AM) transmission principles. Today, we’ll explore a well-designed 27MHz AM transmitter circuit that demonstrates classic RF engineering concepts while remaining accessible to hobbyists.
At its core, this design leverages a 27MHz crystal (CR1) for precise frequency stability, ensuring your transmissions stay on target. This crystal drives the first transistor, Q1 (a 2N2222), which acts as an oscillator, generating the fundamental radio frequency carrier wave. Resistors R1, R2, and R3, along with capacitors C1, C2, and C3, are crucial for biasing and frequency tuning within this oscillator stage.
The amplified and modulated signal then moves to the power amplification stage, utilizing a pair of FETs, Q3 (an IRF610) and Q4 (a 2SK3570). These components are selected for their ability to handle power and efficiently amplify the radio frequency signal for transmission. Inductors L1, L2, L3, and L4, along with capacitors C4, C5, C6, C7, C8, C10, C11, and the variable capacitor VC, form various filtering and impedance matching networks throughout the circuit. Specifically, L4 and the variable capacitor VC (15-250pF) are critical for tuning the output stage to the antenna (ANT), ensuring maximum power transfer and efficient radiation of your signal.
The notes on the schematic provide vital details for constructing the inductors: L4 can be 13 turns on a 6mm air core using 22 SWG wire, and L3 can be 15 turns on a 6mm ferrite core with 22 SWG wire. L1 and L2 are specified as 100uH 0.5W DIP inductors, simplifying their acquisition. These details are essential for achieving the desired inductance values and overall circuit performance. The power output of this 27mhz AM Transmitter is around 5 watts provided that carefully assembled and tuned. 27MHz AM Transmitter
How Modulation Happens in 27MHz AM Transmitter circuit
Let’s focus on the modulation section:
✅ Audio Input Path:
- The audio signal enters through the connector near C6 (1uF).
- This signal is passed to Q4 (2SK3570) — a modulator or audio amplifier transistor.
- This audio path eventually modulates the gate of Q3, the RF final transistor.
✅ RF Path:
- Q1 (2N2222) and crystal oscillator generate a 27 MHz signal.
- Q2 (BD139) amplifies this RF signal.
- The RF is then sent to Q3 (IRF610), a MOSFET power amplifier.
- The RF output of Q3 is modulated in amplitude — and this is where the modulation magic happens.
✅ How Q3 Gets Modulated:
- The drain of Q3 has the carrier signal (from Q2).
- The gate of Q3 receives modulated bias influenced by Q4 and the audio input.
- When the audio signal is positive, it increases Q3’s gate voltage → higher output amplitude.
- When the audio signal is negative, it reduces Q3’s gate voltage → lower output amplitude.
- This varies the amplitude of the carrier — classic AM
Inductor L4, specified as 13 turns on a 6mm air core using 22AWG wire, forms part of a pi-network filter. The accompanying variable capacitor VC (15-250pF) allows for precise tuning and impedance matching to various antenna configurations. This adjustability proves essential for optimizing power transfer and minimizing reflected power.
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