This ceramic resonator controlled QRP transmitter delivers 1 watt of power across the 80-meter band (3.52 to 3.59 MHz). The circuit uses a single 3.58MHz ceramic resonator for frequency stability. This makes it perfect for beginners who want to build their first low-power transmitter.
1W QRP Transmitter Circuit Features and Specifications
This QRP transmitter operates from a +12V DC supply and produces approximately 1 watt of RF output power. The frequency range covers 3.52 to 3.59 MHz, placing it squarely in the 80-meter amateur radio band. The ceramic resonator provides reasonable frequency stability for QRP operation, though not as precise as crystal control. The circuit includes a keying input for CW operation and uses common, readily available components throughout.
Oscillator Circuit
The oscillator section centers around the 4069 hex inverter CMOS IC configured as a square wave oscillator. The ceramic resonator connects between pins 14 and 7 of the 4069, with the resonator determining the operating frequency at 3.58MHz. Resistor R5 (270R) provides the bias needed for oscillation, while capacitor C4 (100n) decouples the power supply. The 4069 contains six NOT gates, and multiple gates are connected in series to provide sufficient gain and drive capability for the following stages. The output from this stage is a square wave rich in harmonics at 3.58MHz.
Buffer and driver
The buffer and driver section uses a 2N3906 PNP transistor to isolate the oscillator from the power amplifier stage. This prevents loading effects that could shift the oscillator frequency. The transistor receives its input signal through coupling capacitor C2 (100n) from the 4069 oscillator. Resistor R4 (2k2) serves as the collector load, while the emitter connects to the positive supply through the keying circuit. When the key is closed, the circuit completes and allows the QRP transmitter to operate. The 2N3906 provides both buffering and some voltage amplification before driving the final amplifier stage.
Power amplifier
The power amplifier stage employs a VN10KM N-channel enhancement mode MOSFET. This device can handle several watts of power dissipation, making it suitable for QRP applications. The MOSFET gate connects to ground through resistor R6 (1M), which provides a DC path while presenting high impedance to RF signals. The RF drive signal couples to the gate from the previous stage, forward-biasing the MOSFET during positive signal peaks. The drain connects to the +12V supply through RF choke L1, which provides DC power while preventing RF from entering the power supply. The source connects to ground through a 100-ohm resistor, providing some negative feedback and current limiting.
The output network consists of coupling capacitor C6 (680pF) which blocks DC while passing the RF signal to the antenna connection. The RF choke L1 prevents RF energy from flowing back into the power supply, while also providing the DC path for drain current. This simple arrangement works well for QRP power levels and provides reasonable harmonic suppression.
1W QRP Transmitter – MOSFET Alternatives and Substitutions
The original VN10KM MOSFET has specifications including 60V drain-source voltage rating, 270mA continuous drain current capability, 1W power dissipation, and gate threshold voltage between 0.5-2.5V. Several alternatives work well in this circuit. The VN10KN provides nearly identical specifications and serves as a direct replacement. The IRF510 provides much higher capability at 100V and 5.6A with 43W power dissipation, but requires proper heatsinking and may produce 2-3 watts output with this circuit. When using the IRF510, increase the gate bias resistor R6 to 2.2M and add gate protection.
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