The Mighty Mite 40 Meter QRP CW Transmitter is a legendary low-power Morse code transmitter widely built by amateur radio operators for portable, minimalist, and educational operation. Operating around 7.023 MHz, this compact rig delivers approximately 5 watts RF output, making it ideal for QRP (low power) CW contacts on the 40-meter band.
This design, originally distributed by Far Circuits, demonstrates elegant analog RF engineering using discrete components and transformers, making it an excellent learning platform for homebrew enthusiasts.
QRP CW Transmitter –Crystal Controlled Oscillator Stage
At the heart of the transmitter is a 7.023 MHz quartz crystal (Y1) driving a 2N3904 transistor (Q2) in a stable Colpitts-style oscillator configuration. The crystal ensures excellent frequency stability for CW operation. The tuned transformer T1 provides impedance matching and harmonic suppression before feeding the driver stage.
Key features:
- Fixed frequency operation for rock-solid CW
- Temperature-stable oscillation
- Low phase noise suitable for narrowband Morse reception
RF Driver and Buffer Amplifier
The oscillator signal is fed into a 2N3053 RF driver (Q3), which increases signal level and isolates the crystal from load variations. Transformer T2 provides further impedance transformation while capacitors C4 and C5 form part of the interstage tuning network.
This stage ensures:
- Clean waveform drive
- Reduced frequency pulling
- Proper excitation of the final amplifier
Final Power Amplifier and Keying Circuit
The power output stage uses a rugged RF transistor controlled by a BD138 keying transistor (Q1), allowing smooth, click-free CW keying. The final stage delivers approximately 5 watts when powered from a 13.8V supply.
A low-pass filter (L2, L3, C11–C13) suppresses harmonics and provides proper antenna matching for 50-ohm systems.
Output Low-Pass Filter and Antenna Matching
The T37-2 toroid inductors and high-quality capacitors form a 7 MHz low-pass filter, ensuring compliance with spectral purity requirements and protecting nearby receivers from harmonic radiation.
This network:
- Suppresses 2nd and 3rd harmonics
- Matches transmitter output to the antenna
- Improves overall signal quality
Attribution and Copyright
Circuit design and original documentation:
Far Circuits – Mighty Mite 40 Meter QRP Transmitter
All schematic rights belong to the original designer and Far Circuits. This article is an educational review and technical explanation only.
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