In the world of QRP (low-power) amateur radio, simplicity and innovation often go hand in hand. The Curumim Transceiver, World’s Smallest QRP CW Transceiver, designed by PY2OHH, stands out as one of the smallest working QRP CW transceiver ever built. Operating on 7.030 MHz (40-meter CW band), this tiny circuit demonstrates how minimalist design can still achieve functional communication on HF.
This circuit embodies the essence of homebrewing — a single-transistor transmitter combined with an audio amplifier for receiving, all powered by 12 V and capable of driving a headphone or small speaker.
Curumim Transceiver – Circuit Overview
The schematic consists of two main sections:
- Transmitter Section (Upper part with 2N3904)
- Receiver/Audio Amplifier Section (LM386-based)
Let’s break down the working of each.
Transmitter Section – 2N3904 Crystal Oscillator
At the heart of the transmitter is a 2N3904 NPN transistor configured as a crystal-controlled oscillator.
- Crystal: The 7030 kHz crystal determines the operating frequency (CW QRP calling frequency).
- 120 pF capacitor and 5.6 µH inductor form a feedback network to sustain oscillation.
- The 18 kΩ resistor biases the transistor, while a 100 nF capacitor provides supply decoupling.
- The 390 pF capacitor couples RF output to the antenna.
The CW key in the emitter circuit controls the transmission. When pressed, it completes the circuit, allowing oscillation and radiation from the antenna. When released, the transmitter stops, and the circuit switches to receiving mode.
Receiver Section – LM386 Audio Amplifier
The LM386 audio amplifier serves as the receiver’s core.
- The 10 kΩ resistor and 100 nF coupling capacitor feed the RF signal (picked up through the same oscillator circuit when the key is open) into the LM386 input pin (pin 2).
- The LM386 then amplifies the weak CW signal.
- A 10 µF capacitor between pins 1 and 8 increases the amplifier gain to about 200.
- The amplified audio output appears at pin 5, driving a headphone or small speaker through a 10 µF coupling capacitor.
Thus, when the CW key is open, the circuit acts as a direct conversion receiver, and when pressed, it becomes a crystal-controlled CW transmitter — a true minimalist transceiver!.
Curumim Transceiver – Working Principle
When the CW key is open, the transceiver is in receive mode. The transistor circuit acts as a regenerative detector, picking up weak RF signals from the antenna and converting them into audio frequencies. These audio signals are then amplified by the LM386 and heard through the headphones.
When the CW key is pressed, the transistor enters oscillation, generating an RF signal at 7.030 MHz that is fed to the antenna for transmission. During this time, the receiver section becomes inactive, as the strong RF field from the transmitter dominates. This elegant mode-switching system allows the Curumim Transceiver to function as both transmitter and receiver without additional relays or switches.
The Curumim Transceiver operates at 7.030 MHz, a popular CW calling frequency in the 40-meter band. It uses a single transistor for RF generation and an LM386 for audio amplification. The entire circuit functions on a 12-volt supply and produces an output power of approximately 100 to 200 milliwatts.
Sensitivity is sufficient for copying strong CW signals, and the audio output is adequate for headphones or small speakers. The design’s minimal component count and compact form make it ideal for experimentation, education, and portable QRP operations. Builders can assemble it easily on a small piece of perfboard or even construct it using dead-bug style wiring.
Source – https://www.qsl.net/py2ohh/trx/curumim/curumim.htm
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