The High-Sensitivity Regenerative Receiver is a classic single-transistor radio that combines simplicity with surprisingly high sensitivity. By using controlled positive feedback, the circuit operates just below oscillation, dramatically increasing gain and selectivity. With the correct coil and tuning capacitor, it can receive medium-wave broadcast, amateur HF bands, and shortwave broadcast stations using only a 3-volt supply.
Because this is a single-tuned regenerative design, frequency coverage depends almost entirely on the inductance of the main tuning coil. Each coil is therefore optimized for one band or a portion of a band.
Regeneration Mechanism and Feedback Path
In this circuit, regeneration is achieved by magnetic coupling between two windings. The main tuned coil resonates with the 140 pF variable capacitor and selects the desired frequency. A smaller tickler coil, coupled to it, carries RF from the MOSFET drain back into the tuned circuit. When the phase is correct, this feedback reinforces the signal, increasing effective Q and gain.
The amount of regeneration is controlled by the MOSFET bias and by the degree of magnetic coupling. Reversing the tickler coil connections changes the feedback polarity and determines whether oscillation can occur.
Understanding the Core Circuit
At the center of this receiver is the 2N7000 N-channel MOSFET. The circuit uses a single-tuned topology, so its operating frequency is set by the resonant combination of the 140 pF variable capacitor and the main tuning coil.
RF energy captured by the antenna is coupled into this LC tank, where the desired frequency is selected. The 1000 pF capacitor together with the 10 MΩ resistor provides the correct gate bias and AC coupling for the MOSFET, while the 4.7 kΩ drain resistor and the 3 V supply establish the operating current. Regeneration is produced by the tickler coil, the small secondary winding magnetically coupled to the main coil. A portion of the amplified RF from the drain is fed back through this winding to the gate, increasing the effective gain and selectivity until the circuit operates just below the point of oscillation.
This receiver follows a fundamental design rule: one tuned coil corresponds to one band. Wide frequency coverage is only possible by switching coils. Performance is best when each coil is designed specifically for a narrow frequency range, rather than attempting to cover multiple octaves with a single inductor.
2N7000 regenerative receiver –Mastering Frequency Coverage
The most critical design rule for this project is that it is a single-band receiver. To change bands, you must change the coil.
Medium-Wave Performance Using Ferrite Rod Coils
For the AM broadcast band, ferrite rods provide high inductance and excellent signal capture. With the original 635 µH coil and a 140 pF tuning capacitor, the receiver covers approximately 530 kHz to 1.6 MHz and exhibits strong sensitivity to both local and distant stations. For this specific band, a ferrite rod is highly recommended as it acts as its own high-efficiency antenna, pulling in distant stations with ease.
Typical Ferrite Rod Turns
Ferrite losses increase rapidly above about 7 MHz, which limits their usefulness for higher shortwave bands.
Shortwave Coverage with Air-Core Coils
Air-core coils are preferred for frequencies above the medium-wave band because they offer low loss, high Q, and excellent stability. They also provide smoother regeneration control and sharper selectivity, which is important for CW and SSB reception.
Assuming a 22 mm former, 26–28 AWG enamelled copper wire, and a 140 pF variable capacitor, the following turn counts serve as good starting points.
Air-Core Coil Turns for Popular Bands
Fine tuning is done by compressing or expanding the winding or by adding and removing turns
2N7000 regenerative receiver –Use of Ferrite and Toroidal Cores
Ferrite rods are ideal for medium wave and the lowest HF bands, but their losses become significant at higher frequencies. Air-core coils are therefore recommended for 40 meters and above.
Ferrite toroids such as FT-140-43 or FT-240-43 can be used, but they have lower Q and broader bandwidth due to core losses. While they allow compact construction, they reduce sensitivity and make regeneration more difficult to control. Powdered-iron toroids (such as T50-2 or T68-2) offer better performance than ferrite if a toroidal form factor is required.
2N7000 regenerative receiver –Practical Construction Considerations
The tickler coil typically uses one-fifth to one-tenth the number of turns of the main coil. It should be placed close to the tuned winding for adequate coupling, and its polarity must be chosen to ensure positive feedback.
Mechanical stability is important, as regenerative receivers are sensitive to vibration and hand capacitance. A grounded metal front panel and short lead lengths improve frequency stability. The power supply should be well decoupled, as regeneration amplifies any noise present on the supply line.
A very effective first build targets the 40-meter band using an air-core coil of approximately 17 turns on a 22 mm former, with a four-turn tickler. This combination provides stable tuning, strong CW reception, and easy adjustment of regeneration.
Performance Notes and Fine Tuning
This receiver performs very well on AM and CW (Morse code), and with careful adjustment it can also demodulate SSB signals, although tuning becomes quite critical. At frequencies above about 10 MHz, hand capacitance starts to have a noticeable effect, and simply moving your hand near the tuning control can cause the received frequency to shift. Using an insulated or plastic extension shaft on the variable capacitor helps reduce this problem and improves tuning stability.
It is also important to keep in mind that a regenerative receiver can begin to radiate a small amount of RF energy if the feedback is advanced beyond the oscillation threshold. For this reason, the regeneration control should be set only as high as necessary for good reception, and the circuit should always be operated with care and in the true experimental and educational spirit of radio.
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