Regenerative Receiver: Regen Radio

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The regenerative receiver, regen radio was a popular form of radio receiver in the 1920s and 1930s.

As a result this form of radio deserves its place in this summary of the different types of radio available.

Regenerative receiver history

The regen radio was one of the many inventions in radio technology that was made by Edwin Armstrong. He invented and patented the regenerative circuit while he was at college, in 1914.

Although the invention of the regenerative receiver is generally credited to Armstrong, others contested this. Lee de Forest filed a patent in 1916 and he took out a lawsuit that lasted over 12 years. This bounced back in the courts, finally ending in the US Supreme Court at which point Armstrong lost.

The regenerative receiver was widely used in the 1920s and 30s because it was able to provide high levels of gain and selectivity with a small number of valves or tubes. As costs of these devices were high, and they often ran on batteries, minimising the number of stages was key. As a result the regenerative receiver was a popular radio technology.

The regenerative receiver was particularly popular with radio hams. As they had to build all their equipment in the 1920s and 30s, the simpler construction of the regen radio meant that they were more achievable than the superhet that was really only just beginning to be used.

Regenerative receiver basics

The RF amplifier has a feedback loop that feeds a proportion of the output back to the input so that the signals around the loop are in phase. In this way, any signal that is in the amplifier will be repeatedly amplified, and this can increase the levels of gain by factors of 1000 or more.In theory feeding back the output to the input should provide infinite gain, but in reality factors including the saturation of the amplifier, and phase delays mean that this cannot be achieved in reality.

The other factor of importance is the selectivity. As there is a tuned circuit in the feedback amplifier, the gain increases around the point of resonance rather than away from it. This means that the Q of the coil is effectively multiplied, providing far higher degrees of selectivity.

The improvement in selectivity can also be seen by understanding that the regeneration introduces a negative resistance element into the circuit. This means that the overall resistance within the circuit is reduced. As the Q of the resonant circuit is equal to the reactance divided by the resistance, the Q of the circuit is greatly increased, producing the visible improvement in selectivity.

In this way the regen radio overcomes many of the disadvantages of the basic TRF, and has a performance level that is not far short of the superheterodyne receiver in many aspects.

Operation of the regen receiver

The regenerative radio receiver requires a little more skill to operate it than the more normal superheterodyne receivers.

The regen has what is termed a regeneration or reaction control. This determines the degree of feedback introduced around the circuit.

The adjustment of the level of regeneration or reaction enabling the level of feedback to be controlled. The way that this is controlled along with the tuning enables the receiver to be used to receive different modes of transmission.

• AM reception:   For the reception of AM using a regenerative receiver, the feedback regeneration or reaction control is adjusted to provide the maximum gain without allowing the circuit to oscillate. Also the point just before oscillation may result in a little added distortion, so it may be necessary to back the control off very marginally for optimum reception. At this point the level of feedback not only provides additional gain, but also additional selectivity that is sufficient for most situations. It may be that under some circumstances exceedingly strong signals are audible across a wide section of the band.
• Morse / CW reception:   When using the regenerative receiver for the reception of Morse or CW signals, the level of feedback is adjusted so that the circuit oscillates. By tuning the receiver a few hundred Hertz away from the signal, the oscillation in the receiver mixes with the incoming signal to provide a beat note, thereby providing an intermittent audio tone as the Morse signal is turned on and off tor present the Morse characters.
• SSB reception:   For single sideband, SSB reception, the regenerative receiver again needs to be set into oscillation. This oscillation acts as a beat frequency oscillator / carrier insertion oscillator and reintroduces the suppressed carrier for demodulation. In this way the regenerative receiver is able to resolve SSB signals. Typically the receiver tuning will need to be adjusted to the correct side of the signal so that the signal sounds intelligible.

Regenerative receiver advantages / disadvantages

The regenerative radio receiver has many advantages that mean it was used in many applications for many years. However it also has some disadvantages that need to be remembered when considering it use.

Despite its disadvantages, the regenerative receivers till has some benefits, although as other types of receiver offer higher levels of performance and are easier to use. As a result the regenerative receiver is not in widespread use these days.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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