From 1896 when Guglielmo Marconi invented the wireless telegraph until 1901 when he added a tuned circuit to the receiver the radio receiver was just an antenna connected to a wire which went to a coherer detector. After Marconi added a tuned circuit the receiver had a little selectivity. It was not until 1906 when Lee de Forest added the grid to the diode and the receiver was able to have something more then just passive devices. With the grid RF amplifiers could be added before the detector and audio amplifiers after the detector which was a great improvement.The tuned radio frequency (TRF) radio became the receiver of choice. The problem was that each stage had to be tuned separately and the tubes had to have low amplification or they would oscillate. The TRF remained the only way to add amplifiers ahead of the detector until Edwin Armstrong conceived the idea of superhetrodyne during WW1.
In 1914 Edwin Armstrong, an amateur radio operator, patented the regenerative detector. This was a real break through in radio receiver progress.
The regenerative receiver uses a positive feedback which greatly improves both the sensitivity and the selectivity. In the earlier days the receiver had, compared to today’s receivers, a very large coil for the input tuned circuit which fed the grid. It had a. coil of just a few loops of the wire in the place circuit. The coil in the plate circuit, called a tickler coil, would be physically moved in the proximity of the input tuned circuit until the positive feedback was almost enough to cause oscillation when receiving an AM station and it was adjusted just into oscillation to receive a code station. The oscillation would produce a heterodyne, two signals mixed in a nonlinear circuit which result in an output of the sum and the difference of the two input frequencies, which could be heard as a tone this allowed true CW to be used. Up to then only MCW could be used.
The gain of a regenerative receiver using just one tube can be up to about 20,000 and if a MOSFET, not developed until many years later, is used a gain of 100,000 is possible.
Then in 1922 Mr. Armstrong patented the Super-Regenerative receiver. The super-regenerative detector uses a “quenching” frequency usually about 20 KHz to 30 KHz though other frequencies are also used. Using the quenching frequency to quench or stop the detector’s oscillation there is no need to adjust the tickler coil. This makes the receiver easier to operate and increases the sensitivity. Because they are inexpensive and easy to make these receivers are still being built and used though these are usually kits.
One of the biggest disadvantage of the regenerative and the super-regenerative receivers is they radiate a lot of noise over a wide area of the frequency on which they are being used. Back in the Sixties Heath Kit had a little 10, 6, and 2 meter transmitter receiver kit called the Tenner, Sixer, and Twoer. These units used a super-regenerative receiver. In 1965 I had a twoer installed as a mobile. That was the year the United States Coast Guard transferred me from Brownsville, TX to Coast Guard Radio Station New Orleans (NMG). My wife and I were driving around Metery (just across the river from New Orleans) looking for a place to live. We had our infant daughter with us. The Metery Police at that time were using 150 MHz radios. As we were driving I saw a police car sitting on the side of the road. I was very careful to obey all laws as I passed him. I came to a full stop at the stop sign. And when I pulled out from the stop sign he followed me and shortly on came the blue lights (I had never seen blue lights before but I guessed they meant the same as the red ones) so I pulled over. He stopped right behind me and one officer got out of one side of the car and the other from the other. They approached the car with hands on their guns, though they were still holstered, and one grabbed my door with his free hand and the other grabbed my wife’s door. The questions started flying, who was I, why was I there, and finally what is that radio. It took years before I understood why they pulled me over. My tower opened up their squelch when I passed by and again when they came up behind me and they wanted to know why.
Just as a little added note. Erwin Armstrong invented the FM transmitter and receiver a few years after the super-regenerative receiver.