Regenerative and Reflex Receivers
The antenna of a radio receiver is supposed to receive energy from passing radio waves but is not supposed to radiate or send out radio frequency energy. Radiation of energy is presumed to take place only from the aerials of transmitting stations. Yet a majority of radio receivers in use are capable of radiating energy which hampers or completely spoils the reception of other receivers within a wide radius.
Any feedback of energy from receiver circuits which are in an oscillating condition will cause radiation when this feedback reaches the antenna circuit. The antenna is tuned more or less closely to the frequency to which the receiver is tuned and the antenna then radiates this frequency.
If the antenna of a radiating receiver sent forth only the frequency to which the receiver is tuned, things would not be so bad. But it almost invariably sends out at least two frequencies because of the two points of resonance that exist in coils which are quite closely coupled. Not satisfied with radiating two frequencies the receiver will also send out harmonics of the received frequency, these harmonics being at twice the received frequency, three times the received frequency, etc.
A receiver does not radiate sufficiently to cause harm unless one or more of its tubes are oscillating. The oscillating condition is brought about by pushing regeneration or "volume" too far so that regeneration gives way to oscillation. It is fortunate that a receiver other than a superheterodyne operated with any tubes oscillating will not give satisfactory reception to its operator. If the operator is sufficiently experienced to recognize the cause of his own trouble, he will take steps to stop the oscillation provided his receiver has the necessary control over oscillation. About the only type of receiver from which radiation cannot be prevented is the superheterodyne. This is partly because the oscillator tube, which must oscillate to operate the receiver, is coupled almost directly to the antenna.
A superheterodyne operated with a loop antenna does not do a great deal of harm with its re-radiation because the loop is an inefficient radiator and its radiated energy travels for only a few yards in any direction. Regeneration in the loop of a superheterodyne or any other receiver makes this re-radiation reach far enough to bother at least some of the neighbors. A superheterodyne operated with an outdoor antenna makes itself a nuisance to all other receivers within a considerable distance.
Regenerative receivers, especially those of the single-circuit variety, are among the worst offenders in the matter of re-radiation. Any receiver which uses regeneration in the tube immediately following the antenna will re-radiate badly when regeneration is carried so far as to cause oscillation. A stage of radio frequency amplification which is properly balanced and placed between the antenna and the tube using regeneration will quite effectively prevent re-radiation. This is one of the chief advantages of properly built and properly balanced Neutrodyne, Browning-Drake, Roberts and similar balanced receivers. But when these receivers are not properly balanced and kept balanced they are as bad as any others.
The easiest way to locate a distant station on the dials of a regenerative receiver is to turn the regeneration control to a point which causes oscillation, then to rotate the tuning dials until the carrier wave of the desired station causes a heterodyne whistle with the oscillations of the receiver. Regeneration may then be brought about by stopping oscillation and the station will be received satisfactorily. But during the process of locating the whistle, the oscillating receiver is acting as a transmitter and spoiling the reception of neighbors operating their receivers near that frequency.
Whether a certain receiver re-radiates may be determined by a simple test with the help of someone within a short distance who also has a receiver. The two receivers are tuned to the same frequency, tuned to receive the same station at the same time. The regeneration control of the receiver to be tested is then set at its highest point to produce maximum regeneration. The tuning dial or dials are then turned back and forth across this setting a number of times. If the other receiver gives vent to a series of whistles and squeals as the dials are turned on the first one, the receiver being tested is re-radiating and is capable of causing much interference.
Whistles and squeak heard in a receiver may originate either in the same receiver or in others which are re-radiating. If the pitch of the whistle rises and falls while the tuning dials and other controls remain unchanged, the interference is coming from another receiver. But if the whistle remains at exactly the same pitch until the receiver controls are moved and then rises and falls with movement of the controls, it indicates that the receiver being tested is oscillating and is undoubtedly re-radiating.