The antenna tuning was an important part of the tuning circuitry of early radio receivers. Furthermore, most antenna-tuned radios were not overly selective on their own, so a trend developed where a cascade of tuned RF amplifiers were placed before the detector for greater selectivity. Each of these amplifiers initially had its own tuning knob. By the mid-1920s, the average home AM receiver featured three tuning knobs. However, the knobs really could not be effectively coupled together for single-knob tuning.
While with the aid of tight quality control, any number of tuned RF amplifiers could be made that all tuned the same way simultaneously, the first tuning circuit — the antenna tuning — could not be controlled. After all, who knew what kind of antenna would be used with the receiver? And, since the antenna length defined the properties of the first tuning circuit, the best that could be done in many cases was to design a radio with two tuning knobs: an antenna tuning knob and a knob that tuned everything else.
An alternative fix was to tune everything with one knob, while equipping the radio with a trimmer capacitor(s) to allow the user to compensate for the antenna — and, perhaps, any irregularities in the rest of the tuning circuitry related to lack of quality control. The antenna trimmer knob, at least, would still need to be tweaked regularly by the user as the antenna circuit invariably refused to tune the same as the rest of the radio.
Isolating the Antenna
One possibility that presented itself to manufacturers of AM radios was that of isolating the antenna completely from the receiver. Various schemes presented themselves. One, beautiful in its simplicity, was to simply feed the antenna into an untuned amplifier that in turn fed the rest of the circuit. This simple scheme eliminated antenna concerns altogether; the antenna was left completely untuned and was isolated from the tuning circuitry. (Incidentally, now that the antenna was no longer tuned as part of the receiver, it could be made as long as desired without compromising the selectivity of the set.)
In the final tally, however, the most common solution was to heavily isolate the antenna from the receiver and, as an extra measure, the Q of the antenna tuning was lowered with the aid of a resistor connected between the antenna and ground or shunted across the secondary of the antenna transformer. This worked and was used for years, but there was a better way, which is still sometimes seen today.
Tuned Loops
A loop antenna can be used with a radio and tuned with a parallel capacitor; in fact, it lends itself quite readily to this application as it is naturally highly inductive. Though this phenomenon was known and even used, albeit rarely, in the 1920s, it was still uncommon largely due to poor antenna efficiency. A loop antenna of any reasonable size tends to be less efficient at AM frequencies than a long wire antenna. Furthermore, efficiency was critical as the RF amplifiers used in the 1920s had very low gain; the type 01A triode, a hallmark of most 1920s receivers, could only provide a gain of up to about 8 times, and was somewhat unreliable. For this reason, the antenna efficiency needed to be high for the average receiver to perform well.
As RF amplifiers were improved, however, the lower efficiency of the loop antenna could be made up with higher amplifier gain. This opened up a new possibility: the tuned loop antenna.
As the loop antenna could be incorporated into the receiver, its characteristics could be carefully set to match up with the rest of the tuning circuitry in the radio allowing for compactness and one-knob tuning. This method became common in the 1940s, and has been in use since.
These days, loop antennas are very small, high-Q devices usually wound around a ferrite core. However, one still can see these loop antennas working with a miniature tuning capacitor wired in parallel across them, quietly serving both as an antenna and as the preselector for the radio. And as such, these loop antennas are working both as the antenna and as a fundamental part of the radio receiver itself.
ARSI 