Antennas As Part of a Receiver: Part 1
An antenna that is too short will have a low efficiency. This efficiency can be redeemed in part by retuning the antenna. The tuning process often consists of a set of inductors and capacitors, the idea being to artificially shift the short antenna’s natural resonance to the desired frequency. As we have mentioned before, especially as in connection with the Chu limit, while the efficiency can be increased by tuning the antenna in this fashion, the bandwidth of the antenna will suffer.
There is, however, an interesting application of these narrow-band, artificially tuned short antennas. Though it is common to think of an antenna as at least semi-independent of the device it is used for, a narrow-band antenna can be incorporated into a receiver to tune it.
Historic Uses of Tuned Antennas: Direct Coupled Antenna
In the earliest days of radio, tuning was done almost exclusively with the antenna. The antenna circuitry would have a fairly high Q, which is a measure of the quality and hence selectivity of a resonance circuit, allowing its use for radio tuning. The circuitry would also tend to have a high output impedance — decidedly not 50 ohms. At the time, this was not a problem as the amplifier and detectors used then also had very high input impedances.
There were various methods of tuning the antenna in use by the early 1920s. A common method would be to attach the antenna to one side of a variable capacitor. The other side of the capacitor in turn was directly attached to one side of a variable inductor. The other side of the variable inductor was then attached to ground. Signal to the detector was picked off at the junction of the capacitor and inductor. Both the inductor and the capacitor were part of the tuning. The antenna was an integral part of the system; varying its length would also change the tuning of the receiver. Usually, the antenna was an inverted L. As the frequencies used in these early days were quite low (AM band or, often, even lower) the antenna by default was much shorter than would be the ideal length for low-frequency reception, and, as a result of the tuning, would have a narrow bandwidth. Such a circuit as described is still not very selective, prompting designers to use other methods of tuning and attaching the antenna to the radio.
It was known that the longer the antenna the more signal would be received, but, at the same time, the less selective the tuning circuitry would be. This is not surprising, considering that longer antennas are more efficient — and have better bandwidth. This better bandwidth in turn would wreck the selectivity of the radio tuning circuit.
Historic Uses of Tuned Antennas: Loosening the Reliance on the Antenna
Often a transformer was used to semi-isolate the antenna from the receiver. One lead of the primary of the transformer would then be connected to the antenna and the other lead to ground. Often the transformer primary was equipped with a multipoint switch in order to select the number of turns used. For high-end applications, a variable capacitor was also placed in series with the antenna.
The secondary of the transformer was usually shunted parallel with a capacitor to form a resonant circuit for tuning purposes. The secondary was then connected to the detector or, occasionally, an RF amplifier. The coupling between the primary and secondary of the transformer was often designed to be variable in order to loosen the coupling between the antenna and the tuning circuit on the secondary. The degree of coupling set both the selectivity and sensitivity of the circuit. A tightly coupled antenna would tend to spoil the Q of the tank circuit on the secondary, but would also allow more signal into the detector.
For the average receiver, in the end most antenna tuning circuitry simply consisted of a transformer with fixed coupling between the primary and secondary. The secondary was tuned, but there were only a couple of primary connections available to accommodate for the different sizes of antennas that could be used with the receiver. Such a circuit with fixed antenna coupling was particularly vulnerable to loss of selectivity with longer antennas.
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