Nostalgia Air: Signal Tracing

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Antenna Post

If the receiver is tuned to a strong local station, the signal should be heard at this point.

Isolating and simplifying the input circuit, we have a circuit as shown in Fig. 2.

Fig. 2. First r.f. amplifier stage.

The secondary side of the antenna coil is a seris circuit consisting of inductance, capacitance, and a small amount of resistance. Series resonance is utilized to obtain maximum voltage across the condenser.

To understand that this is a series resonant circuit, we remember that a series circuit is one in which all the components are in series with the source. Here the source voltage is the e.m.f. induced in the secondary by the current in the primary. The cuircuit may be redrawn as shown in Fig. 3.

Fig. 3. The equivalent circuit of Fig. 2.

E_S is the voltage induced in the secondary coil. If the Frequency of E_S is varied, the current I becomes a maximum at resonance. The voltage E_L and the voltage E_C also reach maximum value at resonance. This is shown in the curve of Fig. 1.

Fig. 1. Resonance curves of the tuned circuit illustrated in Fig. 2.

   In the receiver, resonance is obtained not by carying the frequency but by tuning the resonant circuit to the frequency desired. For that setting of the tuned circuit, the desired signal will set up a greater voltage than is produced by any other signal having the same intensity but of a different frequency.

   When a voltage appears across the secondary of the antenna coil it means that there is a voltage across the primary also, and this is the voltage from antenna post to ground. Because resonance is established in the secondary rather than in the primary, the secondary circuit is a necessary factor in any discussion involving the primary voltage.

Trouble
   If no signal is heard it means that there is no voltage across the primary of the antenna coil. Usually this indicates a short circuit, either in the primary or in the secondary.

   It might be suggested that the primary could be open. To pusrsue this line of thought, the following experiment is instructive. Take any ordinary antenna coil, even a very early type, and connect it between the antenna and ground.

   If the probe is now applied to ANT, a signal will be heard. More than one signal may bve present if there are several strong local stations. The simple coil is not selective. It offers a high impedance to all voltages at radio frequency so that any r.f. voltage picked up by the antenna is available for amplification.

   In the average neighborhood there is also a 60-cycle field that sets up a voltage in the antenna circuit. At this low frequency the impedance of the antenna coil is very low and the 60-cycle hum voltage is usually neglible.

   Next, let us remove the antenna coil and substitute a resistor of high value, say one megohm. Now we have a high impdeance to ground but it is an impedance that is nondiscrimnating as to frequency. The small antenna currents set up IR drops across the resistance and these IR drops are independant of the frequency. Consequently, the voltage set up by any signal depends only on the field strength of that signal (assuming that the antenna itself is nonselective). Usually, it will be found that the 60-cycle field predominates and a strong hum will be heard. A powerful local station may be heard faintly through the hum and noise.

   Finally, we remove the resistor and thus create an open circuit between antenna and ground. The impedance at 60-cycles is now very great. At radio frequencies the impedance is relatively lower because the stray capacitances (and the inpu capacitance of the tester) offer a path to ground. Now the hum will be pronounced and no r.f. signal will be heard.

   From these experiments it can be understood that the operation of tuning is simply this: The antenna circuits are so adjusted that r.f. waves of the desired frequency set up a considerably greater voltage on the grid of the first tube than do wave of any other frequency. The tuning is done usually be means of a series resonant circuit inductively coupled to the antenna.