Antennas for Amplitude-Modulation Radio Receivers

Vertical Straight-Wire Receiving Antennas. The oncoming radio signal wave, which is vertically polarized in amplitude-modulation broadcast reception, induces a signal voltage in each elemental length of the antenna as shown in Fig. 15(a). These voltages add vectori-ally¹⁸ and produce a current flow as indicated in (b). The current in the primary of the antenna coil induces a voltage in the secondary which is the voltage impressed on the tuned-radio-frequency amplifier circuit previously discussed.

Loop Receiving Antennas. Many amplitude-modulation broadcast receivers utilize built-in loop antennas. There are many advantages to this, including the possibilities of noise reduction (Chapter 14).

Figure 15. The oncoming radio-frequency signal waves induce voltages in each unit length of antenna wire. The vector sum of these voltages is assumed to be concentrated at one point, and it forces an alternating signal current through the antenna coil of the radio-receiving set as shown by the arrows in (5). This current induces a signal voltage in the secondary of the antenna coil.

Whether the loop receives signal because the vertical wires (or parts thereof) are parallel to the arriving electric lines of force or because the plane of the loop is perpendicular to the arriving magnetic lines of force is a common question, and one which is discussed in reference 14 (see also page 576). The signal voltage induced in the wire loop causes a current to flow around the loop which is tuned to series resonance with the capacitor (Fig. 16).

The voltage drop across the capacitor is the signal voltage to be amplified (page 499). On page 500 it was explained that a superheterodyne radio-receiving set contained a tuned-radio-frequency amplifier. In many superheterodynes this amplifier is omitted, and a tuned circuit or the tuned loop is used to give some initial selectivity. In very inexpensive (and often unsatisfactory) superheterodynes all tuning is omitted preceding the frequency converter. Such sets are bothered with the reception of image frequencies and with cross modulation. If the tuning ahead of the converter is omitted and if a 1,000,000-cycle station is being received, a station outside the broadcast band on an "image frequency" of 1,910,000 cycles will also be received. Cross modulation¹² is a form of interference caused by the modulation of the desired carrier by an undesired signal.

Figure 16. Simplified circuit of a loop antenna. Many turns of wire are used at low radio frequencies.