Errors Due to Background Voltages

Author: J.B. Hoag

Often, extraneous or background voltages are induced in a simple loop antenna. These enter in equal strength from all directions. They, therefore, act in the same manner as the pickup voltages from a vertical antenna, as discussed in the preceding section. If the spurious voltages are in-phase with those of the loop, a response curve like that of Fig. 34 I (at the left) will be obtained.

Fig. 34 I. Effect of background voltages when in-phase (left) with the loop voltages, and at 90° (quadrature, on the right)

It should be obvious that the two minimal or zero intensities of reception are not diametrically opposite each other. In the figure, they are 15° off the vertical, on each side of the loop; but note that they are on the left of the vertical in both cases. In case the spurious and loop voltages are in quadrature (90°) (right figure of Fig. 34 I), the minimae will be opposite each other but will not be sharp. Then, the correct line of propagation will be obtained, but the accuracy-will be greatly reduced because the minimae are not sharp.

The in-phase errors are corrected by balancing the loop with, say, condensers, and by using an electrostatic shield, as in Fig. 34 J.

Fig. 34 J. A balanced and shielded loop

The out-of-phase voltages are frequently caused by the re-radiation of the energy of the oncoming wave from nearby conductors, which produce small currents, which re-radiate as feeble waves of the same frequency as that being received directly from the transmitter. With the loop installed in a given setting on an airplane or on a ship, compensating loops of wire are added to the main loops, nearby rigging is broken up with insulators, and, if necessary, a vertical antenna is added such that its voltages counteract the spurious ones. Finally, for 1° accuracy, the dial of the compass is calibrated by direct visual observation of the position of a nearby transmitter, located, for example, on a ship which circles the receiver.