(c)
Instantaneous voltage. The amount of voltage present at any given instant in an
electric circuit. Differences in instantaneous voltages, or the lack of differences, give an
antenna its directivity.
(d)
Magnitude. How big, or small, strong, or weak, something is. For our purposes
magnitude refers to the strength of an electric or magnetic field.
(e)
Plane parallel to the wavefront. This concept is the biggest single stumbling
block to discussion of wave theory the average uninitiated person finds. With reference to
the path of the wave, even though the wave front (first part of the wave) is spherical by the
time it has reached a reasonable distance from the transmitting antenna, the portion of the
wave we intercept with our antennas is assumed to be a straight flat plane. If the wave is
traveling toward us in a straight line the wave front is at right angles to the direction of travel.
Look at a wall. The line to the wall (or move exactly from the wall to you) is the wavepath, and
the wall is the wave front. If you were an antenna with your plane parallel to the wavefront,
you would be turned so that the wave front would hit both arms at the same time, if the wall
(wave front) went over you (the antenna).
(f)
Plane perpendicular to the wave front (Figure 2-3). The loop antenna has been
turned so that the incoming wave strikes one vertical arm before it strikes the other. If the
antenna face is at the true perpendicular the voltages in use in the receiver are strongest,
because the differences in the voltages in the opposing arms of the loops are greatest. This
is because the wave will be at different intensities (strengths) for each arm of the loop.
Figure 2-3. Plane perpendicular to the wave front.
(g)
Spacing. The physical distance between the antenna elements. Spacing is
usually measured in terms of the wavelength, which means that any antenna which is spaced
properly for one frequency will not necessarily be spaced properly for another. Spacing is
one of the principal limiting factors in determining the frequency range of a directional
antenna.
2-5
IT0302
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