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DA25 • 7
one radio frequency (or RF) cycle. Since the charge traverses the wire twice,
the length of wire needed to permit the charge to travel the total distance in
one cycle is λ / 2, or one half the wavelength. Therefore, the shortest resonant
wire length will be one half wavelength long.
Let’s consider a “half wavelength” example to help it make sense. Picture a
trough with barriers at each end. If a rubber ball is rolled along the trough from
one end to the other it will hit the end and bounce back. When it bounces
back, it will hit the near barrier and bounce again. This will continue until the
ball runs out of energy and stops. If however, whenever the ball returns to the
near barrier it is given a push just as it starts away, its back and forth motion
can be kept up indefinitely as long as the impulses are timed properly. In other
words, the rate or frequency of the impulses must be adjusted to the length of
travel and the rate of travel. If the timing of the impulses (the push) and the
speed of the ball are fixed, the length of the trough must be adjusted to “fit”. In
the case of the antenna, the speed is constant. This leaves the alternatives of
adjusting the frequency or the length of wire to match a given frequency.
Antenna Gain
Another performance specification common with
antennas is their gain, usually given in dBi units. To
understand this concept, let’s explore the “i” in the dBi
unit as an isotropic source. Imagine a point in space as
a source of a radiating signal. The signal would then
expand spherically from the point source. If we then
move a given distance from the point source, the power
would be distributed uniformly in all directions. The
power density is uniform about an isotropic source and
thus is related to the surface area of a sphere (area =
4x π x radius
2
). Although this is not practically
possible, it is the basis for an antenna gain specification. The gain of an
antenna is usually referenced in comparison to this type of source in a decibel
unit with a logarithmic relationship. Without getting hung up too much on
logarithmic theory, suffice it to say that an increase of 3 dB is equal to twice
the power being present . An increase of 10 dB is equivalent to a gain factor of
10. For example, a 1 Watt signal with a gain of 3 dB equals two Watts, while
the same power with 10 dB gain is 10 Watts.
Although we are using our discone as a receiving antenna, the rules of
antenna gain are reciprocal so we can count on at least a 3 dBi improvement
in the signal power over the entire frequency range.