Figure 4. In some antennas, an array of radials is used.
length. For example, on car antennas, the ground plane is
the metal surface of the car surrounding the antenna. In
some antennas, an array of radials is used (see Figure 4).
Note that there are four or more conductors of λ/4 mounted at the base of the vertical segment of the antenna.
These may be wires on lower frequency antennas or short
horizontal conductors in higher frequency antennas.
In any case, the ground plane antenna is also resonant
at its frequency of operation. The resistive value of its feed
point is about 36 ohms. Normally, we use a 50-ohm coax
cable to feed the ground plane, as it is a close match and
works well. The radiation pattern for the ground plane is a
circle, meaning that it is omnidirectional and transmits and
receives equally well in all directions.
Common Antenna Types
Most radio applications use a dipole or a ground plane,
but there are many other special types of antennas. Almost all
of them are some variation of either the dipole or the ground
plane. A good example is the widely used Yagi. Take a look at
Figure 5. This antenna uses a dipole as the antenna, but parasitic elements are added to shape the radiation pattern.
The longer element next to the dipole — or driven element — is called the reflector. It actually does reflect (at least
redirect) some of the energy from the dipole in the forward
direction. The shorter element is called a director. It further
helps focus the radio wave in the forward direction. The end
result is that the dipole energy is concentrated and directed
in one direction, making the antenna directive. The radiation
pattern looks something like what you see in Figure 6.
The Yagi antenna has what we call gain. It actually
boosts the signal level because the electromagnetic energy is focused in one direction. For example, if your transmitter puts out one watt of power and you connect it to a
Yagi, the actual effective radiated power (ERP) may be five
times that — five watts. The receiver doesn’t know the difference between a plain old five-watt transmitter on a sim-
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