Open Communication
You can get a coax cable
with a 75-ohm characteristic
impedance that is a very
close match to the antenna’s
impedance. RG-59/U is one
example, as is RG-6/U,
which is commonly used in
cable TV systems.
As it turns out, most
people use a 50-ohm coax
with a dipole because it is
the most common impedance used for inputs and
outputs on receivers and
transmitters. There are lots
of different types of 50-ohm
cable, like RG-58/U, RG-
8/U, and RG-213/U. There are many
others. Even with the difference
between 50 and 75 ohms, this mismatch doesn’t matter that much. As
it turns out, the impedance of the
dipole at resonance varies considerably with the height above the
ground. The 73-ohm value doesn’t
really become the actual impedance
until the antenna is over one wavelength above the ground. This is pretty high at the lower frequencies and
is rarely attained. That is why the
antenna is such a good match to the
common and cheap 50-ohm cable.
The dipole is actually a directive
antenna. That is, it does not radiate
the electromagnetic waves equally in
all directions. The radiation pattern is
actually a figure-eight pattern, as
shown in Figure 2. Looking down on
the antenna from above, the maximum horizontal radiation pattern is
at a right angle to the line of
the antenna wire. If you can
think in 3-D for a minute, visualize that figure eight as a
doughnut-shaped pattern.
Looking at the pattern, there
is actually very little radiation
from the ends of the antenna.
Now you can see why it is
important to orient the antenna for the desired direction of
transmitting or receiving.
Incidentally, if you mount
the antenna vertically, its antenna pattern is a circle. We call
this omnidirectional. It radiates
Figure 2. The radiation pattern is actually a
figure-eight pattern.
λ/2 = 468/fMHz = 468/10 = 46. 8 feet
Each section of the dipole then is
one-half that length or 46.8/2 = 23. 4
feet or about 23 feet and five inches.
This antenna will be resonant to
10 MHz. It acts just like a resonant
inductance-capacitance-resistance
(LCR) circuit. At the operating frequency, it appears to be a resistance
of about 73 ohms. It does have a certain amount of bandwidth, but — if you
stray too far from the resonant point —
the antenna begins to look like complex impedance that is a combination
of resistance and reactance, capacitive at lower frequencies and inductive
at the higher frequencies.
You also need some way to connect the antenna to the receiver or
transmitter. This is done with a transmission line. The most widely used
type of transmission line is coax cable.
or receives equally well in all directions. This is a pretty tricky thing to do
physically with long wire antennas, so
this technique only works for short
antennas used at high frequencies.
That brings up one more key
antenna fundamental: polarization.
Polarization is defined as the direction of the electric field with respect
to the Earth. If the dipole is mounted
horizontally, the polarization will be
horizontal. Mounting the antenna
vertically produces vertical polarization. Ideally, you want both the transmitting and receiving antennas to
use the same polarization for maximum signal strength, but, as it turns
out, as the wave travels over long distances it is rotated a bit, so you can
still get reception of a vertically
polarized signal on a horizontal
antenna and vise versa.
Another common antenna type
is the quarter-wave (λ/4) ground
plane (see Figure 3). What it
amounts to is half of a dipole, mounted vertically. The center conductor of
the coax attaches to this vertical conductor while the shield of the coax
connects to ground. The ground (or
Earth) acts like the other half of the
dipole. The nice thing about a ground
plane is that the length of the antenna is half that of the dipole. This is a
big deal at the lower frequencies.
For example, the length of a
dipole for an AM radio transmitter at
650 kHz (0.65 MHz) will be:
λ/2 = 468/fMHz = 468/0.65 = 720 feet
Figure 3. Another common antenna type is the
quarter-wave (l/4) ground plane.
The actual formula for a quarter
wave ground plane is:
λ/4 = 234/fMHz = 234/0.65 = 360 feet
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Note, if we use a quarter-wave ground
plane, the length is only 360 feet.
Think of the cost difference between
a 720-foot tower and a 360-foot tower
— big difference.
In most applications, the Earth
is not used as the other side of the
dipole. Instead, at the higher frequencies, we often use a large metal area
— larger than one-quarter wave-
APRIL 2005
