Open Communication
home brew. It is a never-ending battle.
For you shortwave listeners, antennas are also critical. Although you can
often get by with short inside antennas
if you have a good receiver for just listening. If you have ever tried an outside
antenna, you know that the longer and
higher it is, the better the reception. If
you are into scanners, you know that
an outside antenna up high is the ticket to maximum coverage.
Today, most of the antennas we
encounter are built-in. We have car
antennas, cell phone antennas, and
those in family radios and CBs. If you
use any wireless networking products
— like Wi-Fi in a laptop — the antennas are built in, but are just as important. And just try to get a good satellite TV signal without a dish antenna
pointed in the right direction. Anyway
— and pardon the pun — you get the
picture. That seemingly worthless
collection of wires, conductors, and
copper patterns are the key to good
communications.
You can also calculate wavelength in
feet using the formula:
However, if the antenna is wire or
some other thin conductor, this has to
be modified to:
λ = 984/fMHz
λ/2 = 468/fMHz
The dipole has a length of one-half
wavelength, or λ/2. This length for wire
antennas is computed with the formula:
λ/2 = 492/fMHz
Let’s say you want an antenna
to receive the WWV time signal at
10 MHz. A dipole for that frequency
will be:
Antenna Basics 101
Most antennas are some variation
of the basic antenna that we call a
dipole. A dipole is a pair of conductors
that, together, are a half-wavelength
long at the operating frequency (see
Figure 1). A wavelength is the distance
between adjacent peaks or troughs of
the radio wave fields. This distance is
dependent upon the frequency of the
signal. You can calculate wavelength
represented by the lowercase Greek
lambda — λ — with this formula:
λ = 300,000,000/f
The 300,000,000 is the speed of
light in meters and f is the operating
frequency in Hz. Since our antennas
are mostly used at the higher frequencies, we can use the formula:
λ = 300/fMHz
The wavelength is in meters. For example, a 150 MHz signal’s wavelength is:
λ = 300/150 = 2 meters
APRIL 2005
Circle #41 on the Reader Service Card.
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