| MadSci Network: Engineering |
Greetings:
Reference: Y.T. Lo, S.W. Lee, Antenna Handbook,
Van Nostrand Reinhold Co., New York, 1988.
There are many different types of antennas and you did not describe
the one that
you are using so I will discuss the most common types in use today.
The band of frequencies (bandwidth) covered by an antenna is generally
specified as
the bandwidth between the one half power points (also called the minus
3 decibel or -3dB
points). The antenna can operate beyond the half power points;
however, the signal might
be to weak. The antenna is usually tuned to be optimum in the middle
of the bandwidth. In
your case that would be 420 MHz. The wave length in meters is equal to
300 divided by
the frequency in megahertz (MHz). For your antenna the midband
wavelength is
300/420 = 0.714 meters or 71.4 centimeters (28.12 inches).
The most common antenna in use with portable communications sets is
the
quarter wavelength monopole antenna (whip antenna)fed by a
transmission line at one
end. The length of a monopole antenna of this type would be 71.4 cm
divided by 4 =17.9
cm (4.5 inches).
A second common antenna is the quarter wavelength monopole on a
ground plane. A ground
plane is a disk shaped metal plate at the base of the monopole. The
transmission line shield
is connected to the ground plane and the center conductor passes
through the center of the disk and
is connected to the monopole. This antenna would also be 17.9 cm high
and the ground
plane disk might be about 17.9 cm in diameter.
The third most common antenna is the half wavelength dipole.
The common
rabbit ears antenna is similar to this type of antenna. The dipole
antenna consists of two
collinear quarter wavelength long rods fed by a transmission line in
the center. This
antenna would be 2* 17.9 = 35.8 cm (14 inches ) long.
Various versions of these antennas might have the rods formed into
cones and disks
(discone antennas), with the points of the cones at the
transmission line. The
cones and ground planes are often wire grids instead of solid metal to
reduce the weight
and wind resistance.
From the reference book we find that molopole antennas have a 16%
bandwidth,
monopoles with a ground plane have a 40% bandwidth and a dipole has a
34% bandwidth.
If your 420 MHz antenna is a monopole, 16% gives a - 3dB bandwidth of
67.2 MHz or 386.4 MHz to
487.2 MHz. So now you can see why your antenna covers the 406 to 416
MHz band. It is
well with in the - 3dB bandwidth of a monopole and the reception is
probably less than minus
one dB down from the 420 MHz operation. One dB loss is not perceptible
to human ears and -3dB
is only slightly noticeable.
An optimum quarter wave antenna at 410 MHz would have a length of 300/
(410 *4) =
18.3 cm which is only 4 millimeters longer than an optimum 420 MHz
monopole.
You might ask why do we have so many different designs of antennas and
the simple
answer is that different antennas , such as the dipole, have greater
effective capture areas
and intercept more power from a passing radio wave than the simple
monopole antenna.
The cone shaped antennas have greater bandwidth , often as high as
100%. Also different
antenna designs have radio beams in different directions which produce
antenna gain. Antenna gain
is discussed in the Mad Science Archives.
Best regards, Your Mad Scientist
Adrian Popa
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