Antenas

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ANTENNAS FOR LOW POWER APPLICATIONS
By Kent Smith

Introduction:
There seems to be little information on compact antenna design for the low power wireless field. Good
antenna design is required to realize good range performance. A good antenna requires it to be the right
type for the application. It also must be matched and tuned to the transmitter and receiver. To get the best
results, adesigner should have an idea about how the antenna works, and what the important design
considerations are. This paper should help to achieve effective antenna design.
Terminology:
Wavelength - Important for determination of antenna length, this is the distance that the radio
wave travels during one complete cycle of the wave. This length is inversely proportional
to the frequency and may becalculated by: wavelength in cm = 30,000 / frequency in MHz.
Groundplane - A solid conductive area that is an important part of RF design techniques. These
are usually used in transmitter and receiver circuits. An example is where most of the traces
will be routed on the topside of the board, and the bottom will be a mostly solid copper area.
The groundplane helps to reduce stray reactancesand radiation. Of course, the antenna line
needs to run away from the groundplane.
dB (decibel) - A logarithmic scale used to show power gain or loss in an RF circuit. +3 dB is twice
the power, while -3 dB is one half. It takes 6 dB to double or halve the radiating distance,
due to the inverse square law.
The Basic Antenna and how it Works.
An antenna can be defined as any wire, or conductor,that carries a pulsing or alternating current. Such a
current will generate an electromagnetic field around the wire and that field will pulse and vary as the
electric current does. If another wire is placed nearby, the electromagnetic field lines that cross this wire
will induce an electric current that is a copy of the original current, only weaker. If the wire is relatively
long, in termsof wavelength, it will radiate much of that field over long distances.
The simplest antenna is the “whip”. This is a quarter
wavelength wire that stands above a groundplane. The
most common examples are found on automobiles and are
used for broadcast radio, CB and amateur radio, and even
for cellular phones. This design goes back to the 1890's
when Marconi set out to prove that radio signalscould
travel long distances. To be successful, he had to stretch a
long wire above the ground. Due to the low frequencies,
thus a long wavelength, the wire had to be long. He also
found that the wire worked better when it was high above
ground.

Basic
Full-size
Whip:

1/4
wavelength

All antennas, like any electronic component, have at least two connection points. In the case of thewhip,
there must be a connection to a ground, even if the groundplane area is nothing more than circuit traces and
a battery. The whip and groundplane combine to form a complete circuit. The electromagnetic field is set up
between the whip and the ground plane, with current flowing through the field, thus completing the circuit.
Ideally, a groundplane should spread out at least a quarterwavelength, or more, around the base of the
whip. The groundplane can be made smaller, but it will affect the performance of the whip antenna. The
groundplane area must be considered when designing an antenna.
A quarter-wave whip is not a compact antenna. At 1 MHz, in the AM Broadcast band, one quarter of the
wavelength is about 246 feet, or 75 meters. At 100 MHz, in the FM Broadcast Band, it isnearly 30 inches
(75 cm). This dimension continues to shrink at higher frequencies, being nearly 3 inches (7.5 cm) at
1000 MHz. A simple formula for the quarter-wave (in cm) is: 7500 divided by the freq. (in MHz), or for
inches: 2952 / freq. (in MHz). This formula is only a starting point since the length may actually be shorter
if: the whip is overly thick or wide, has any kind of coating,...
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