ANTENNAS FOR LOW POWER APPLICATIONS

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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, a designer 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 be calculated 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 reactances and 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



Antenna Characteristics:
Gain:
An antenna that radiates poorly has low “gain”. Antenna gain is a measure of how strongly the antenna
radiates compared to a reference antenna, such as a dipole. A dipole is similar to a whip, but the
groundplane is replaced with another quarter-wave wire. Overall performance is about the same. An antenna
that is 6 dB less than a dipole is -6 dBd. This antenna would offer one half the range, or distance, of the
dipole. Compact antennas are often less efficient than a dipole, and therefore, tend to have negative gain.


Printed Circuit Whip, or “Stub”
The whip can be made as a trace on a printed circuit board (PCB). This is very practical at frequencies over
800 MHz. At lower frequencies, a full size whip may be too long, even when wrapped around a few
corners. The length of the whip should be 10 to 20% shorter than the calculation, depending on the
dielectric and the thickness of the board. In most cases, 15% shorter is close enough. If the unit is to be
hand held, the antenna can be made a little shorter, to compensate for the effect of the hand.
long, in terms of wavelength, it will radiate much of that field over long distances.