RIM 1802G, Руководство интегратора

Страница: 61 / 76

Selecting an antenna
Integrator Guide 61
The voltage standing wave ratio (VSWR) is another way to express the ratio of incident power (from
the radio modem) to reflected power (into the radio modem).
VSWR equation
VSWR = 1 or RL = -
∞
dB is a perfect match. In practice, imperfections are inevitable, which means
that VSWR is greater than 1 and RL is a negative number.
VSWR and RL normally vary as a function of frequency.
Antenna size
The optimal antenna radiation efficiency is produced by an antenna measuring one wavelength, l.
The value of l for the RIM 1902G or RIM 1802G is calculated by dividing the speed of light c = 3 x 10
8
m/s by the center frequency.
Antenna lengths of
λ
/2,
λ
/4, and
λ
/8 also work well, and usually result in a relatively well-matched
antenna.
λ
/2 or
λ
/4 can be electrically “shortened” by adding load matching elements to control the
antenna match. However, this shortening reduces the antenna efficiency and, therefore, the effective
radiated power.
Selecting an antenna
The antenna is one of the most important components of a wireless communication system. The
proper antenna maximizes the coverage area of the RIM 1902G or RIM 1802G.
The antenna that you choose should suit your project’s needs. There are many different antenna
types and options that can meet your engineering and user requirements and remain within budget
constraints. RIM strongly recommends that you use an experienced antenna provider to realize the
highest gain possible. A well-designed antenna solution can maximize your application’s efficiency,
coverage area, and battery life.
Antenna manufacturers have designed and manufactured a wide variety of antennas for use on the
GPRS network and for other radio frequency (RF) systems that operate in the same frequency range.
RIM does not recommend specific antennas because the choice of antenna is application-dependent.
VSWR
=
+
1
P
P
1 -
P
P
reflected
output
reflected
output