Range Consideration
There are a number of factors that can affect the range of the wireless modems, including:
•
The environment or space between the modems
•
The types of antennas used.
P/N 1815-760 REV A 10/07
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Fresnel Zone
•
The mounting of the antennas.
Environment
We found that up to about 500 feet, a visual line-of-sight
worked fine with the standard dipole antennas that come
with the wireless modems. While visual line-of-sight may
be fine for many applications, better range can be
achieved by observing the Fresnel Zone – the football
shaped tunnel between the two sites that provides a path
for RF signals.
If the antennas are mounted just barely off the ground, the earth will obstruct half of the Fresnel zone.
Buildings, trees (that may grow later) or any other obstructions will further decrease the
communication range. For optimum range, set up the antennas to allow a free and clear Fresnel
zone. The diameter of the Fresnel zone is typically 16-feet when the antennas are 1000 feet apart.
Antenna Types
We suggest using high-gain antennas
(DoorKing P/N 1815-741) when a range of
greater than 500 feet is desired. High-
gain antennas focus the RF energy if a
particular direction. How much energy is
focused in the particular direction is
referred to as antenna gain. A flashlight
whose beam can be adjusted by twisting
the lens illustrates this.
A low gain antenna cannot receive signals from a long distance, but they can receive signals from a
wider area. High-gain antennas, on the other hand, can receive signals from a longer distance, but
the area they receive them from is smaller.
Use high-gain antennas in applications where longer range and more focused transmissions are
required.
•
P/N 1815-741 High-gain Antenna Kit (includes 20-feet coax cable).
Antenna Mounting
When mounting an antenna, care should be taken to make sure it is as far away from metal objects
as possible. If the antenna is mounted too close to nearby metal, the metal has the potential to
interfere with the way the antenna radiates the RF signal and thus decrease the range.
In some cases, a coax cable must be used to connect the antenna to the wireless modem. All coax
cables add losses to the system. For any given cable, the longer the cable, the greater the loss
becomes. Often, a longer serial cable to the DoorKing system or a longer USB cable to the PC can
be used to minimize the length of the coax cable.
