Antenna And Feedline Selection; Antenna System; Feedline Selection - GE MDS 4790M Installation And Operation Manual

10
1. Systems installed in lightly populated areas are least likely to encounter
interference; those in urban and suburban environments are more likely to
be affected by other devices operating in the 400 MHz frequency band and
adjacent services.
2. Directional antennas should be used at the remote end of the link. They
confine the transmission and reception pattern to a comparatively narrow
beam, which minimizes interference to and from stations located outside the
pattern.
3. If interference is suspected from another system, it may be helpful to use
antenna polarization that is opposite to the interfering system's antennas. An
additional 20 dB (or more) of attenuation to interference can be achieved by
using opposite antenna polarization.

3.2 Antenna and Feedline Selection

Antenna System

The antenna system is perhaps the most crucial part of the system design. An
antenna system that uses poor quality feedline, or is improperly aligned with the
companion site, will result in poor performance, or no communication at all.
Generally speaking, an omni-directional antenna is used at the master station
site and the remote radios use directional antennas.
GE MDS can also furnish antennas for use with the radio. Consult your GE
MDS representative for details.

Feedline Selection

For maximum performance, a good-quality feedline must be used to connect
the radio to the antenna. For short-range transmission, or where very short
lengths of cable are used (up to 8 meters [26 feet]), an inexpensive coax cable
such as Type RG-213 may be acceptable.
For example, 100 feet (30 meters) of RG-58A/U cable (commonly used for
frequencies below 100 MHz) has an insertion loss of 5 dB at 450 MHz. A 5
watt transmitter operating into such a feedline would produce only 1.6 watt at
the antenna; a similar loss in receiver sensitivity would result, and no amount
of gain within the receiver can recover the signal lost in the feedline.
Conversely, a 100 foot (30 meters) length of 1-5/8 inch cable has a loss of 0.52
dB at the same frequency, but its cost is many times greater than RG-58A/U.
For systems covering short distances, feedline loss is relatively unimportant,
and 6 dB or more of loss may be completely acceptable. For systems designed
for maximum range however, each dB of loss directly affects signal-to-noise
ratio at the receiver. It is good practice to keep feedline losses as low as
possible and certainly under 3 dB. Remember that for each 3 dB of feedline
loss, half the transmitter power is lost, and twice the receive signal power is
needed to produce the same signal-to-noise ratio.
The RG-8A/U cable is a widely available and inexpensive feedline that is
suitable for systems with short ranges or those with short feedlines. For longer
feedlines and lower losses, Andrew HELIAX
similar products are a good choice. Table 2 shows the length of various types
of cable and the resulting degradation in signal strength.
Mount the antenna and feedline securely to the supporting structure to avoid
damage from wind and ice loading. Refer to the instructions provided by the
antenna and feedline manufacturers to ensure a safe and reliable installation.
MDS 4790M (MPT 1411) I/O Guide
™
semi-rigid coaxial cable or
MDS 05-3628A01, Rev. B