MDS 05-3438A01, Rev. F
MDS 4790/9790 Series I/O Guide
9
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 improp-
erly aligned with the companion site, will result in poor performance or
no communication at all.
Generally speaking, an omni-directional antenna (
Figure 5
) is used at
the master station site. This type of antenna provides equal coverage to
all of the remote sites. At remote stations, directional antennas such as
Yagis, are typically used.
Figure 5. Typical Omnidirectional Antenna for Master Station
(Shown mounted to mast)
Feedline Selection
For maximum performance, use a good-quality feedline to connect the
radio to the antenna. For short-range transmission, or where short
lengths of cable are used (up to 8 meters [26 feet]), an inexpensive coax
cable such as Type RG-213 might 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 pro-
duce only 1.6 watts at the antenna; a similar loss in receiver sensitivity
would result, and no amount of receiver gain can recover the signal lost
in the feedline.
Conversely, 100 feet (30 meter) of 1-5/8 inch cable has a loss of 0.52 dB
at the same frequency, but its cost is considerably higher than
RG-58A/U.
For systems covering short distances, feedline loss is relatively unim-
portant, and 6 dB or more of loss might be 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
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