Specifications

MDS 05-3438A01, Rev. E MDS 4790/9790 Series I/O Guide 9
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, a good-quality feedline must be used 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 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 watts 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 meter) length 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 may be completely acceptable. For sys-
tems 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.