Pendulum GPS-12, User Manual, Page: 51 / 62

considerations could force you to find a place for
the antenna situated far from the receiver and
high above ground level. We will discuss these
matters presently, but the implication in brief is
that new system components may have to be in-
troduced, like low-loss coaxial cables and line
amplifiers.
Coping with Interference
There are mainly two sources of interference
that may affect the location of the antenna.
They will be treated below. Careful inspection
of the proposed site can often reveal the most
apparent ones.
n
Multi-path Reception
Reflections from metal surfaces nearby can
cause the wanted signal to enter the antenna
from two or more directions with more or less
equal strength. Then the phase relationship de-
termines whether the resulting signal will be
amplified or attenuated. At worst it can vanish.
Since the GNSS satellites are not stationary the
conditions can be compared to the fading prob-
lem on the shortwave bands. The directive sen-
sitivity of the antenna resembles a hemisphere
with its base plane going horizontally through
the antenna, provided it is mounted vertically
on top of a mast. Therefore you must avoid lo-
cations near reflective surfaces beside or above
the antenna.
n
High-frequency Interference
Strong signal sources in the vicinity, like cellu-
lar radio transmitters, can cause interference or
saturation in the antenna amplifier. Even
though the systems do not operate at the same
frequency the problem has still been observed
from time to time and must not be neglected.
Standard Configuration
The majority of installations can do with the
standard configuration, consisting of one
satellite antenna, one 20-m antenna cable, and
the frequency reference itself, the GPS-12X. In
many cases, the distance between the antenna
and the receiver could be less than 20 m.
Normally excessive cable length is easy to tol-
erate. Do not shorten the cable, unless abso-
lutely necessary, because the high antenna gain
of 36 dB requires the signal to be attenuated by
at least 6 dB before entering the receiver.
Deviations from the Standard
Configuration
n
General Principles
The system configurations that cannot directly
use the standard options provided by Pendulum
can be listed in different ways. One simple
method of classification used here is to isolate a
number of cases and treat them separately. If
several steps have to be taken to arrive at a solu-
tion that satisfies all requirements, then it
should be possible to derive the individual
components from the list by means of combina-
tion. The method is illustrated by some practi-
cal examples later in this chapter.
The most important design parameter is the ca-
ble loss at the receiving frequency. There are a
number of cable types to choose from, but for
the sake of simplicity one normal and one
low-loss type have been selected:
Cable type Attenuation per 100 m
Standard (RG-213)
Low loss (LMR400) Approx 40 dB
Approx 17 dB
The common laboratory cable RG-58 has been
excluded, as its losses only allow relatively
8-3 Antenna Installation
Appendix