Vicom Spectracom Instruction Manual
Technical Note:
Outdoor GPS Antenna Installation Considerations
www.spectracomcorp.com 1 | Timing & Synchronization Technical Note
Many Spectracom timing and synchronization products utilize an embedded GPS receiver as a primary reference. Typical
installations utilize an outdoor active GPS antenna to collect and transmit GPS satellite radio signals to the receiver via an RF cable.
This technical note describes the main considerations for a successful outdoor GPS antenna installation for a timing application:
•Locating the antenna
•Connecting to the antenna
•Using ancillary products
•Evaluating signal attenuation to validate cable length
•Using signals in addition to GPS L1
Locating the GPS Antenna
The GPS antenna must have a direct view of GPS satellites with an unobstructed line of sight to the sky. Rooftops that are clear of
other structures or geographic features overhead, with views to the horizon, generally make good installation locations. Such a clear
view allows the antenna to track the maximum number of satellites throughout the day. Installations with obstructed views may
experience reduced reception quality and may not be able to simultaneously track the maximum number of satellites. An
observation angle of 70° from the vertical axis (20° above the horizon) usually offers good performance. Contact Spectracom if your
antenna location’s view of the sky is restricted for mitigating approaches.
When installing your GPS antenna, select a site at which the antenna will not become buried in drifting or accumulated snow. It
should not be covered by foliage or placed in a position where it could become obstructed in this way. Whenever possible, avoid
placing the GPS antenna in close proximity to broadcast antennas or high power transmitters.
Connecting to the Antenna
A coaxial cable transmits GPS signals from the antenna to timing receiver. It also conducts 5 volts from the receiver to power the
antenna and other elements of the antenna system if necessary. The choice of cable is on the basis of attenuation characteristics,
weathering ability, temperature rating, and UV resistance. Type N female connectors are used on the antenna and any other
element in the antenna cable system, therefore male connectors terminate cables. While Type N female connectors are used on
many receivers, smaller form factor products can require the use of alternative connectors. Therefore a different connector may
need to terminate the cable to the receiver or an adapter can be used. The application of weather proofing sealant or tape is a good
idea for all outdoor connections.
www.spectracomcorp.com 2 | Timing & Synchronization Technical Note
Examples of inline amplifier and
surge suppressor
Do not allow the antenna cable to be placed in standing water, as water may permeate through the coax jacket over time. On flat
roof installations, the cable may be suspended by cable hangers or placed in sealed conduit. GPS antenna cable can be installed
through a conduit either by itself or with other cables. The conduit dimensions must be large enough to accommodate the “N” type
connectors, which have a diameter of about 0.8 inches (20 mm), and the GPS cable itself, which has a diameter of about 0.4 inches
(10 mm) for LMR400 equivalent. Using a conduit that is too small or whose bend radius is too tight will prove difficult during
installation (and may actually damage the cable). The maximum bend radius of the antenna cable can be as little as 1 inch (25 mm)
and as large as 4 inch (100 mm) depending on the type of cable.
Using Ancillary Products
Several items are available to improve the functionality and reliability of a GPS antenna system. You can leverage the installation of a
single GPS antenna across several GPS devices by the use of an RF splitter. A surge suppressor is strongly recommended to protect
indoor equipment against lightning damage. Install a surge protector and properly connect it to earth ground at the point where the
antenna cable enters the building. It is best to calculate cable distances from the antenna to the surge suppressor and surge
suppressor to receiver so two connectorized cables can be ordered at the proper length. This saves time and eliminates the risks of
splicing cable and installing connectors in the field.
For installations that require a long cable run, an inline amplifier can be used to ensure
signal levels at the receiver. The amplifier should be placed at a point where the
antenna gain has been reduced to 10 db. It should also be placed a minimum of 5 ft
(1.5 m) from the surge suppressor to protect it from lightning damage.
All ancillary items used in the GPS antenna cable system are required to pass 5 volts
from the receiver to the antenna (nominally 27 ma). This power can be used for other
items in the system.
www.spectracomcorp.com 3 | Timing & Synchronization Technical Note
Evaluating Signal Attenuation to Validate Cable Length
The question of maximum cable length is dependent on the sum of the gain (and losses) of all the items in the GPS antenna system
and the tolerance for minimum gain required for reliable operation of the receiver. A good way to evaluate these considerations is
the following equation:
Antenna Gain −Cable Loss −1 db/Surge Suppressor −0.5 db/Connector 20 db/Inline Amplifier ≥Minimum Receiver Gain
Gain of antennas commonly used by Spectracom timing systems:
Model Gain
8230 40 db
ANT-35 35 db
8225S 33 db
8225 30 db
Coax loss is proportional to length. Typical attenuations of Spectracom supplied cable at the GPS L1 frequency 1575 MHz are:
Model Type Attenuation
per 100 ft
Attenuation
per 10 meters
CAL7xxx LMR400 5.5 db 1.7 db
KX4NN-xx RG213 10.0 db 3.5 db
KX15NN-xx RG58 27.6 9.2 db
15 db is a typical value for minimum receiver gain. This value is slightly dependent on the receiver and the quality of the antenna
location (clear view of the sky, free from interference, etc.). At the same time, a receiver could become saturated if there is not
some attenuation of the signal from the antenna. Again, depending on the receiver, the max gain at the receiver will be in the range
of 26 to 33 db.
GNSS Signals, Beyond GPS L1
GPS antenna systems may need to accommodate a variety of signals as required by the receiver capability. For instance authorized
users of SAASM receivers utilize signals at different frequencies (L1 and L2). While there had been limited alternatives in commercial
applications, the proliferation of Global Navigation Satellite Systems (GNSS) now offer the benefit of redundancy by receiving signals
from different systems and at different frequencies. A common application today is to combine GPS L1 signals at 1575 MHz with
GLONASS L1 signals centered at 1602 MHz. Spectracom-provided antenna systems support combinations of L1 and L2 or
GPS/GLONASS L1. Other antenna systems should be checked for compatibility with the required range of GNSS signals.
For More Information Contact:
USA | 1565 Jefferson Road, Suite 460 | Rochester, NY 14623 | +1.585.321.5800 | [email protected]
FRANCE | 3 Avenue du Canada | 91974 Les Ulis, Cedex | +33 (0)1 64 53 39 80 | [email protected]
UK | 6A Beechwood | Chineham Park | Basingstoke, Hants, RG24 8WA | +44 (0)1256 303630 | [email protected]
October 15, 2013 – TN07-101 (D)
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