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Many of our products,
such as our GPS10RBN GPS disciplined, rubidium frequency
standard use a GPS antenna. Here are answers to
the questions we get asked about GPS antennas.
What GPS Antenna
is the best one to use
The best type of
antenna is an active type operating from 3-5 VDC.
These antennas have about 24 dB gain and are powered by
feeding DC up the inner wire of the coaxial lead.
Precision Test
Systems supply a very small antenna which is only
40 mm square and has a magnetic mount on it. This
allows it to be easily mounted to a steel structure.
The operating voltage is anywhere from 3 to 5 volts.
The advantage of this wider operating voltage range is
that any slight loss of voltage in the coaxial cable
will not affect the antenna's operation.
Where should it
be positioned?
The GPS antenna
should be placed on the roof of the building with a full
360 degree view of the sky. This is the perfect
location. If a 360 degree view is not available,
that the antenna should be positioned for the best view
possible.
Some
experimentation should be carried out if a full view is
not available. Often good results can be obtained
on one side of a pitched roof.
The GPS receivers
in our products are very sensitive and it is possible to
get some type of GPS reception from a windows edge.
But it is likely the GPS receiver will flag errors and
so this is not recommended.
How much loss
can be tolerated in the antenna cable ?
The gain of the
antenna plus any amplifier or splitter used, plus any
cable loss should be in the range 18 to 36 dB.
The antenna
supplied by Precision Test Systems has a gain of 24 dB.
This takes into account the five meters of RG174 that is
supplied with the antenna.
Therefore the
maximum loss in any extra cable can only be 6 dB (24-6 =
18 dB).
What is the
best cable to use?
Many people use
RG58 but this should not be used since it is only rated
to 1 GHz. GPS frequencies operate at 1575.42 MHz,
so any cable used must be designed for this frequency.
We recommend the
following cables:
- HDF200.
Diameter 5 mm. Loss 0.45 dB / m.
- 7808A.
Diameter 6.1 mm. Loss 0.32 dB / m.
- HDF400.
Diameter 10 mm. Loss 0.185 dB/m.
Should I use an
antenna amplifier?
As already
mentioned above, only 6 dB loss can be tolerated in the
antenna cable. If longer cable lengths are needed,
then an inline amplifier should be used.
Precision Test
Systems manufacture the GPS35 antenna amplifier.
This gives a minimum 33 dB of gain and is very easy to
use. Simply connect between the antenna and the
GPS receiver. The GPS35 is powered by the GPS
receiver inside the unit. 5V DC is fed up the inner wire
of the coaxial cable. This powers the GPS35.
The GPS35 then feeds the same voltage to the active
antenna.
The amplifier
should be positioned at the antenna end of the
installation.
The GPS35 has
filters that only allow the GPS frequencies through.
Therefore, signals outside the GPS band are rejected.
This improves overall performance of the system.
What is the
longest cable lengths that can be used?
Taking into
account all that's been said above, the maximum cable
lengths that can be used between the antenna and the GPS
receiver are:
- Using HDF200
cable. 13 m without an amplifier. 86 m
with an inline GPS35 amplifier.
- Using 7808A
cable. 18 m without an amplifier. 122 m
with an inline GPS35 amplifier.
- Using HDF400
cable. 32 m without an amplifier. 210 m
with an inline GPS35 amplifier.
What if I need
longer cables lengths than mentioned above?
If longer cable
lengths are needed two amplifiers could be used.
However it will probably be cheaper and easier to use a
fiber optic connection.
We supply a fiber
optic antenna connection as an option to our products.
The standard antenna is connected to an optical
transmitter. Then thin fiber optic cable is used
to connect the optical transmitter to the optical
receiver. So the optical transmitter is at the
antenna end and the optical receiver is at the GPS
receiver end.
Fiber cable
lengths of well over 1 km can be used with minimal loss
of signal.
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