GPS Repeater: Antenna and Coaxial Cable Specification
To ensure that a GPS repeater receives a good input signal, we must specify an appropriate antenna and coaxial cable.
The GNSS/GPS antenna has a built-in low noise amplifier (LNA) which boosts the signal before it travels down the cable.
Typically the antenna is a Trimble or Tallysman device, with an antenna gain factor of 35 to 40dB.
The LNA requires DC power, which is obtained from the GPS repeater at the other end of the coaxial cable.
The repeater emits 5V DC through its RF connector, which travels up the cable to power the LNA.
The location where the antenna is installed is important. It should ideally be in a place where it has a full 360 degree view of the sky.
This is covered in more detail in the installation guide.
The coaxial cable
The coaxial cable inserts signal loss through attenuation, causing a drop in signal levels over its length.
For cable lengths up to 40 meters, we specify RF240.
This has a solid copper core and is double-screened, ensuring a low loss characteristic while being extremely flexible and immune to Electro Magnetic interference (EMI).
At 1575 MHz, the frequency of GPS L1 signals, the loss incurred is 0.3 dB per meter.
Which means that RF240 is a great alternative to the commonly available RG58 cable, which inserts 0.6 dB per meter, has a stranded copper core, and has a single earth screen.
For distances longer than 40 meters, we have a number of options.
We can specify RF400, which is similar in construction to RF240, but is physically bigger.
RF400 can be used for cable runs up to approximately 70 meters without amplification.
By comparison, RF240 has an outer diameter of 6 mm, while RF400 measures 10 mm.
Signal level calculation
As an example, if the antenna LNA inserts 38dB gain and the RF240 cable is 30 meters long, the overall signal gain from antenna to repeater input will be as follows.
Assume signal frequency is 1575 MHz:
- Antenna gain 38 dB
- Cable loss –10 dB
- Overall link gain = 38 – 10 = +28 dB
Which is perfect as the repeater input should be at least 16 dB higher than the outdoor signal level.
For longer distances between antenna and repeater, we can insert a GNSS inline amplifier to boost the signal.
The amplifier should be installed as near to the antenna as practically possible.
This ensures the signal input to the amplifier is strong and clear of electrical noise before it is launched down the long coaxial link cable.
Even with an inline amplifier, it is generally not practical to install cable over distances longer than about 100 meters.
In which case we can use GPS-over-fibre technology to increase the distance to several kilometres, if required.
Fibre optic cable is made from an inert material, and signals carried within are immune to electro magnetic interference (EMI).
More details of GPS over fibre technology here.
For more information on any of the technologies mentioned here, please get in touch.