Novatel OM-20000077 user manual Appendix C, Antenna Specifications, Cable Selection

Appendix C

Antenna Specifications

The GPS antenna is an important part of the total system performance and its selection should be based on your application. The L1 GPS receivers include a Low Noise Amplifier (LNA) before the RF ASIC. This +20 dB LNA permits reasonable performances with a passive GPS antenna. But depending on the cable loss between the antenna element and the L1 GPS receiver, and position accuracy requirements, a +12 dB up to +36 dB active GPS antenna may be needed.

This appendix is divided into characteristics for high end active geodetic antennas and passive antennas.

We also offer the coaxial cables required between the GPS antenna and the SUPERSTAR II. You will also find in this section, different coaxial cables required in your GPS system. The end of this section includes detailed antenna drawings.

C.1 Cable Selection

The interconnecting cable between the GPS antenna and the receiver is of prime importance for the proper performance of the system. Three parameters should be considered:

1.Loss

2.Isolation

3.Outer diameter

The bigger the outer diameter, the lower the loss. The loss increases with the length of the cable and decreases with extra isolation. If the highest accuracy possible is not required, receivers can accept a total cable loss of 3 dB. Depending on the cable type, this could represent a cable run from 2 m up to 10 m.

Table 11 on Page 56 details the specifications of the RG-58 Low Loss Cable (RG-58/U LLDS80) used in the GPS antenna cable 217-601730-XXX. The RG-58/U LLDS80 is a custom-made low loss noise coaxial cable. It is a double-shielded cable similar to Belden Type 9310 but with the improvement of having 85% minimum coverage of the second shield versus the Belden at 55% coverage. The electrical characteristics are included in Table 11.

Table 11: Coaxial Cable Specifications
Center Conductor	#20 Bare Copper wire, Resistance - 33.1 ohms per Km
Insulation	Polyethylene
Inner Shield	Aluminium Foil - 100% coverage
Outer Shield	Tinned copper braid - 85% coverage, Resistance 45.9 ohms per Km
Jacket	Black PVC
Nominal Impedance	50 ohms
Nominal Vel. of propagation	66%
Nominal Capacity	101.7 pf per meter
Attenuation	@ 1000 MHz: 44.3 dB per 100 m (or 54 dB @ 1575 MHz)

Table 12 shows the minimum and maximum cable length when using GPS antenna cable (drawing 217- 601730-XXX) normally required between the receiver and the chassis case of your system. Please see Page 60 for the cable drawing.

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