What is the Maximum Coaxial Cable Length Allowed for GNSS/GPS Antenna Feed?
Some articles say 50 meters, others say 95 meters before adding in-line amplifiers. However, the total coax cable feed length depends on the characteristics of the system (Antenna, Power Feed, Cable Type and Receiver Sensitivity)
For temporary passive installations (no in-line amplifiers) often used with portable test sets for on-site timing testing purposes, the total antenna cable length allowed depends on the type of coaxial cable you plan to use. Thick and hard coaxial cables used for permanent antenna drops have low loss and may allow longer reach.
However, the calculations required to come up with a recommendation are not simple, since they depend on:
- The active antenna's low-noise amplifier (LNA) Gain
- The Active antenna's supported voltage feed (typical 5V for portable antennas)
- The receiver's sensitivity
- The receiver's voltage and current feed
- Antenna position and whether a full unobstructed sky view is possible
- The minimum required performance for the receiver (signal quality in dB-Hz, effective aperture, etc.) to achieve the required accuracy and stability
Some people in the industry recommend the use of in-line amplifies for cable feeds longer than 50m, while others say longer than 95m. However, without all the information above, it is hard to say what those recommendations are based on. Is it with the use of thick and hard coaxial cables with very low loss? There are many types of low-loss 50 Ohm coaxial cables that can be used for GNSS applications, but a lot of them would be impractical for temporary field-testing applications. For example, they may be too rigid and require larger type N or TNC connectors. For portable applications, the best thin and flexible coaxial would be preferred. One that doesn't inflict mechanical stress on the portable test set or its SMA connector.
VeEX's Coaxial Extension Recommendations
For temporary passive installations, using VeEX's test sets supporting the GNSS Receiver option and the multi-band antennas supplied by VeEX, we suggest experimenting with the use of <30m (100 ft) extensions made of low-loss 50 Ohm LMR-195 cable (similar to RG-58 and RG-142) or <15m (50 ft) of LMR-100A (similar to RG-316 or RG-174). They offer a good compromise between low loss and flexibility. (You can experiment with longer high-quality low-loss cables, to see if they fit your application requirements and expectations.)
- The 50 Ohm LMR-195 cable has an attenuation of about 0.5 dB/m @ 1500-1800 MHz
- The 50 Ohm LMR-100A cable has an attenuation of about 1 dB/m @ 1500-1800 MHz
In general, we recommend the use of the lowest loss and shortest extension cable possible, with the least number of adapters. A single SMA male to SMA female extension cable, made of thin, flexible and low loss coax is recommended. Always use the test set's GNSS Signal (C/No) table to verify that the meter is always receiving at least four good quality satellite signals with >36 dB-Hz.
The use of thick coaxial cables with hard TNC or N to SMA adapters are not allowed for use with VeEX's GNSS receiver, due to mechanical stress. If hard cables are used (e.g. permanent installation), we recommend using a 1 or 2m flexible converter cable on the GNSS receiver side, to release the mechanical stress and avoid any damage to the receiver.