Configuration for an Outdoor Antenna with Time Servers
The Configuration for an Outdoor Antenna with time servers is crucial when network timing reliability is at stake!
What if the antenna needs to be more than 250ft from the GPS time server? There are two factors that limit the length of the cable between the time server and antenna.
First Factor: Signal loss in the cable Antennas used with most GPS receivers have a gain rating. This gain rating has nothing to do with helping a GPS receiver obtain signal lock in a given location. A higher gain rating will not make up for an antenna location that can’t see the GPS satellites well. What it is for, is to overcome the signal loss in a cable. The GPS L1 frequency, which is the primary frequency we are interested in for our time servers, is 1575.42 MHz.
The RG58/LMR195 cable that TimeMachines® sells loses about 13dB per 100 ft of cable at the GPS L1 frequency. If the antenna amplifier is 40dB, and there is 250ft of cable, the loss is approximately 30dB, leaving about 10dB of margin. If a splitter is added to the signal path, then another 4-7dB can be lost, plus all the cable connectors add a bit more loss. If the total loss in the cable path exceeds the antennas 40dB gain by much, then the GPS likely will not be able to lock and obtain the timing information required to operate.
Overcoming the signal loss can be done by changing to a higher grade cable, such as LMR400. LMR400 has roughly half the loss of LMR195. It is more expensive, larger diameter, and less flexible, but enables longer cable runs. TimeMachines® does not sell LMR400, but an internet search will reveal many sources.
Second Factor: DC Loss to the Antenna The second factor that must be overcome is the DC loss from the time server to the antenna. TimeMachines® time servers can put out a maximum of 5V DC. This is required by the antenna to power its internal amplifier circuit. If this voltage is not present, or below a minimum value, then the antenna won‘t work at all. The amplifier simply doesn’t turn on and no GPS signal is present in the cable. When the cable runs become longer than 250ft, there is more voltage lost in the cable between the time server and the antenna. If the cable becomes long enough, the voltage will “droop” enough to degrade or eliminate function.
This is more easily overcome than GPS signal loss. Using a TimeMachines® powered splitter, allows putting a higher voltage onto the antenna cable such that more voltage gets to the antenna. The Tallysman antennas that TimeMachines® resells are capable of working from 2.7 to 16 Vdc. The power injectors come with a 9V supply. This voltage is sufficient to overcome the DC loss of a longer cable run. The powered splitter has no gain of its own, it simply functions as a way to inject a higher voltage on to the antenna cable.
The Result: Both the signal loss from the antenna to the time server AND the DC voltage loss from the time server to the antenna must be overcome to extend the cable length much beyond 250ft. If both are not addressed, then signal issues will be a problem and the time server may not function.
