GPS Antenna Cable Delays at N8URI measured the electrical length of the two GPS antennas using an Agilgent 8712ES network analyzer. These measurements are from the clock-room ends of the LMR-400 cable that connect to the two distribution ampliers. For fun, I used two methods to measure the delay:
- Cable fault mode, with velocity factor set to 1.0. This uses a swept signal and lots of math to show cable discontinuities; the antenna at the end of the cable is usually easy to spot since it has a large mismatch over the swept frequency range.
- Delay mode measuring S11 (the time it takes the signal to get from the analyzer to the end of the cable, then back again); this is twice the actual electrical length. My VNA does not go to the 1.575 GHz frequency of the GPS system, so I used a sweep from roughly 1100 to 1200 MHz.
GPS Antenna 1 -- Aeroantenna
The cable fault length of 134.16 feet equates to 136.43 nanoseconds. Taking one half of the mean delay of 273 nanoseconds, we get 136.5 nanoseconds. The two measurments correspond well.
This antenna is connected to an HP 58517A 8-way splitter, which has about a 14 ns delay. So, with that in mind, I am using a cable delay of 140 ns at the splitter outputs for this system.
GPS Antenna 2 -- Motorola Timing 2000
This antenna is connected to an HP 58535A 2-way splitter, which has about a 22.5 ns delay. So, with that in mind, I am using a cable delay of 165 ns at the splitter outputs for this system.