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How can one measure the delay of a GPS antenna with preamp & filter?

Question asked by drkirkby on Feb 8, 2015
Latest reply on Feb 16, 2015 by drkirkby
Special GPS reveivers are optimised for timing applications, so can be hooked up to a computer to make an NTP server. Such reveivers are often used to provide an accurate 10 MHz frequency reference in a lab to. They should be fitted with an outside active antenna clear from reflections for best results.  These timing receivers have the facility to set the delay between the GPS receiver and the antenna so that the receiver knows the time accurately.  

How could one measure the delay between the GPS antenna and the end of the antenna cable? Obviously one could do a rough estimate of the delay of the coax by its length and velocity factor, but the velocity factor is never known very accurately. But the antenna will have an integrated pre-amp and SAW filter centred around 1575 MHz, so just measuring the coax is not enough. 

I considered one could make a simple dipole, excite that from port 1 of the VNA and measure S21. One could easily calculate the delay between the two antennas,  but it is not clear to me how to measure the delay when the filter will have a narrow bandwidth centred around 1575 MHz - one cant simply use the time domain option when the usable bandwidth is very narrow.  

Could one use port extension? I suspect not, as they assume that the delay is that of a 50 Ohm transmission line, not a sharp filter.

I suspect a VNA is the right tool for this, but I can't work out how to do it properly. A typical run of cable maybe 20 m, so is many wavelengths.

Dave  

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