Dana... I think that you are neglecting two important mitigating factors.
1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those reflections quite a lot from one end to the other. It amounts to 2 - 3 dB in 25', depending on cable quality. 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order of 20% of the incident power, about 7 dB. I am rounding a lot here just to keep the math easy...for me. By the time a reflection has made the round trip from the receiver back to the antenna and them back to the receiver, which is how the delay would have to manifest itself, it will be down at least 15 dB from its original self, and probably more. Given the coding of GPS signals which allows several satellites to share a common frequency band, that is not going to be much of a problem. And if only one end of the path actually is 75 ohms, then there won't be a delayed signal. Tom Holmes, N8ZM -----Original Message----- From: time-nuts <time-nuts-boun...@lists.febo.com> On Behalf Of Dana Whitlow Sent: Friday, June 05, 2020 9:01 PM To: Taka Kamiya <tkami...@yahoo.com>; Discussion of precise time and frequency measurement <time-nuts@lists.febo.com> Subject: Re: [time-nuts] ThunderBolt question I'd like to point out that mismatches at the ends of an antenna cable *can* cause trouble. When both ends are mismatched, each bit of detail in the signal gets partially reflected back and forth, each time delayed by the round trip propagation delay in the cable, and so you have something like multipath going on. Fortunately the successive reflections get weaker with time, generally quite rapidly. Since many GPS users seem very concerned about multipath resulting from poor antenna placement, I think this factor should be considered as well and not just get swept under the rug. The amplitude of the "multipath" resulting from cable mismatches depends on the product of the voltage reflection coefficients at the two ends of the cable. If either end is perfectly matched, then the quality of the match at the other end is not significant vis-a-vis apparent multipath problems and only affects transmission loss. But when there is a mismatch on both ends, then the length of the cable comes into play as well. A longer cable means more delay between successive reflections, which is just like multipath involving longer delays between the direct and the reflected signals. Cheers, Dana (K8YUM) On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts < time-nuts@lists.febo.com> wrote: > 50 ohm / 75 ohm question is really irrelevant in this kind of thing. > Trmble itself says in manual, not to be concerned with this apparent > mismatch. > In my particular case, I have a home lab standard and existing system. I > have an antenna and network of distribution amplifiers. They are all 50 > ohms and N connectors. Some ports have BNC adapters attached. I have > pretty much standardized everything to SMA, N, or BNC. > > I boxed a power supply, T-bolt, and buffer amp in a metal case. I bought > a short cable (RG58) that goes from F to BNC. On back of the case, I have > BNC to N adapter. I also have a few adapters that goes from F to BNC for > the test bench. It really doesn't matter what you use, as long as it makes > a solid connection. > > Advantage of F connectors and RG6 are, cheap, abundant, and low loss for > the size. Advantage of having house standard is, less adapters and less > headache..... > > --------------------------------------- > (Mr.) Taka Kamiya > KB4EMF / ex JF2DKG > > > On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario < > ka3...@comcast.net> wrote: > > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > connector for the antenna input, and BNC connectors for the 1 pps and 10 > MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 > Ohms and they just used the F connector to distinguish it from the > others? What do people do, just use a 50 Ohm antenna? > > Thanks > > Robert DiRosario > > KA3ZYX > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.