I have used these "Hockey Puck" GPS antennas with 100' (thats feet) of RG-174.
No problem locking up. Granted, the antenna needs an unobstructed view, but the antennas today work a lot better. I have a Laptop in the shack, near an inner wall, with one of these GPS/GLONASS USB units. I consistently see 8 or more satellites and get a 3D fix. That's on my desktop with an SB-220 looming over the antenna. Try it, you will be surprised. 73, Dick, W1KSZ On Fri, Jun 5, 2020 at 6:43 PM Tom Holmes <thol...@woh.rr.com> wrote: > 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. > _______________________________________________ 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.