[time-nuts] ThunderBolt question

Sat Jun 6 11:13:24 UTC 2020

It's one thing to maintain lock in a multipath environment, quite another
thing
to get "full" accuracy of GPS measurements of PVT.

An interesting difference between my scenario of poorly matched impedances
and "ordinary" multipath is this:  In the poor matching scenario, all the
received
signals will be impaired identically, while in the ordinary multipath
scenario,
signals from different satellites will suffer different (and time-varying)
multipath
impairments.  I'm not at all sure what effect this difference will have on
final
outcome, but my gut feel is that the case where all signals are impaired
identically
could lead to worse effects.

Dana

On Fri, Jun 5, 2020 at 8:43 PM Tom Holmes <tholmes at 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-bounces at lists.febo.com> On Behalf Of Dana
> Whitlow
> Sent: Friday, June 05, 2020 9:01 PM
> To: Taka Kamiya <tkamiya9 at yahoo.com>; Discussion of precise time and
> frequency measurement <time-nuts at 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 at 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 <
> > ka3zyx at 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
> >
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