[time-nuts] ThunderBolt question

Sat Jun 6 01:56:36 UTC 2020

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 <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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