[time-nuts] ThunderBolt question

[Tom Holmes]

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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