[time-nuts] Re: Trimble GSPDO - ISSUE

[Dana Whitlow]

I'm not worried about the F connector itself- rather, it's
irresistible temptation to use
wrong-impedance cables, often rather long ones.

Agreed that LNA inputs don't tend to make good matches, but that's what the
splitter
largely addresses.  It's the wrong impedance of the source that foils the
benefit of the
splitter, and which most people ignore altogether.  I think we'd get along
better if the
antenna output impedance were 75 ohms, F connectors were used, and 75 ohm
splitters were used.  Then good isolation would be preserved at low cost no
matter
what load impedances were present on the splitter's outputs.  The simple
mismatch
loss between 50 ohms and 75 ohms is pretty negligible in this context.

When I see an attenuator with only about 15 dB return loss at L-band, it
goes straight
into the trash.  To me, the anticipated main benefit of having a nanoVNA is
that I'll
be able to identify such defective items at flea markets before spending
money on it.
That is, if there are ever any more flea markets (because of Covid).  I'm
not optimistic.

Dana



On Wed, Jul 28, 2021 at 7:58 AM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> If you measure the input impedance of most low noise receivers, it’s not
> going to be 50 ohms pure resistive. There are multiple reasons for this.
> Since the “load” is not a match to either 50 or 75 ohm cable, it’s not that
> big a deal.
>
> The amp in the antenna with it’s post filtering may or may not present a
> 50 ohm source. They rarely hit a 14 db return loss spec in any case. Indeed
> there are a lot of attenuators out there that don’t get to 14 db at 1.5
> GHz.
>
> Trimble did a *lot* of testing before they put the F connector on the
> TBolt.
> They could find no impact at all.
>
> Bob
>
> > On Jul 28, 2021, at 8:23 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> >
> > Most general purpose passive splitters for the L-band regime are
> > essentially variations
> > on the Watkinson splitter, which provides good isolation between the
> output
> > ports, if
> > (and only if) the input port is driven from a matched source.
> >
> > So if your GPS antenna, whether active or passive, has a good output
> match
> > then
> > the Samsung's input impedance will not affect operation of the Trimble.
> > But, again,
> > this is true only if the antenna's output impedance is matched to the
> > splitter's
> > characteristic impedance (including consideration of any impedance
> > transformation
> > in the transmission line between the antenna and the splitter).
> >
> > I cringe at the cavalier treatment given to line impedances used in GPS
> > systems.
> > AFAIK, all GPS equipment is designed for 50 ohms, yet a great deal of
> such
> > equipment is provided with 'F' connectors, which invite the use of
> > inexpensive RG-6
> > line, which is 75 ohm cable.  In addition to problems with impedances and
> > loading,
> > It's a built-in source of multipath, all at no extra charge!
> >
> > Dana   K8YUM
> >
> >
> > On Tue, Jul 27, 2021 at 3:01 PM Don Cross <cosinekitty at gmail.com> wrote:
> >
> >> On Tue, Jul 27, 2021 at 2:54 PM ordnit via time-nuts <
> >> time-nuts at lists.febo.com> wrote:
> >>
> >>>
> >>> Any guess? Do I have to throw the unit from the window? For your
> >>> reference, I have a samsung unit connected to same antenna via a
> >>> splitter. The GPSDO is performing very well.
> >>>
> >>>
> >> Hi Ordnit,
> >>
> >> I'm hardly an expert, but I wonder about the splitter. Is this a
> passive or
> >> active splitter? If passive, it could be the input impedance of your
> >> Samsung is much lower than for your new Trimble and it is making the
> signal
> >> seen by the Trimble too weak. If you are willing to disconnect the
> Samsung
> >> temporarily, perhaps you could try connecting your antenna to the
> Trimble
> >> only and see what happens.
> >>
> >> Don
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