[time-nuts] Re: Trimble GSPDO - ISSUE

[Bob kb8tq]

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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