[time-nuts] how to feed multiple GPS receivers ?

[Bob kb8tq]

Hi

On a GPS distribution system the issue of 75 vs 50 ohm cable is pretty much a 
non-issue. A lot of people have run 75 ohms (like Trimble on the TBolt) and had
no problems from this.

The “TV” splitters are generally spec’d to 2 GHz, so well beyond GPS frequencies.
Again, it should not be an issue. There are also cheap multi-port splitters on the 
auction sites. 

DC blocks *are* a good idea. DC loads may or may not be needed. It’s a try it and see
sort of thing. Some have been known to simply drop a 50 ohm load on one port of a 
8 way splitter and let that be the DC load. You *can* run into issues doing this ….
(one device feeds 12V and another is a 3.3V feed …. smoke results …). Cable TV 
DC blocks are dirt cheap ….

Ideally you want the antenna as high as you can get it. In an urban area with various
constraints, that may or may not be very high. Coax losses plus splitter losses should
be considered when designing something like this. An amplifier ahead of the splitter 
is not at all unusual. 

With the arrival of world of low cost multi-band GPS devices … put up a multi-band
antenna. At the very least L1 / L2 (including Glonass coverage). Ideally L1 / L2 / L5. 
Much easier to do it right the first time.

Bob



> On Feb 24, 2019, at 4:59 PM, shouldbe q931 <shouldbeq931 at gmail.com> wrote:
> 
> After starting off with a single GPS module on a RaspberryPi, and
> discovering the issues of just having a single clock... I now have
> three GPS receivers (and a fourth on the way) of various types, each
> with their own antenna. I'm aware that I'm heading off down an
> interesting road that will consume ever more of my time, but at the
> moment it's still fun (-:
> 
> To remove the difference of multiple antennas (different skyviews
> etc), I'm thinking about using a single antenna and "distributing" the
> feed to the receivers.
> 
> I should also add that I'm doing this in the UK (in central London),
> and as inexpensively as possible.
> 
> Reading back through the mailing list several people appear to have
> used SatTV splitters with GPS signals to apparently good results, and
> I've ordered one of these
> https://vision-products.co.uk/8-way-single-port-dc-pass-splitter/
> which if I understand it correctly, will passthrough DC from one port,
> and block DC from the other ports. From other reading of the mailing
> list, it looks as if I will also need to present a DC load to the
> other receivers, and if I've read corectly this would be a "Bias tee"
> used with a resitive load instead of a DC voltage, but am not sure if
> I have understood correctly.
> 
> I'm aware of impedance matching issues with using a mix of 75 ohm and
> 50 ohm parts, but as I said, I'm doing this as inexpensively as
> possible, and a 58536A is way over budget at the moment.
> 
> Any suggestions gratefully received.
> 
> Cheers
> 
> Arne
> 
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