[time-nuts] GPS antenna splitter buyers' guide

Stewart Cobb stewart.cobb at gmail.com
Sat Jun 29 20:53:48 EDT 2013


(If this shows up twice, I apologize.  Gmail seems to have switched me to
sending HTML-formatted mail by default, and the time-nuts server doesn't
like that. Anyone know how to switch back permanently?)

It's possible to use standard microwave power dividers as GPS antenna
splitters, but they have several drawbacks.  Management of DC to the
antenna LNA creates one set of problems. Insertion loss and lack of RF
isolation between outputs can present additional problems.  In professional
settings, it's common to use purpose-built GPS antenna splitters.

HP/Agilent/Symmetricom GPS antenna splitters are fairly common on the
auction site.  Unfortunately, they were designed and specialized for
all-Agilent systems and they don't necessarily play well with others. You
can get GPS splitters that work better with a mix of GPSDOs, and generally
pay less as well because you're not buying the Aglient name.

The two most common (in the US) makers of purpose-built GPS antenna
splitters are GPS Source and GPS Networking. Their devices are more or less
interchangeable. One of these (the latter, I think) was formerly known as
WR Incorporated.  Googling WR no longer gets you any useful information, so
stuff with that name tends to be cheaper at auction than the others.  (Full
disclosure: I know several of the principals in this business.  But I'm
sure they'd want me to tell you to buy their stuff new rather than used.)

Purpose-built GPS splitters generally consist of an RF power divider and a
method of managing the DC bias sources for the antenna LNA.  They often
contain an RF amplifier to make up the loss in the power divider itself (10
dB for an 8-way splitter) and the loss in a potentially long cable run from
the antenna.

Some models are built with high RF isolation between ports.  This is done
by adding (say) 20dB to the amplifier gain, and then inserting a 20dB
attenuator between the power divider and each port.  This adds 40 dB of
isolation between ports, which keeps the various receivers from interfering
with each other.  (Some common receivers through the 1990's would leak high
levels of local oscillator signals out their antenna ports, which would jam
other receivers connected to the same antenna.)

If the splitter says a receiver port is "DC blocked", that port is
generally connected to ground through an RF choke and a resistor of 200 to
500 ohms.  This keeps the "antenna fault" detector on many receivers happy.
If the port says "DC thru", that port is DC-connected to the antenna port
to power the antenna LNA. That port will generally also power the
splitter's internal amplifier, if it has one. Sometimes the combined load
can overload the receiver's DC output current limit.

It's common for larger splitters to have a separate power source
(wall-wart) to power the antenna and the internal amplifier.  In this case,
all the receiver ports will be "DC blocked."  This configuration is most
convenient for a time-nut lab, so that you can swap out any receiver you
want without interrupting the others.

These devices are fairly simple internally, and the various ordering
options (amplifiers, isolation, DC blocking, etc) were typically done by
stuffing the correct set of parts onto a common PC board. The ones I've
worked on all had a two-layer PCB, with parts and microstrip on one side
and mostly ground-plane on the other side.  If you find one for sale that's
not quite configured the way you want, you can probably reconfigure it with
a bit of thought and some surface-mount soldering.  The cases come apart
easily, with conventional screws along the sides, sometimes hidden under
the labels.

These splitters were available new with SMA, TNC, and N connectors, and
sometimes BNC as well.  BNC connectors can often be replaced with TNC
connectors without too much trouble (and should be replaced; BNC connectors
are not suitable for precision GPS installations).

If possible, get a splitter with one or two more ports than you think you
will need.  There's always another GPSDO to test, and you may be able to
borrow some professional survey equipment and connect it to your spare
antenna feed to get an accurate antenna position.

Hope this helps.

PS: The irony of doing a post like this is that more people will bid on
these things now that they know more about them, and prices will rise.  But
I already have all the splitters I need :)


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