[time-nuts] WTB: GPS Antenna Splitter

Bob Camp kb8tq at n1k.org
Wed Oct 8 14:12:40 EDT 2014


There are antennas out there that draw as much as 100 ma in “normal” operation. The overload threshold on GPSDO’s and GPS modules varies from 20 ma to 200 ma. It seems to depend on exactly what antenna the manufacturer decided to use as a standard. Undercurrent threshold varies from zero (as it there isn’t one) to 20 ma. Again, it seems to depend on what they were targeting when they made the box you are looking at. In some cases different years production of the same box has different limits (or the limits are not very precise).  It’s also possible that some of what I’ve seen is defective, since it’s auction site used pulls / salvage. 

Yes you read that correctly, a 20 ma antenna will be “under current” on some modules and “over current” on others. 


On Oct 8, 2014, at 11:31 AM, Tim Shoppa <tshoppa at gmail.com> wrote:

> e.g. HP58540A will raise the red alarm LED if DC antenna current is less
> than 5mA [open] or bigger than 50mA [short]. I thought this was in common
> with Z3801A but I don't see it mentioned in the Z3801A book.
> Page 15 of M12+ users guide says: "The M12+ is capable of detecting the
> presence of an antenna. The receiver utilizes an antenna sense circuit that
> can detect under current (open condition), over current (shorted or
> exceeding maximum receiver limits), or a valid antenna connection. " then
> goes on to note that an external bias-T will make this detection
> meaningless. It lists the current limits as 15mA [open] and 80mA [short].
> Tim N3QE
> On Wed, Oct 8, 2014 at 11:22 AM, Dave M <dgminala at mediacombb.net> wrote:
>> Thanks, Tim,
>> I have wondered about the available antenna current from GPSDO units, but
>> haven't been able to find a spec on any of them, or at least, the GPS units
>> that I have. I have an HP Z3801A and a Nortel GPSTM NTBW50AA. Have you (or
>> anyone) any information on typical available antenna current, or alarm
>> conditions for antenna overcurrent?  I know the Nortel unit spec says that
>> it can survive a shorted antenna/cable without damage, but no info on
>> alarms or current limits.  My guess is that the unit won't see any
>> satellites and will just go into holdover.
>> The Z3801A must have an active antenna connected (no passive antennas
>> allowed), but no specs in the manual on antenna current limits.
>> I guess I could substitute an active load (transistor or FET) for the
>> antenna so I can vary the current drawn, and see what alarms pop up.
>> Thanks for more info,
>> Dave M
>> Tim Shoppa wrote:
>>> At the very low end, a F-cable splitter for like Cable TV will likely
>>> (*) get the job done. Many of us are already using RG-6 type cable
>>> for the GPS antenna run anyway.
>>> (*) Some GPS units may get confused if they have active sensing on the
>>> antenna preamp current and this goes out of tolerance.
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