[time-nuts] WTB: GPS Antenna Splitter

mike cook michael.cook at sfr.fr
Tue Oct 7 01:15:44 EDT 2014


Le 7 oct. 2014 à 03:09, Bob Camp a écrit :

> Hi
> 
> Missed the survey question… 
> 
> If a ns in free air is about 1 foot (30 cm), then you probably want a survey that is better than 6” to keep the error down. You do not want to have the antennas on top of each other, so yes, the GPS will need a survey / location each time you change antennas. If you go with the 10’ spacing, then you will get some pretty big jumps without switching the location. 
> 
> Bob
> 
> On Oct 6, 2014, at 4:01 PM, Dave M <dgminala at mediacombb.net> wrote:
> 
>> Does anyone in the group have, or can point me to, a low-cost (but not 
>> cheap) 2-port splitter for a GPS antenna?  Those on Ebay are rather 
>> expensive.

 I use micro-circuits power splitters, ZN4PD1-50-S+, to which I add DC blocking and load to all but one output which is used to power the antennas. Attenuation is low and port isolation adequate (at least I can detect no performance loss with 4 receivers connected). I have three now in service and haven't paid more than $60 a piece including transport from the US. I have also used 2 port ZN2PD-2000s, no relegated to the spares box. 

>> 
>> I have two GPSDO units, and have both an older timing antenna and a new 
>> choke ring antenna (Thanks, Pete L).  I already have one 2-port splitter 
>> (working well), but my intent is to connect both antennas through the 
>> splitters and a couple coaxial relays so that I can, with the twist of a 
>> switch, allow me to run each GPS from a different antenna, or both from the 
>> same antenna.  I would like to gather some data as to the differences 
>> between the two antennas.  I know I could switch the connections manually, 
>> but I like the idea of a switch to sort of automate the connections, and I'd 
>> need another splitter anyway.
>> 
>> Before I go to the trouble and expense of building upon this idea, are there 
>> any comments as to the value of the project?

I haven't put coax switches in the paths but I do manually switch new receivers between the three different antennae that I have to see how they respond. I can detect, via the signal strength bars in the various GPS utilities, that there are differences in antenna performance, and receiver sensitivity, but have not seen any significant survey position differences from spec, though I only have patch antennae. I have done ADEV measurements and don't see any  significant difference between different antenna sources. Before today I have not done any investigation on the effect of different antennae on PPS phase. So I am just eyeballing that of a ublox 6M as I type this. With all 3 antennae I do not see more than a few nano seconds phase offset using the same receiver . I am using a T-Bolt PPS to trigger the scope. However, the PPS does does slew around in time, maybe due to the relatively poor sky view that I have, or T-Bolt PPS issue. I have two receivers on the scope ( a Trimble SMT is the other which has been kept on the same antenna during the test) and both are showing the same issue. For a time-nut this looks like a useful field of investigation. 

>> Some questions come to mind:
>> I'm thinking about mounting both antennas on the same mast, at the same 
>> elevation, just separated by a couple feet.  Any problems that I should be 
>> aware of by putting both antennas so close together?  Will that small 
>> distance have a noticeable effect when switching a receiver from one antenna 
>> to the other?  Will the GPS notice the difference and want to do another 
>> survey?

I have never seen any survey restart on antenna disconnect/reconnect . No effect on the T-Bolt, or Z3801A and I think that this only occurs on request or on power up depending on available date or receiver design.

>> 
>> Thanks for your comments.
>> Dave M 
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