[time-nuts] WTB: GPS Antenna Splitter

[Bob Camp]

Hi

I have not (yet) seen a GPS antenna that is spec’d to be close spaced to another GPS antenna. I would not recommend putting them closer than about 5 feet. If you do, they will interact with each other and you will not be able to get a “antenna A” to “antenna B” comparison. You will always be looking at some combination of A reflected back and forth to B. Close mounting is a common error when setting up things like factory test systems. It does indeed create problems. A *lot* of people are surprised at the spacing requirements on a timing grade antenna. 

If you want “Time Nuts Grade” accuracy, you need the best 48 to 96 hour survey you can get. If the survey is off, you will see more drift in a day’s cycle than if it’s correct. 

If they are mounted on a tower, put one 5 meters above the other. Then when you switch antennas, just bump the survey value +/- 5 M as required.

Bob


On Oct 8, 2014, at 11:40 AM, Dave M <dgminala at mediacombb.net> wrote:

> Bob, Thanks for that.  My intent was to mount the antennas on yardarms on a mast.  Both antennas will be at the same elevation, just separated horizontally by a couple feet (not 10ft).
> My concern (and I think you answered it) was how far apart could I place the antennas without having to do another survey when I switch them around.
> As an alternative, I could enter the antenna coordinated manually into each GPS when I switch antennas.  That should avoid a new survey each time. Then, the distance between the antennas shouldn't matter.
> 
> Thanks for the info!!
> Dave M
> 
> Bob Camp wrote:
>> 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 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?
>>> 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?
>>> 
>>> Thanks for your comments.
>>> Dave M
> 
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