Hi As always with any simple question - the answer is "that depends ...".
What are you using for a receiver? Is it "sensitivity challenged"? A modern receiver will be a *lot* more sensitive than the receiver in an old HP GPSDO. The next layer to that onion is feed line and splitters (or not). All that is made more complex by the fact that you can put to much gain in front of some receivers. Where is the antenna located? Is it "multipath challenged"? An antenna on the top of a 100 foot tall mast with no obstructions anywhere has a very different situation than one at ground level with dozens of reflecting objects nearby. What is and isn't a reflector at 1.5 GHz is not always obvious. There are a wide variety of multipath situations you can run into. Some receivers are better at rejecting stuff than others. What are you trying to do? Are you after carrier phase data or conventional reception? Carrier phase is less forgiving in general. Is your objective timing or navigation? I'll assume it's fixed location timing. How does your GPSDO (or what ever) handle the incoming data? How good is it at rejecting odd looking inputs?. All of that can vary depending on what the recent history into the unit has been. How good can you measure? Put another way, is there a "good enough" timing value below which a change could not be seen? Are you averaging readings over a day or over a few minutes? What are you comparing the timing to? Are you comparing to an independent standard or to another GPS? Are you sky view challenged? Are you far enough down in a hole that only a portion of the sky is visible? Some common urban settings are quite poor for seeing more than a couple satellites all the time. Some very poor ones are lucky to catch a couple every so often. Are you overload challenged? Antennas have a wide variety of filtering in them. Some receivers have a lot of filtering, others have very little. If you are located next to a cell tower, filtering in both is a good thing. These days there are a surprising number of gizmos that come and go spraying RF all over the place. The kid next door who talks on the el-cheapo cell phone for 4 hours at a time is one.... Hard to believe, but that's far from a complete list. Some of what's there is pretty specific, other stuff is going to apply to just about every setting. In some cases you could compare the best antenna made to a bent coat hanger and the difference while very measurable would not be statistically significant. In other cases a metric that is multipath focused would be so very well to make a decision. In still other cases it's sensitivity and only sensitivity. Simple answer - no one number, no 60 second test, no free lunch :).... More than you would ever really *want* to know about GPS orbits and the like: http://www.kowoma.de/en/gps/index.htm A good look at what you *could* be doing with GPS, but might not be: http://www.boulder.nist.gov/timefreq/general/pdf/1424.pdf They seem to like tracks in the 15 minute range. Simple experiment: Take two (hopefully) identical receivers, two fixed and widely separated crummy antenna locations. Similar sized antennas with different stuff inside them. Run antenna A in location A, B in B. Collect pps output data for a few weeks. Look at the pps wander around. Swap A to B and B to A. Collect data for a few more weeks. Swap A back to A and B back to B. More data. Repeat A to B and B to A. Look for an obvious winner. The winner changes depending on which data sets I use, and how I look at the data. That's not to say that the antennas are equal. They most decidedly are not. One is better at some things, the other at other things. Which one wins is situational. No simple answer.... Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Hal Murray Sent: Wednesday, March 24, 2010 2:00 PM To: [email protected] Subject: [time-nuts] GPS antenna setup: how good? How do I tell which of two setups is better? For example, how much does adding a pie pan help? Is there some simple parameter I can look at that tells me an antenna goodness value? If not, what's a reasonable recipe to come up with a number or compare two antennas? What's the appropriate time scale to use when thinking about that problem? I haven't (yet?) looked at any of the satellite position and signal strength data. Can I do something like wait until a satellite is about to go directly overhead and plot the signal strength while occasionally swapping the antennas? Are the signal strength readings reasonably noise free and/or repeatable from run to run which may be a few days apart? Assuming it goes high overhead, how long is a satellite within view? (ballpark) -- These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
