Hello All,
Over the past couple of years I have seen several references indicating that the reason all GPS units intended for precision survey applications use a choke-ring antenna is that a properly designed GPS choke ring antenna will cut multi-path reception down very significantly. Other references I have seen indicate that using a choke-ring antenna results in a moderate improvement in GPSDO performance.
Can any list members comment on this? If a choke-ring antenna does improve GPSDO performance, what degree of improvement can be expected? Enough to be worthwhile? Would switching to a properly designed choke-ring antenna on my Trimble Thunderbolt GPSDO be worthwhile? My house is surrounded by large trees and my roof-top GPS antenna does not have a foliage free view of the sky until about 40-degrees above the horizon. I have set a horizon elevation mask of 30 degrees in my T-bolt software but have wondered if multipath through the remaining tree-tops may still be a slight problem.
Only very rarely do used GPS choke-ring antennas show up on eBay and they typically ask around $600 to $800.
My good friend Bob Johnson, WB4JZM, tracked down some references to GPS choke-ring antennas which he sent to me (Thanks, Bob!!) and I have taken the liberty of appending some of his post below.
So, can any list members shed any light on the efficacy of switching to a choke-ring antenna on a GPSDO unit?
Mike Baker
WA4HFR
Gainesville, Florida
Bob said:
It looks like this article describes an antenna that is easier to design and manufacture than a choke-ring antenna, with similar performance:
http://mwrf.com/Articles/Print.cfm?ArticleID=5490
The standard choke-ring design comes from JPL, and may be public domain in the U.S. Certainly there must be a published patent somewhere that will give enough detail to build one (otherwise, it isn't a valid patent). They are normally milled from a solid aluminum billet, but you could use other fabrication methods and make your own. And you seem to have enough test equipment to test it…
An article on choke-ring theory: http://javad.com/jns/index.html?/jns/technology/Choke%20Ring%20Theory.html
Improvements on the JPL design from the same company: http://javad.com/jns/index.html?/jns/technology/Single-Depth%20Low-Multipath%20Choke%20Ring.html
A choke-ring design with improved reception of low-elevation signals: http://www.novatel.com/Documents/Papers/3D_choke_ring.pdf
This looks like an excellent article. Of interest in this article are the photos of different choke rings they evaluated: it appears that the critical factor is the depth of the grooves, and there is some disagreement about the optimum depth, so obviously that isn't critical either! In other words, build your own, it will almost certainly provide some benefit, possibly more than an expensive commercial unit.
A patent that appears to have enough detail to build your own: http://www.freepatentsonline.com/6278407.html
Possibly the greatest difficulty in building your own is the actual GPS antenna: for best results, you need to know the phase center of the antenna, and mount it at the center of the choke ring (for that matter, the antenna needs to have a single phase center: but real-world designs have different phase centers in different dimensions). And the choke ring should be designed for a specific antenna: you can't just buy a choke ring off eBay and stick any old antenna on it and get optimum results (although it is likely to be an improvement over the same antenna without a choke ring if you mount it correctly).
Discussion of phase center and precision GPS work: http://www.ngs.noaa.gov/ANTCAL/images/summary.html