[time-nuts] Choke-Ring Antennas for GPSDO use?

[Michael Baker]

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:
-----------------------

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/Index.cfm?Ad=1&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

and 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
------------------------------------

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