Just in case anyone is doing a search in the future, I did order and install the Harxon GPS-500 mini survey antenna and installed it on a trimble RTK system and it works perfectly.  I see that you can now source these antennas from china via ebay for somewhere in the $150-$200 range vs the $1000+ for the Trimble AG25 which have a real problem with environmental sealing.  My next project is to install the GPS1000 which set me back an extra $25 but is a newer model with supposedly better specs.  About the only drawback is that you have to provide your own magnet and a 90 degree antenna adaptor.