[time-nuts] Antenna problems
k6rtm at comcast.net
k6rtm at comcast.net
Sun Aug 22 17:09:56 UTC 2010
Antenna gain and placement make a lot of difference. Yesterday I swapped out my old Trimble active patch antenna (24 - 26 dBc) for an HP/Symmetricom 58532A antenna (30+ dBc). It's on the roof of our two story house, at the end of probably 20 meters of reasonable quality feedline (LMR400).
As Warren will attest, my signal levels (which weren't too bad before), are now much better, with >48 dBc readings in the central ring, elevations above 60 degrees, with a relatively clear view of the sky. As the manufacturer's literature suggests, the Thunderbolt was designed for high gain antennas, and is pretty damn deaf in comparison to most modern designs. But then part of being a time-nut is getting equipment considered obsolete by many to outperform its original specifications.
I'm still doing signal strength measurements (in Lady Heather, S - A - D, and Z for Zoom if you want to see a larger plot). Over 36 to 48 hours, that will show me if I still have problems with nearby trees.
I also use an HP/Symmetricom GPS splitter, the 58535A. The splitter, antenna, and the antenna mount (as well as the thunderbolt) were all sourced from eBay. There are more of the 58532A antennas available from the same seller (in Hong Kong) currently, at a reasonable price. Unfortunately, the folks selling the splitters seem to know what they have, and have priced them accordingly.
Putting on my VHF/UHF ham radio hat for a moment, you know that good feedline and good connectors are a must. Sloppy practices and poor quality in feedline and connectors can easily cost you 6dB or more at the relatively low frequency of 1.5 GHz, not tolerable in such weak signal work as GPS.
And take Warren's advice -- I think he has quite the sideline tuning up Thunderbolts around the world! He's given me many useful suggestions, and more interesting questions to ponder.
Best 73 de Bob K6RTM in Silicon Valley.
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