On 9/15/13 1:36 PM, Hal Murray wrote:
j...@westmorelandengineering.com said:
Well, I need something that I can put outside, in the weather, with my
verticals, and other antennas. I am a Ham radio enthusiast, and I want
something I can properly mount and can be an all-weather device and can live
happily 'in the farm' so to speak.
I split GPS antennas into 3 clumps.
At the low cost end are the small "mouse" or "hockey-puck" type units,
usually with a magnetic mount. They typically come with 10 or 15 feet of
thin (lossy) cable. Ballpark price is $10.
In the middle are the typical cones that you see on cell phone stations. The
Lucent 26 dB ones are common on eBay. Ballpark price is $50. The same or
very similar thing is also available with different brand names. Some of
them come with a pipe mounting setup such that the coax and connector is
inside the pipe and out of the weather.
http://www.megapathdsl.net/~hmurray/time-nuts/Lucent-Antenna.jpg
At the top end are the choke ring antennas intended for surveying. They are
mostly out of my price range so I haven't looked carefully.
----------
I haven't seen a GPS antenna without an amplifier, but I haven't been
looking. They also include a filter. See the LightSquared flame-wars for a
discussion of filters.
I think the choke ring antennas usually let L1 and L2 through while most
others are L1 only.
Or, it might be that the choke ring is "tuned" for L1, but not L2/L5.
Multiband choke rings are more complex than single band ones. the
classic Dorne Margolin/JPL choke ring is pretty straighforward, and, in
fact, one can do the nested cake pan thing to get pretty close.
The multiband choke rings have segments and steps. The Leica ones I've
used are termed "artichokes" because that is what they look like.
Topcon has some really funky looking ones with mushroom shaped rods
sticking out.
The other important consideration is the sensitivity of your receiver. Every
couple of years a new generation comes out that is a few dB better than the
previous ones. (Has anybody seen a Moore's Law type graph?)
I'd find "few dB" hard to believe. The NF of most LNAs these days is
sub 2 dB, so changes are going to be in the "tenths of a dB" range.
Modern receivers are sensitive enough to work indoors with a non-fancy
antenna, at least most of the time. YMMV etc, and "indoors" probably doesn't
include buildings with a lot of steel. It doesn't cost much to try.
Sensitivity probably isn't the issue. Multipath is probably the
dominant error source.
If you have an old recycled GPSDO such as a TBolt or Z3801A, the receiver is
much less sensitive and a good antenna position helps a lot. Of course, it
also depends upon what you want to do and/or how nutty you are feeling.
My Z3801 uses an antenna with a built in LNA, which is typical.
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