On 9/15/13 4:33 PM, Bob Camp wrote:
Hi
All of the ones I've opened up or seen dis-assembled have had the
ceramic plate antennas in them. That very much surprised me early on,
since I *assumed* they had something fancy inside based on their
shape.
I don't know that the thermal expansion effects on the ceramic or the
antenna elements would be all that huge (it's a wideband low-Q device,
after all), compared, say, to the CTE effects on the coax, or the
temperature effects on components inside the LNA.
No argument about the filtering, I'm not sure if the temp-co of
filter delay on an exposed antenna makes it a plus or a minus….
I'd guess that the crossed drooped dipole or quad helix or quad spirals
might have a more consistent phase center as a function of look angle.
And a bigger radome is always good, because "crud" on the radome is
farther away and will have less effect.
Hi Bob, Many of the midrange antennas have one or more significant
differences from the cheap "pucks". Firstly they generally have
better filtering, many pucks have none. This is important if you
are co-located with transmitters.
Yes. We have some timing type antennas at work which are effectively
jammed by a cellphone near by.
And, the geodetic GPS folks are not looking forward to wider deployment
of LTE/4G, which uses bands closer to the GPS bands (presumably the 1700
MHz ones), although keeping the phones 10 meters or so away seems to be
enough to fix that (barring Light Cubed rising from the dead carcass of
Light Squared)
Secondly many use quad-helix
antenna elements rather than the off-set feed ceramic patches in
the pucks. The heical elements have better control of the radiation
pattern and along with a larger radome are less likely to be
affected by external contamination. I also wonder how the tuning of
a cheap ceramic patch holds up over the range of temperatures seen
by a fixed antenna. Modern receivers compensate well for poor
antennas, try using an early receiver on an internal patch and you
won't get great results.
You might be able to design a patch that "self compensates".. the
antenna gets bigger as temperature goes up, but also moves away from the
ground plane AND the effective epsilon gets lower because the material
is less dense. In any case, it's a small effect. ALumina has a CTE of
6-8 and Copper is about 17, so the thermal effect of an air dielectric
copper antenna might be bigger than a plated onto alumina one.
Robert G8RPI.
________________________________ From: Bob Camp <[email protected]> To:
Discussion of precise time and frequency measurement
<[email protected]> Sent: Sunday, 15 September 2013, 22:19
Subject: Re: [time-nuts] Req: Decent GPS Antenna Active/Passive
Recommendation
Hi
Worth noting:
The "mid class" antennas are not a lot different electrically than
the "low end" antennas. The main differences are mechanical:
1) You get a much more weather tight housing 2) You get a rational
way to mount the antenna 3) There's a connector on it so you can
put a good piece of coax on it 4) The housing *may* be more immune
to snow / ice buildup and bird nests
RDR Electronics on the usual auction site appears to be selling
some nice ones at the moment.
Bob
On Sep 15, 2013, at 4:36 PM, Hal Murray <[email protected]>
wrote:
[email protected] 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.
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?)
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.
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.
There is yet another dimension. GPS receivers come in two modes:
navigation and timing. Navigation units need 3 or 4 satellites
to figure out where (and when) they are located. The timing
units assume they are not moving and that they know their
location. They should be able to maintain timing with only 1
satellite.
-- These are my opinions. I hate spam.
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