[time-nuts] Req: Decent GPS Antenna Active/Passive Recommendation

Jim Lux jimlux at earthlink.net
Sun Sep 15 23:55:20 UTC 2013


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 <lists at rtty.us> To:
>> Discussion of precise time and frequency measurement
>> <time-nuts at febo.com> 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 <hmurray at megapathdsl.net>
>> wrote:
>>
>>>
>>> john at 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.
>>>
>>> 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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>>
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