[time-nuts] GPS Antennas

[Björn]

https://www.ngs.noaa.gov/ANTCAL/#

Look at say an old Ashtech ASH700936D_M which you can find calibrations for both SCIS and “Snow” radomes and without radome for the same antenna.

I have seen more recent radomes that are hemisphere-shaped. However these are more prone to be favored bird view points.

Radomes, like the Aeroantenna SPKE with a plastic “spike” to discourage birds are less ideal for super high accuracy applications due to its non uniform signal delay impact.

See also clear acrylic domes OSOD/OSOS

http://www.euref.eu/symposia/2015Leipzig/p-02-01-Ohlsson.pdf

https://kb.unavco.org/kb/article/unavco-resources-radomes-520.html

/Björn 

Sent from my iPhone

> On 7 Sep 2019, at 02:54, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> Hi
> 
> Based on the papers I have seen, there is no “magic” shape that can be ignored. 
> Part of the issue is that the cover is going to be in the near field of the antenna.
> As noted in another post, the impact is more complex than a simple delay. ( consider 
> that signals bounce off the cover material …. ).
> 
> Also consider that if I’m going to 3D print the device, I have a limited number of 
> materials available. Teflon isn’t going to run through a 3D printer. Unless you paint
> the gizmo it will not be water tight so humidity will get into the act as well. 
> 
> =====
> 
> A “good” GPS antenna should have roughly the same phase center regardless of
> the angle to the sat it’s receiving. Indeed GPS antennas in general get this right.
> There is a modest variation with angle, but it’s not all that great. Units of mm are not 
> uncommon. It will be greater on a multi band design. The impact of your cover also 
> will be greater on a multi band device. All the various delay /. loss / reflection parameters
> are frequency dependent…..
> 
> Bob
> 
>> On Sep 6, 2019, at 5:57 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>> 
>> 
>> kb8tq at n1k.org said:
>>> The gotcha with putting anything on top of a GPS antenna is that it impacts
>>> the phase  shift from the sat’s. How important that is gets quickly into just
>>> how picky you happen to be. 
>> 
>> How picky do I have to be before that is significant?  How geeky would I have 
>> to be to measure it?
>> 
>> Would a hemispherical cover provide equal delays at all elevations?  I think 
>> the solidly mounted USGS fault/volcano monitoring setups have a round top.
>> https://www.usgs.gov/media/images/gps-tracks-ground-deformation
>> 
>> Do any antenna manufacturers advertise uniform phase delay?
>> Do any software vendors advertise that their software works better with 
>> specific antennas?
>> 
>> 
>> 
>> -- 
>> These are my opinions.  I hate spam.
>> 
>> 
>> 
>> 
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