[time-nuts] Anyone have experience with this antenna?
kb8tq at n1k.org
Tue Feb 6 16:13:41 EST 2018
Since we are talking about an L1 / L2 antenna here, a reasonable assumption
would be that the target is something better than an “average result”. If you construct
a cover out of a piece of PVC pipe (as shown in the original drawing), your worst
case path has a foot or so of PVC in it compared to a best case path with well under
a tenth of an inch. That’s going to give you a bit of variation ….. Add some dirt or water
or ice to the equation and who knows what the result might be.
> On Feb 6, 2018, at 3:45 PM, Michael Wouters <michaeljwouters at gmail.com> wrote:
> I can see why the geodetic community would worry about antenna phase centre
> variation when a radome is installed but is it really an issue in timing
> applications? The few papers I've read suggest PCVs of less than 10 mm, or
> equivalently, 30 ps. This is at the level of precision available from
> post-processed, carrier phase time-transfer but invisible in the 1 pps
> coming out of your receiver, even with a good sawtooth correction. Am I
> missing something?
> On Wed, 7 Feb 2018 at 4:14 am, Bob kb8tq <kb8tq at n1k.org> wrote:
>> There are “cell site” specific GPS antennas on the market. Panasonic has
>> had one out
>> for quite a while. I’m sure there are several others.
>> One issue with doing any sort of “cover” for a precision antenna is
>> distorting it’s pattern.
>> Plastic (or whatever you use) will have different properties than air. A
>> path through a blob
>> of “not air” will change the effective path length. That impacts the
>> timing and thus the
>> navigation solution. If you are worried about 2mm sort of pattern
>> accuracy, things get
>> tricky. Early on, there was a big “throw out the radomes push when this
>> was first noticed.
>>> On Feb 6, 2018, at 6:15 AM, Bo Hansen <timenuts at rudius.net> wrote:
>>> Besides the RF characteristics it may also be worth considering the
>> quality of the plastics used. Over time water ingress may become an issue.
>> Fours years after the installation of a CN brand antenna, sourced locally
>> so probably not counterfeit either, we had to replace it at OZ7IGY
>>> RF wise 42 dB of gain IS an issue. Again at OZ7IGY, with 12 carriers in
>> the air especially 13 cm and 23 cm, blocking and IMD were an issue before
>> we mounted a BPF. I have taken apart the above mentioned antenna, a
>> Motorola antenna and an eBay "hockey puck" antenna. The best design was
>> clearly the Motorola one because it had a BPF after the pre-amp - probably
>> because it was designed by RF competent people too. Each of the other ones
>> had two FETs/MMICs in series and then a BPF. Of cause if no nearby carriers
>> are in the air it may be less of an issue.
>>> So designing a really good antenna and pre-amp may be a business
>> opportunity. There are many hi IP3 MMICs available designed for GPS and the
>> like purposes. SAW BPFs with <1 dB loss are available fairly cheap so one
>> before the FET/MMIC with a 1 dB NF is the way to go. A DIY radome using
>> standard materials from any hardware shop is attached.
>>> Bo, OZ2M
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