[time-nuts] Anyone have experience with this antenna?

[Michael Wouters]

That does represent a limiting case but it's a bit pessimistic. The longest
path is for very shallow incident angles eg a 3 mm thick and 150 mm radius
disk gives an angle of only about 1 degree. At 10 degrees, the path is
about 20 mm; with a refractive index of 1.5, the path is only 10 mm longer.
10 degrees might be what you set in the receiver's elevation mask.

A mm thick layer of dirt is just going to be roughly another mm of plastic;
worse if it's absorbed moisture, true.

You have a point about water. Water has about 10 times the refractive index
of plastic (real part of n)  so this is more of a worry. A 1 mm film will
have triple the path through a nominal 3 mm of plastic so at 10 degrees
incidence there is now about 40 mm extra path which you might see in
post-processing. But you're not going to see that in the 1 pps.

Cheers
Michael

On Wed, 7 Feb 2018 at 8:14 am, Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> 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.
>
> Bob
>
> > 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?
> >
> > Cheers
> > Michael
> >
> > On Wed, 7 Feb 2018 at 4:14 am, Bob kb8tq <kb8tq at n1k.org> wrote:
> >
> >> Hi
> >>
> >> 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.
> >>
> >> Bob
> >>
> >>> On Feb 6, 2018, at 6:15 AM, Bo Hansen <timenuts at rudius.net> wrote:
> >>>
> >>> Hi
> >>>
> >>> 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
> >> www.oz7igy.dk
> >>>
> >>> 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
> >>>
> >>> <GPSAnt_DIY_Radome.png>_______________________________________________
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