On Thu, 29 Aug 2019 12:59:06 -0400
Dan Kemppainen dan@irtelemetrics.com wrote:

As a general answer, as this kind of question pops up again and again:

For time-nuts home use, any GPS antenna with a suitable band-filter
and a decently shaped radiation pattern is good enough.

The band-filter is nothing but a (cheap) SAW filter in the antenna,
so that's easy to take care (especially in China). The radiation
pattern is a bit harder, but most antenna architectures are quite
benign. Especially if they are not optimized to be ultra small.

Most people will cite stuff like phase center variations. But honestly,
most time-nuts don't have the means to measure that accurately.
E.g. a bad antenna has a phase center variation of 1-2cm. Let us assume
it's 3cm, to make numbers simple. That would mean a worst case time
offset for a single satellite of 100ps. Ie that would result in a worst
case error (assuming there is no averaging, which isn't true) of 100ps.
Compare this to the (AFAIK) best acheived time-transfer uncertainty
using GNSS common view with integer-PPP is ~180ps. Most GPSDOs we have
wont be even that good and are rather in the 1-10ns offset and TDEV
in the order of 1ns.

			Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neal Stephenson

On Fri, 6 Sep 2019 10:07:29 -0700
Nick Sayer via time-nuts time-nuts@lists.febo.com wrote:

I don't recommend 3D printing. The materials used for 3D printing
are quite hygrophile, ie suck up a lot of humidity. This will give
you varations in the refraction index of the cover depending on
the air humidity.

Instead use some thin plastic sheet and thermoform it.
ABS is a good candidate for ease of thermoforming, with reasonable
RF performance. PTFE would be better (almost ideal) in terms of
RF performance, but is quite a bit more difficult to thermoform.
Though, considering how cheap PTFE sheets are, I would probably
try it anyways.

BTW: an indication of what the cover does on the phase center
can be found in [1]

		Attila Kinali

[1] "The Effect of Antenna Covers On GPS Baseline Solutions",
by Braun, Stephens, Ruud, Meertens, 1997
https://kb.unavco.org/kb/article.php?id=185

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neal Stephenson

Hi

There are a lot of 3D printers out there that happily print ABS ….

Bob

On 10/15/19 8:17 AM, Attila Kinali wrote:
On 10/15/19 8:17 AM, Attila Kinali wrote:

antenna. If

I’ve got

presumably just

useful if

You might try a funnel, upside down.

Yes, vacuforming is easy, if you have the equipment.  Hemispheres are
pretty easy (box with a hole in the top and a vacuum cleaner for suction

• heat lamp or broad hot air gun to heat the plastic), but you might be
  able to shop around and just buy something in a shape that works.

Check the craft stores (e.g. Michaels in the US) - one can find all
sorts of plastic shapes in various sizes -  For hemispheres, too, you
can get rigid plastic balls (used for decoration) and cut them in half.
Obviously, one would probably not want the pretty ones with the metallic
film<grin>

Styrofoam (expanded polystyrene) and other foams are also widely
available, and if the shape isn't what you want, that's what bread
carving knives and microplanes are for.

One might also be able to find someone who could fabricate it for you -
here in the Los Angeles area, I'd go looking for a "prop maker" -
someone who makes props for film and video, they usually have a lot of
good fabrication capability (including vacuforming equipment).

On Tue, 15 Oct 2019 11:29:08 -0500
Bob kb8tq kb8tq@n1k.org wrote:

I am not sure that's a good idea.
The way how filament printers work will lead to lots of
pores where humidity can accumulate. Not something you want.

Thermoforming sheets, on the other hand, gives a smooth, closed
surface. Additionaly, the tensile strength of the sheet after
thermoforming is higher than that of filament printed structures.
This means a radome can be thiner, which leads to less lensing,
which is the major source of phase center shift.

		Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neal Stephenson

Attila wrote:

"Bob kb8tq <kb8tq at n1k.org
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com> wrote:

I am not sure that's a good idea.
The way how filament printers work will lead to lots of
pores where humidity can accumulate. Not something you want.

Thermoforming sheets, on the other hand, gives a smooth, closed
surface. Additionaly, the tensile strength of the sheet after
thermoforming is higher than that of filament printed structures.
This means a radome can be thiner, which leads to less lensing,
which is the major source of phase center shift.

		Attila Kinali"

Also, how would 3D printing plastics hold up to UV from sunlight? I don't know much about the materials used, but this is mostly an outdoor application, so it has to endure the elements too.

Ed

On 10/15/19 8:19 PM, ed breya wrote:

As a middle school shop teacher commented: "Paint covers a multitude of
sins"

Hi

The standard process after printing ABS is to vapor the part. That
forms a solid surface.

Bob