On 10/15/19 8:17 AM, Attila Kinali wrote:
On 10/15/19 8:17 AM, Attila Kinali wrote:
> On Fri, 6 Sep 2019 10:07:29 -0700
> Nick Sayer via time-nuts <[email protected]> wrote:
>
>> I’m late to the party, but my go-to antenna is the Gilsson marine
antenna. If
>> it has a downside, it’s that it has a flat top instead of a cone.
I’ve got
>> one mounted outside at our vacation home near Lake Tahoe, so we’ll see
>> whether snow loading becomes a problem shortly. The fix would
presumably just
>> be a 3D printed plastic gnome’s cap, though even that might not be
useful if
>> we get snow like we did last year.
You might try a funnel, upside down.
>
> 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.
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).
>
> 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
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