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Questions about covering an antenna with plastic - RF Cafe Forums
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Michael
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Post subject: Questions about covering an antenna with plastic
Posted: Tue Sep 13, 2005 10:39 am
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Good day. We have a microstrip patch antenna
that is operating at 1 GHz. Everything works very
well when the antennna is in free space. But due
to environmental considerations we need to cover
the antenna. We are looking at a couple of different
methods and I would like some input (articles, books,
websites, personal experience, etc.) on these methods.
Method 1) Totally encapsulate the antenna with
potting material. The dielectric constant for the
potting would be 3 to 5. The thickness of the material
is still under discussion. Method 2) Use
a plastic cover over the antenna. The plastic would
also have a dielectric constant of 3 to 5. There
will be an air gap between the antenna and the plastic
cover. The gap could be anywhere from 0 to 5 mm
(For a gap of 0 mm, the plastic cover would actually
be touching the antenna). The thickness of the plastic
has not been decided. Thank you for your
time and help. Michael
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Steve R |
Post subject: Covering your antenna
Posted: Wed Sep 14, 2005 11:09 am
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Can't recommend any books but..............
1. Your epsilon R is an issue but this is not
the major concern (see 2 below). With epsilon R
greater than 1 your antenna will be de-tuned to
some degree depending on how near to the actual
high Z (high E field) bits the higher Er material
is. but you can account for this by offsetting the
tuned frequency when in free space OR you have some
method of tuning once the cover is on. In any case
its not a great issue. 2. The thing you need
to watch for is the Tan delta or dielectric loss
of the materia you put on or near the radiating
elements. Materials like PTFE are nice as they have
very low tan delta. If your not careful you can
waste quite a lot of power in a poor dielectric.
Note that some coloured plastics have fillers that
are carbon based (definately not good). Hope
thats somthing to go on with
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kanling |
Post subject:
Posted: Wed Sep 14, 2005 2:42 pm
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Colonel |
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Joined: Mon Jul 18, 2005
4:31 pm Posts: 32 Location: Baltimore, MD
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Remember that the antenna was (I assume) tuned in
air. The wavelength in plastic will be smaller.
So, for the potted option, since the plastic is
right on the antenna, the ant dimensions might need
to be changed slightly to maintain tuning.
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cbw003 |
Post subject: Re: Questions about covering an antenna with plastic
Posted: Wed Sep 14, 2005 7:57 pm
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As mentioned previously by the other responders,
dielectric is less of an issue than loss tangent
or tan delta (or loss of the material). GE Plastics
has posted on their website different material properties.
Generally, Lexan141 or the like (a type of poycarbonate
plastic) is ok to use. A lot will depend on
your application. Is the antenna meant to be a stand
alone base station type or is it meant to go into
something like a cell phone or laptop computer,
etc. Antenna efficiency or gain will go down
with whatever material is covering the antenna.
How much degradation is dependent on the material
properties of the material being used.
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Graham |
Post subject:
Posted: Wed Sep 28, 2005 7:43 am
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Colonel |
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Joined: Fri Sep 02, 2005
7:25 pm Posts: 34 Location: Hampshire UK
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Additional to all the good stuff so far, glass reinforced
plastic radomes use polyester rather than epoxy.
Polystyrene is also low loss, but may not be
robust enough. You need to consider sunlight degradation.
The right exterior paint is the solution. I once
stuck a piece of white PVC (push-on system) 40mm
wastepipe into a microwave oven, along with a glass
of water. It stayed cold, so I used it for a mandrel
support for a 2.4GHz helical antenna. Whatever the
loss tangent, it seemed not to matter.
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HJF |
Post subject: antenna encapsulant
Posted: Thu Oct 06, 2005 3:29 pm
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Hi, This may sound pretty hokey, but I had a
small printed loop (~2"x2.5") that had to be ruggedized.
I tried the liquid electric tape route (Wal-Mart).
I just painted it on the entire PCB. The VSWR actually
improved!
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Graham |
Post subject:
Posted: Tue Oct 11, 2005 7:15 am
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Colonel |
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Joined: Fri Sep 02, 2005
7:25 pm Posts: 34 Location: Hampshire UK
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Oh Gosh. That may not be a good sign. Consider
that VSWR is *not* the "goodness factor" that says
it is radiating better. It actually tells you that
less is being reflected back down the feed.
A very lossy antenna, say one that has a dummy
load connected across it, or maybe laying on the
ground in a pile of wet grass, might only be warming
up the leaves, and has an excellent VSWR!
Try an unpainted in a test. Then paint it, but
touch nothing else. Actually, I am thinking that
the paint will not make any difference. There may
be other factors that made the VSWR measure better
second time around.
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Posted 11/12/2012
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