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'[OT]: GPS Case design - test the Box using relativ'
2002\08\02@213606 by Jim

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... simple test for transparency to RF at 1.575 GHz
is to place the GPS antenna inside then outside the
plastic box under consideration then (given the right
module!) read the difference in signal stength (my JRC
OEM modules in one mode do this trick with ease and
the SS value measured is output relative in deciBels) ...

Second test: Place the GPS antenna in the final position
within the plastic box under consideration in the *same*
proximity to the plastic box's top surface and again
measure the signal strength.

Proximity of the antenna to the platic "cover" will affect
antenna pattern directly and therefore affect indirectly
the antenna's utltimate gain in the desired direction.

(I think I saw a previous post along the same lines but
no reference to the box's plastic affecting the antenna
pattern when in close proximity ...)

RF Jim



{Original Message removed}

2002\08\03@135525 by Peter L. Peres

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On Fri, 2 Aug 2002, Jim wrote:

>... simple test for transparency to RF at 1.575 GHz
>is to place the GPS antenna inside then outside the
>plastic box under consideration then (given the right
>module!) read the difference in signal stength (my JRC
>OEM modules in one mode do this trick with ease and
>the SS value measured is output relative in deciBels) ...
>
>Second test: Place the GPS antenna in the final position
>within the plastic box under consideration in the *same*
>proximity to the plastic box's top surface and again
>measure the signal strength.
>
>Proximity of the antenna to the platic "cover" will affect
>antenna pattern directly and therefore affect indirectly
>the antenna's utltimate gain in the desired direction.
>
>(I think I saw a previous post along the same lines but
>no reference to the box's plastic affecting the antenna
>pattern when in close proximity ...)

I'll make a blind guess about the change of angle of view due to the
dielectric plastic affecting the gain negligibly, for a point source at
infinity and an initial angle of view of >60 degrees. ;-)

However it could worsen the SWR by changing the impedance of some
elements. So get a box with a RF window (make a hole in almost any box).
You can buy spare radomes for satellite LNBs and make a hole in your box
to fit that. I hope your windshield does not have an antistatic layer.
Also no antireflex coating or oxide metalization (some toned windows) ;-)

Peter

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