# Material that Degrade GPS Signal Reception

Between clear tempered glass (see through,thick & very strong) Vs. coloured plastic material, which would result in more degradation of GPS signals if used to make a box? The box contains a positioning module and a GSM module with an antenna.

The thickness of the plastic i'm thinking is about 5mm. But of course, its just a rough figure.

If I am to involve tempered glass, my intention is to use the same only on top of the area of the antenna, may be for the botton top half of the box.

Is there other known fairly strong material (may be even rubber) that will do a better job?

This is only a prototype design.

• This can't be answered without the exact composition of the materials being given, but there's a fair chance that unless pigments or fillers change things, neither may be an issue. Consider a test with three receivers and two enclosures, compare performance, then swap the enclosures around. Try to see if any differences move with the enclosure, or stay with a given receiver. – Chris Stratton Sep 29 '14 at 2:43
• A Google search for 'qualities of materials that affect radio transmission' threw this up: digital.library.unt.edu/ark:/67531/metadc5801/m2/1/high_res_d/… – copper.hat Sep 29 '14 at 4:41

There are a few things to consider with a radome over a GPS antenna.

1. the reflection that occurs at the interfaces with air.
2. the loss inside the radome.
3. the near-field effect the radome has on the tuning of the antenna.

Dielectric effects and reflection

The radome is a sheet of dielectric, which has a permeability different to that of air. There will be a reflection of the wave as it enters the material, and as it leaves. In a thin radome, much less than 1/4 wavelength, these two effects can be combined.

The reflection depends on the thickness of the material, and the dielectric constant. Most plastics have an $\epsilon_r$ of about 2.1, and glass is about 4.0 (tempering doesn't matter). For a thin skin, the effect is approximately linear with thickness and dielectric, so a 1 mm radome has half the impact of a 2 mm radome, and a 5 mm plastic radome has the same effect as a 2.5 mm plate glass radome.

I can't find the equations online or in my textbooks, so you'll have to go with my memory for now, which is that for normal incidence, a 5 mm sheet of plastic should not cause a significant transmission loss at GPS frequencies.

But that's not all. At increasing angles of incidence, the plate becomes "thicker" to the incident wave, the reflection becomes stronger and the transmission weaker. Also, the two polarisation components have different transmission coefficients, so the circular polarisation of the antenna will be degraded. A GPS antenna needs to operate with signals from as low as 10 degrees above the horizon, 80 degrees from normal incidence, and at this angle, even a thin sheet of plastic has a significant effect.

The best solution is to have a dome or even hemispherical radome, made from the best thin dielectric. The definition of best is the highest $strength / (thickness * \epsilon_r)$.

2. Losses in the material

If the material is lossy, that will reduce the signal transmitted. This should drive your choice of materials towards pure plastics, or high-glass-fraction composites. Again, in proportion to the thickness of the material - a glass-loaded plastic can be made much thinner than a pure plastic, for the same strength, so it may be even less lossy.

RF Cafe has a table of dielectric constants and loss tangents

The big thing to watch out for is the effect of additives in the plastic. Black plastic is usually bad as it is often coloured with carbon black. Cheap regrind plastic has a particularly high percentage of this black pigment. UV-stabilisers can also increase the loss. Translucent pigments that colour the plastic will have no effect.

Nylon is a common strong plastic that is unfortunately not ideal for radomes, as it absorbs a significant amount of moisture, which raises the dielectric constant, and increases the loss.