[time-nuts] GPS antenna in silicon/RTV encapsulation

Jim Lux jimlux at earthlink.net
Mon Apr 14 23:32:27 EDT 2014


On 4/14/14, 12:11 PM, EWKehren at aol.com wrote:
> Am experimenting with small low cost GPS antennas and am considering as an
> alternative RTV/silicon. Any information on RF attenuation of RTV/silicon
> at 1.6  GHz ?
>

Are you potting the antenna in a solid mass of silicone? Or using it to 
seal an enclosure or what?

pure silicone is very low loss, and it probably has an epsilon around 3. 
It can be loaded with silica (which is also low loss) to adjust the 
mechanical properties and electrical properties.   It can also be loaded 
with other things (TiO2)  which will increase the epsilon, but also the 
loss.

the plastics that are notorious for loss are ones that have metal or 
carbon loading or that are hygroscopic so they pick up water.

In the clear plastics world, Polypropylene, polyethylene and polystrene 
are pretty good.  Polycarbonate isn't as good, neither are various 
acetals (Delrin) and acetates

Here's a chart
http://www.rfcafe.com/references/electrical/dielectric-constants-strengths.htm

or another chart
http://www.eccosorb.com/Collateral/Documents/English-US/dielectric-chart.pdf

here's a whole report from Dow on silcone rubbers as dielectrics

http://www.dtic.mil/dtic/tr/fulltext/u2/656331.pdf

They give quite low loss tangents at 10^9 cps  (which I looked up on my 
cps to Hz conversion chart.. That's in your frequency range) 0.0059 loss 
tangent for Silastic 80.

The trick for you will be knowing what else is in your particular 
silicone resin, and controlling the water content.





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