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

Lester Veenstra lester at veenstras.com
Thu Apr 17 14:09:11 EDT 2014


The classic DIY test of material for RF use is give it 60 seconds in a
microwave oven.
If it gets warm, it’s not a good candidate.

The other more typical concern with "RTV" type materials are the ones using
acetic acid to cure. This outgases, condenses with water vapor to form an
corrosive material on the electronics inside a "sealed" environment. When
the unit mails, and you look inside, you find lots of "green" corrosion
inside, to the point interconnects fail. There are electronic friendly forms
of the sealant that do not have the vinegar odor.


Lester B Veenstra  MØYCM K1YCM W8YCM
lester at veenstras.com

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-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jim Lux
Sent: Monday, April 14, 2014 11:32 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] GPS antenna in silicon/RTV encapsulation

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.h
tm

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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