Author Topic: 915Mhz Lora Moteino and epoxy potting  (Read 1091 times)

warp929

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915Mhz Lora Moteino and epoxy potting
« on: February 01, 2017, 10:25:32 PM »
Hello everyone.  I'm new to RF transceivers and have only been at it for about a month.  Recently I potted a Moteino and my reception went down to about a 1/3 of what it was before potting.  I assume that the potting is the culprit but I'm not sure why.  Could it be that it created shielding from the ground plane?  Below are the specifications of my setup.

My "transmitter" is a Moteino Lora 915MHz with a couple push buttons enclosed in a polycarbonate case with an external 915MHz antenna.
My "receiver" is a Moteino Lora 915MHz with a relay enclosed in a polycarbonate case with the supplied 915MHz wire attached to the Moteino.
Range was 3/4 mile before potting.

I then potted the "receiver" with MG Chemical 832B potting compound up to the point of just covering the Atmel microprocessor.  The antenna was not in the potting compound and remained attached to the side of the polycarbonate case.  (http://www.mgchemicals.com/products/potting-compounds/epoxy-potting-compounds/black-epoxy-832b)
Range became 1/4 mile after potting

Will adding a wire (915MHz 1/4 wave length) from the Moteino ground help.  That ground wire will then be outside of the potting.  My assumption is that it will create a dipole antenna.  Any thoughts on this idea or other possible solutions while keeping the potting?  I know I can use an external antenna but the chance of breaking it is high.  Options that keep the antenna in the polycarbonate enclosure are preferred.  Thank you.

 

Felix

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Re: 915Mhz Lora Moteino and potting
« Reply #1 on: February 03, 2017, 11:53:24 AM »
Never tried potting but that is strange that the epoxy can have such a dramatic effect.
Was it a regular moteino? How about bringing out 2 wires and/or try to connect an SMA for an external antenna. Potting and antenna connection straight on the PCB seems a bit of a tricky mix.

perky

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Re: 915Mhz Lora Moteino and potting
« Reply #2 on: February 03, 2017, 12:13:39 PM »
Never tried potting but that is strange that the epoxy can have such a dramatic effect.
Was it a regular moteino? How about bringing out 2 wires and/or try to connect an SMA for an external antenna. Potting and antenna connection straight on the PCB seems a bit of a tricky mix.
I think it's probably down to having a high dielectric constant, which will basically give it capacitance at RF. There are special potting compounds for RF with low dielectric constants:
http://www.epicresins.com/ElectronicsPotting/rfdevices
Mark.
Edit: The same is true for conformally coating the module. You should choose a conformal coating with low dielectric constant designed for RF.
« Last Edit: February 03, 2017, 12:24:58 PM by perky »

warp929

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Re: 915Mhz Lora Moteino and potting
« Reply #3 on: February 03, 2017, 06:01:41 PM »
Never tried potting but that is strange that the epoxy can have such a dramatic effect.
Was it a regular moteino? How about bringing out 2 wires and/or try to connect an SMA for an external antenna. Potting and antenna connection straight on the PCB seems a bit of a tricky mix.

Yes a standard Moteino.  That was my thought, bringing two wires out but if it doesn't work, I'll end up scrapping the assembly.  Sometimes it has to happen but trying to avoid that.

I think it's probably down to having a high dielectric constant, which will basically give it capacitance at RF. There are special potting compounds for RF with low dielectric constants:
http://www.epicresins.com/ElectronicsPotting/rfdevices
Mark.
Edit: The same is true for conformally coating the module. You should choose a conformal coating with low dielectric constant designed for RF.

Thank you. I talked to Epic Resins.  Very helpful but they have a $500 minimum order.  That's a bit much for me. 
They recommended finding a compound with a dielectric constant of 2.5-2.7.  The MG Chemicals 832B has a dielectric constant of 2.83.  That does not seem that far off.  MG Chem does not mention the type of filler used, maybe that's the issue.

perky

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Re: 915Mhz Lora Moteino and potting
« Reply #4 on: February 03, 2017, 06:12:20 PM »
OK. Sometimes they use filler to reduce the thermal impedance to allow heat out (sometimes also laden with metal oxide particles, not electrically conductive but the overall thermal impedance is lower), but the description seemed to imply it having a high thermal impedance. Maybe it's worth going for one that specifically mentions usage with RF components in the datasheet.
Mark.
Edit: Here's one from Electrolube that's apparently specifically designed for RF:
http://www.electrolube.com/products/polyurethane-epoxy-resins/er1448/resins_epoxy/

« Last Edit: February 03, 2017, 08:26:28 PM by perky »