Author Topic: Antenna recomendation - placed inside enclosure  (Read 879 times)

fgomes

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Antenna recomendation - placed inside enclosure
« on: October 29, 2017, 10:53:32 AM »
Hi, I need to put a moteino inside a plastic enclosure that is mounted on a wall. It is working at 915MHz with an RFM69HW. It will have a 16x02 LCD on its front. I want to make the enclosure as small as possible, but the antenna placement and the interference with the remaining hardware inside the enclosure might be a problem. Do you have any suggestions for this kind of configuration? A simple wire antenna? How much should it be apart from the moteino and LCD?

Thanks in advance!

Fernando

TomWS

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Re: Antenna recomendation - placed inside enclosure
« Reply #1 on: October 29, 2017, 12:25:53 PM »
I have found these ceramic antennae (https://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyANT1204F005R0915A) work surprisingly well.  I don't have any quantitative data to back up that claim, but from my experience, I haven't had a wire antenna work noticeably better at 915MHz.  See https://lowpowerlab.com/forum/projects/moteino-based-drip-irrigation-valve/msg20467/#msg20467, first photo for an example usage.  As you can see they are quite small.

Tom

fgomes

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Re: Antenna recomendation - placed inside enclosure
« Reply #2 on: November 16, 2017, 12:46:43 PM »
Hi Tom,

Thank you very much for the tip! This could be a great solution for the problem I have, I will try a design putting it as far away as possible from the LCD and back wall. Can you share some design information? Matching circuit and PCB layout around the antenna and up to the RFM module mainly? The Yageo datasheet doesn't say much about it :-(

Best regards

Fernando


TomWS

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Re: Antenna recomendation - placed inside enclosure
« Reply #3 on: November 16, 2017, 06:50:27 PM »
Hi Tom,

Thank you very much for the tip! This could be a great solution for the problem I have, I will try a design putting it as far away as possible from the LCD and back wall. Can you share some design information? Matching circuit and PCB layout around the antenna and up to the RFM module mainly? The Yageo datasheet doesn't say much about it :-(

Best regards

Fernando
I didn't add a matching circuit.  I mount the RFM69 module antenna pin as close to the edge of the ground plane as practical and use a 0.050" (1.27mm) wide trace from it to the antenna pin, meeting most of the other placement recommendations in the Yageo datasheet.  With the short run and the matching circuit on the RFM69 module, I'm assuming that it's 'good enough'.  Performance seems to support this assumption.

Tom

fgomes

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Re: Antenna recomendation - placed inside enclosure
« Reply #4 on: November 20, 2017, 08:18:15 PM »
Hi Tom, thanks again for your help! Can you tell me why did you make a 1.27mm wide track? For a 50 ohm microstrip the width shouldn't be around 3mm (for a common FR4 PCB with 1.6mm thickness)?

Best regards

Fernando

TomWS

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Re: Antenna recomendation - placed inside enclosure
« Reply #5 on: November 20, 2017, 08:42:39 PM »
Hi Tom, thanks again for your help! Can you tell me why did you make a 1.27mm wide track? For a 50 ohm microstrip the width shouldn't be around 3mm (for a common FR4 PCB with 1.6mm thickness)?

Best regards

Fernando
I'm just reporting what I've used successfully.  You should tune and adjust to your own needs.

Tom

fgomes

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Re: Antenna recomendation - placed inside enclosure
« Reply #6 on: November 21, 2017, 04:39:43 AM »
Hi Tom, thank you! I was asking because the recommendation I got was to use a microstrip feeding the antenna, and for it to have 50 ohm with an FR4 PCB with the most usual PCB parameters, the track width should have about 3.2mm,  (https://www.eeweb.com/tools/microstrip-impedance, T=1.37mil, H=1.6mm, W=3.2mm, Er=4, Z=50.3 Ohm). Keeping all the PCB parameters and changing the track width to 1.27mm the microstrip impedance will rise to 81 Ohm. But of course, the real results are the ones that are important, particularly with RF where there are many other things that impact on the real world results! I'll have to make a PCB and will keep you posted about the final result :-)

Best regards

Fernando