Custom PCB with RFM69HCW, looking for PCB layout and antenna advice please

[doublec4]

Hi all,

I have created a custom PCB design for a prototype where I've integrated the ATMEGA328 and the RFM69HCW (915MHz). I have done this in the past with success, but in an application where distance wasn't much of an issue (less than 50ft). I had used the Moteino as inspiration for my design.

In this application I am hoping to reliably communicate between modules at distances around 500ft or so. Mostly open air with line of sight. I can't really see having to go through walls... occasionally having to communicate between modules in large concrete tunnels (ie. subway tunnels - would the RF signals "bounce" through concrete tunnels or just get absorbed into the concrete walls?) In any case, I am not an RF expert and I am hoping to employ better PCB and antenna layout practices this time around if I can.

I have a few design restrictions... for example I am trying to fit this PCB boards within an existing plastic enclosure (sort of like a short skinny tube) so I don't have room for one of those big whip antennas inside the enclosure. I could use one of those u.FL adapters to go from PCB to a connector for the whip antenna on the side of the enclosure... but I think I am limited to using a wire monopole or a helical antenna if I am to fit the antenna inside of the enclosure. This is preferable as I would be worried about an external antenna getting damaged / ripped off. The PCB will also be oriented vertically in the enclosure and this is where I believe things are less than ideal for the antenna since the GND plane is now vertical and the wire monopole antenna will also be vertical and not normal to the GND plane.

Additionally, the devices will be sitting mostly on the ground so the PCB/antenna will only be ~6-8" off the ground. A person with a monitoring device (also RFM69HCW module inside) will be within 500ft to monitor the data coming from these devices. So these modules will be sending status updates to a hand held device in someone's hand a few feet off the ground. Therefore I don't have some sending/receiving antenna high off of the ground to take advantage of. The only saving grace is that there won't really be obstacles in the way. Open air line of sight as I mentioned earlier.

So this brings me to my main questions... first, based on the info above, does the wire monopole or helical antenna in the vertical orientation sound like my best bet? Second, I have attached a screenshot of the top section of my PCB layout where the RF module and antenna is located. I used the design layout from Moteino as inspiration for the section from the RFM69HCW module antenna pad to the u.FL pads. Since my PCB layout is obviously different than the Moteino, my radio module placement is slightly different in relation to the edge of my board and my monopole/helical antenna would be going straight up from the point it gets soldered in. I added some via stitching in the GND plane area around the antenna... how does this look to you guys? What would you change or suggest?

Thanks for taking a look!

[Felix]

I would do the SMA like you see on Moteinos and go from there. Either edge mount or through hole if you have the space and constraints allow that type of mounting.
You can start with a monopole and then attach SMA based antennas and see what fits your budget vs performance.

[doublec4]

Thanks Felix. Any advice or comment regarding the GND plane and via stitching configuration around the antenna?

Also, I’ve heard that unused pins on the atmega can act like antennas themselves, picking up interference. Should I just pull them high in the software or physically tie them to gnd or Vcc on the board ?

Thanks!

[Felix]

I suppose your board would then be much more of an antenna than "unused pins" - in any case as a good practice they should be made high impedance in firmware, then they are essentially disconnected from the MCU. But if you follow good practices, wherever your physical constraints allow, then your PCB will be more immune to external noise, there are many videos and tutorials online on this subject.

Yes in a 2layer board it's a good idea when you have split ground planes to stitch them but stitching can have multiple meanings in digital, analog and RF design, in the most general sense you typically want stitching to ensure good return path for ground currents in your board and aid the split ground effect.

[doublec4]

Sounds good, thanks!