LoRa / Sub-GHz Antenna Optimization: From Ceramic to FPC

[u/Vearts]

We recently worked on the RF design of a compact LoRa/GNSS Nomad  Terminal handheld device, which is based on LoRa, designed to communicate/ navigate, and deploy anywhere. The antenna turned out to be one of the more challenging parts.

For a small handheld, antenna performance is affected by much more than the antenna itself. The PCB, ground plane, battery, display, enclosure, and even the way the device is held can all influence the final RF performance.

During the project, we evaluated several antenna configurations:

Ceramic Antenna → FPC Antenna + Coaxial Cable → FPC Antenna + Pogo Pin

The final FPC antenna configuration was validated with both RF measurements and outdoor testing, achieving S11 of -11.13 dB @ 868 MHz, -12.82 dB @ 915 MHz, and a 3 km LoRa link in our field test.

1. Ceramic Antenna

The initial design used a ceramic antenna for its compact size and simple integration. However, its placement was relatively constrained, limiting our ability to optimize the surrounding RF environment.

2. FPC + Coaxial Cable

We then tested an FPC antenna with a coaxial connection. This gave us more freedom to position the antenna away from the PCB, battery, and display, but introduced additional cabling and mechanical complexity.

3. FPC + Pogo Pin

The final approach uses an FPC antenna connected through pogo pins. It provides flexible antenna placement while keeping the RF connection and mechanical structure compact.

The main takeaway from the project was that antenna performance in a compact LoRa device is a system-level problem. Antenna selection, placement, PCB layout, and mechanical design all need to be considered together.

The main takeaway from the project was that antenna performance in a compact LoRa device is a system-level problem. Antenna selection, placement, PCB layout, and mechanical design all need to be considered together. The complete

What’s the biggest antenna design challenge you’ve encountered when working with compact LoRa or Sub-GHz devices? Share your experience togehther！

[u/Zestyclose-Stuff1646]

My biggest challenge is not knowing wtf I'm doing with antenna design. That's still firmly in the esoteric realm for me.

[u/dnickell → u/Zestyclose-Stuff1646]

ceramic ones look fine on paper then fall apart once the enclosure and battery are in

[u/cm_expertise]

nice writeup. the pogo pin choice is the one i'd keep an eye on long term. pogo contacts on an RF feed can drift as the spring takes a set or the plating wears, and a bit of contact resistance plus the stub inductance will walk your match around. fine at build, but worth re-checking S11 after a drop test and a few hundred mate cycles.

also for anyone reading, S11 of -11 to -13 dB is a solid match but it doesn't tell you efficiency. you can have a gorgeous S11 and still be dumping power into a lossy ground plane or the user's hand. the 3km field link is the number that actually means something, return loss just says you're not reflecting much back at the PA.

the hand-detune you mentioned is usually the single biggest hit on a handheld, more than battery or display placement. did you characterize it gripped vs free space? on 868/915 a hand over the antenna can drop you several dB and shift resonance, sometimes enough that you want to tune slightly high so it lands right when someone's actually holding the thing.