Trying to cover 315, 433, 868 and 915 MHz with one Sub-GHz antenna
We’ve been working on the Sub-GHz antenna system for High Boy, and one of the hardest parts has been dealing with a very simple requirement that gets messy fast in practice:
We want one physical antenna to cover 315, 433, 868 and 915 MHz.
That sounds nice on paper, but in a compact device it’s a real challenge.
Those bands are far enough apart that a single small antenna will not naturally behave well across all of them without some help. And once you start shrinking everything into a handheld device, every little detail matters.
The antenna shape matters.
The surrounding copper matters.
The ground plane matters.
Even what sits under an inductor starts becoming part of the problem.
Our current approach is active tuning.
We’re using an RF SP4T switch, controlled by 2 GPIOs, to select between 4 different impedance matching states depending on the band we want to optimize for.
One of those states is an open circuit, with no component to ground, and that one is being used for the lowest band.
The topology currently uses 3 fixed shunt components in the main RF path, plus 4 switchable paths depending on which band we want to optimize for.
We also added ground plane openings under the inductors to reduce parasitic capacitance. It’s one of those details that sounds small until you remember that in RF, the PCB stops being just a place to mount components and starts becoming part of the circuit itself.
What’s been interesting here is that this is one of those problems where every solution comes with a trade-off:
Multiple antennas would take more space.
A broader passive compromise would be simpler, but weaker.
Active tuning gives us more control, but adds switching logic, layout complexity, and more things to validate.
Still validating the design, but this has been a fun RF problem so far.
Has anyone here worked on a compact multi-band Sub-GHz antenna system before? Curious how you approached the matching/tuning trade-offs.
