Last year, a client contacted me regarding his small PCB with a very small ceramic antenna for GNSS at 1.575 GHz. He used the smallest possible ceramic antenna he could find, placed it on a very small PCB and no surprise, it failed completely. Mislead by the small device, the unexperienced designer totally missed the importance of PCB layout and ground size for such small antenna designs.

From the Pulse W3011 antenna data sheet: “Note: Electrical characteristics depend on test board (GP) size and antenna positioning on GP and the size of the ground clearance area.”

That hint is very important, because small ceramic antennas rely on PCB ground for radiation. The W3011 ceramic device itself can be understood as a capacitor, and by adding a PCB cutout with inductance from path length around that cutout we have a resonant structure in the cutout region. With proper placement of that slot radiator on the edge of a large PCB, we can then achieve good radiation performance. The PCB ground plane is guiding radiation and coupling the resonator to free space. Only if we combine all these ingredients properly, we have a functional antenna.

In this analysis report, we will see how layout implementation will change antenna performance. Baseline is the layout recommended in the manufacturer data sheet. PCB dimensions in the datasheet are 80mm x 37mm.

To avoid possible misunderstanding: there is no dielectric resonator mode in the PCB here, the antenna will be fully functional if we use via fences all over the PCB. It is the metal ground plane that is required to guide the waves, the dielectric board material doesn’t have much effect.

Frequency tweaked by cutout size

As already mentioned above, resonance frequency is determined here by the “antenna” capacitance between the two terminals and the inductance formed by path length around the cutout.

It might be unexpected to explain and treat this “antenna” as a capacitor, but it’s really about generating a resonance around the slot, and the “antenna” provides the required capacitance between the two terminals. The plot below shows the resulting magnetic field in the slot region:

Looking at the electric field, we can see that the entire PCB ground plane is radiating:

If we change the cutout size, we change inductance and this changes resonance frequency.

Impedance depends on PCB size

PCB length has almost no effect on frequency, but a massive effect on input impedance. For the model shown below, best matching is achieved around 50-60mm edge length. Smaller PCB length results in larger input resistance at resonance, and this is what broke the client’s design: his PCB was much too small.

Resonance frequency is almost independent of PCB length.

Antenna feed and shunt inductance

The antenna feed is implemented by connecting one device pad to both the input (here: port 1)  and to a shunt sub. This shunt stub connection is the magic to achieve 50 Ohm matching at resonance. Small shunt inductance changes of +/- 0.2 nH have a significant impact on input impedance, so this layout detail must be adjusted with care when using a different PCB size.

 

Very different PCB shape

If we change the PCB to a very different shape, e.g. the rounded shape below, we expect not much change in frequency if we keep the cutout size. The much smaller ground size and shape will however change input impedance at resonance. Note that in this example, the ceramic patch is still located on a straight edge segment.

Simulation results below confirm: resonance is off only slightly, and can be adjusted easily by decreasing the cutout size. However, input impedance at resonance is way above 50 Ohm, external matching is required in this case.

Summary

It sounds very tempting to design very small antennas using very small ceramic antenna modules. However, that ceramic device is only one part of the antenna, and the surrounding PCB must be designed with care. It is possible to dual-use an existing ground plane of a large PCB, so that the antenna device itself requires little PCB area, but the PCB itself must not shrink too much. The smallprint in the antenna datasheet is indeed important: PCB layout is the key here!