Bluetooth antenna on top of large metal object and other RF questions

Question

I've been doing some research on RF antenna design considerations but haven't really found anything close to the situation my device will be in. I am designing a wireless sensor for my FSAE team that will be mounted inside each tire (attached to the wheel rim), similar to this product and reports various measurements to a receiver mounted somewhere on the car. The design process (components, schematic, etc) is pretty much finished except for one area, the antenna.

Since the device will be sitting so closely to a large metal object (aluminum wheel) I am concerned about the effect this will have on the antenna. Below are some other considerations:

• The device is using the NRF52833 (Laird BL653 module with external antenna) and will communicate via BLE (2.4GHz)
• The PCB will be roughly 3cm x 4cm (estimate) with 4 layers, one of which will be a ground plane
• I am open to using any variety of antenna (PCB trace, SMD, patch, whip, dome etc) as long as it is 50 ohm impedance as the BL653 specifies

Now for my questions:

• I understand that antennas use a connected ground plane to aid performance, if I were to connect the PCB ground to the aluminum wheel, would the entire wheel now also act as a ground plane and aid performance as opposed to hindering it if it were disconnected?
• Would a flex patch antenna attached directly onto the aluminum wheel surface be desirable? (with or without the wheel connected to PCB ground as mentioned above)

Thank you so much to anyone who has the time and/or expertise to respond!

Answer 1

A flexible patch antenna affixed directly to the metal wheel will not work well. The conducting surface of the wheel will effectively "short out" the antenna at radio frequencies, whether there is an electrical connection to it or not.

The wheel creates an unavoidable ground plane, so you have two choices:

1. Use a "ground independent" antenna, such as a flex patch dipole, oriented parallel to the rim and elevated above it. It's optimal to elevate it a quarter wavelength (at 2.4 GHz, that's about 3 cm), but it will work decently at half that distance. You could attach the patch to the upper wall of a plastic enclosure, or laminate it with some foam between the antenna and the rim.

2. Use a "ground-dependent" monopole antenna. You can use your PCB ground plane. Connecting that PCB ground to the actual metal wheel might or might not work better. In this case, the antenna element would stick up perpendicular to the wheel rim. Optimal length is a quarter wavelength, but there are ways to shorten that by bending the tip, coiling the element, or adding an inductor in the feed circuit.

Besides temperature, you'll have to take into account shock, vibration, and acceleration in your mechanical design.