There are a lot of discussions lately about the impact of CF decks/enclosures on radio signals. Especially with the VESC WAND people reported disconnecting problems because of the aluminium case of the WAND itself and placing the receiver inside different types of carbon fiber reinforced decks and enclosures.
@Lee_Wright said he will be measuring some RF propagation plots with some professional equipment in the future, but this data isn’t available yet. Because I have no access to a spectrum analyzer and other necessary expensive RF measurement equipment I did a very basic measurement of RSSI values with a smartphone. There are also some questions about range difference from the various Metr Pro antenna type options out there, so I combined these carbon fiber impact measurements with a general range test of different antenna options.
Now you’re probably thinking “I’m standing right on top of the skateboard why do I want more range?”, the answer is pretty straightforward. The difference between the transmit-power and receiver-sensitivity is called link-budget, and increasing distance is only one of many things that can negatively impact this value. Every bit of carbon-fiber in you’re enclosure, every 2.4 Ghz network in your surroundings, even the sunlight or EMI thrown off by nearby electronics subtracts away from this number. With all of these negative factors you want to be sure that your remote is as reliable as possible to ensure your own safety. This isn’t the whole story, and many other factors come into play to get more true to real-world range comparisons but I think we can all agree this section is already too long.
So I did a measurement a few weeks ago and finally put together the pictures and these words to share my observations with you.
The tests were done at a field in the middle of nowhere to have as less radio pollution from other devices as possible. The measurements screenshots were done with the „nRF Connect “ application.
The phone was attached to a cardboard box so my hands did not disturb the readings.
For the first range test five different nRF based bluetooth modules and antenna options were mounted on a 3D printed frame.
- Metr Pro UNITY module with large external antenna („Metr Pro external large“ in the graphs)
- Metr Pro UNITY module with PCB antenna („Metr Pro nRF52840“ in the graphs)
- Metr Pro UNITY module with small external antenna („Metr Pro external small“ in the graphs)
- Standard Metr Pro module with PCB antenna („Metr Pro“ in the graphs)
- Trampa VESC Connect Dongle („VESC BLE UART“ in the graphs)
For the second test (CF impact) the 3D printed frame with the same five modules were placed inside a carbon fibre box. Only the small antenna was mounted outside the box like you can see in the picture. All other modules with their antennas were completely hidden inside the box.
First test // Range data
Second test // Carbon fiber impact
The absolute RSSI values between both tests with and without carbon fiber box can’t be compared 100% because smallest phone movement and phone orientation changes the measurement result. I am quite surprised how much impact the smallest change in phone orientation had, so it isn’t that easy to compare the data against each other. But there is a clear tendency in the relative RSSI values visible that a carbon fiber box reduce signal strength a lot and you should mount an external antenna for the best signal when using any kind of carbon fiber based enclosures. Unfortunately I had no time left to also do 120m/130m/… measurements with the modules outside and 25m/30m/40m/…/… measurements inside the CF box.
But the general range test leads to the following ranking:
- First place goes to the Metr Pro with large external antenna with a range >110m
- Second place goes to the Metr Pro with small external antenna with a range >110m
- Third place goes to the Metr Pro UNITY with PCB antenna with a range of around 100m
- Fourth place goes to the standard Metr Pro with PCB antenna with a range of around 80m
- Fifth place goes to the Trampa VESC Connect Dongle with a range of around 50m
I hope that some of you think these measurements are useful to decide what BT module and antenna option they should get for the most reliable connection in their use case.
Let me know if you have any questions or comments.