Bluetooth Dongle range comparison and CF impact

[hexakopter]

Introduction
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.[1] 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.

The range of the radio signal is correlated with the reliability of the overall radio connection. Let me quote @Deodand ho has explained it well already [2]:

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.

Test setup
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 [3]“ 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.

Measurement data

First test // Range data

2 meters

4 meters

6 meters

8 meters

10 meters

15 meters

20 meters

25 meters

30 meters

40 meters

50 meters

60 meters

70 meters

80 meters

90 meters

100 meters

110 meters

Second test // Carbon fiber impact

2 meters

4 meters

6 meters

8 meters

10 meters

15 meters

20 meters

Conclusion
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.

[1] https://forum.esk8.news/t/some-reporting-issues-with-vesc-wand-under-investigation/10561/249
[2] https://www.electric-skateboard.builders/t/focbox-pilot-new-open-source-universal-remote-by-enertion/97732
[3] https://apps.apple.com/de/app/nrf-connect/id1054362403

[ducktaperules]

Awesome to see some real testing and data. you’ve done a great job here.

[hexakopter]

Thanks.

[ducktaperules]

I guess this proves that if you have carbon fiber enclosure then either Metr with external antenna is a worthwhile upgrade over using an internal antenna.

Also any Metr is better than the VESC connect dongle.

Wonder if you will ever be able to use Metr with the WAND or other NRF remotes?

[rpasichnyk]

Maybe. We think this is possible but requires a lot of time invested in testing and reliability. At the moment it is easier to have 2 dongles.

[Lee_Wright]

This is fantastic testing @hexakopter

It shows relative signal strength for all the popular transeivers and is a great starting point for this thread.

What we must test from here is what the signal strength is at 0.5/1m (the average distance from remote/phone and board) and then the maximum distance that a device will work at. An RSSI value for the given distances is one thing but we do not know if the sensitivity of the receiving device allows it to operate at say -100dBm, I would guess not in that case. Once we have data for signal strength at 0.5m and at maximum usable range then we have fade margin, which is the metric that allows us to truly see how reliable the link will be over reasonable distances. This is the test I was aiming to produce, I was going to simply measure it with a spectrum analyser instead of the app but I think your method is valid enough.

[hexakopter]

@Lee_Wright

An RSSI value for the given distances is one thing but we do not know if the sensitivity of the receiving device allows it to operate at say -100dBm, I would guess not in that case.

Yes thats true. I also wanted to push VESC realtime data trough and see at what sensitivity it still operates fine, but I had no more time. I just connected power to the five modules and no tx/rx so I couldn’t do that test back then anyways.

[Soflo]

@Trampa might be something to look into

[Trampa]

In reality you don’t need 50+ meter range. Connection reliability is a matter of multiple factors including the used method to transfer the data. If you have a good signal over 1Km distance, but the signal is prone to interference that is not good. If you have a signal that is reliable over 30meter and interference is not happening at all that is a lot better and the option to go for. As Lee pointed out, we are always close to the receiver, so terrible range is not really needed as long as the signal is always strong enough to get through to the other side. Riding in urban environments with tons of interference from other RF devices is the challenge. There was a group ride in London recently and the Wands didn’t cut out a single time although the list of other Bluetooth devices you get listed in the App is beyond any scale.
There is also 5G around there. That is the worst environment to put a remote into. If the riders do not face any interference and have a stable signal at all times that is a good indicator for a very very high reliability.
With PPM we did get cutouts in London, with BLE we do not see such things. There is still room to make the receiver side stronger and we might make a device that has twice the output power.
Another thing Hexacopter should point out is Certification. As soon as you mount a random antenna any certification is not valid any longer. So he needs to test modules with similar certification standards against each other.

[rpasichnyk]

The main goal was not to compare different modules against each other.

It was to get some real numbers, to make some real field test with some real numbers! Because so far on this forum it was just a talk with no data to back it up.

I think we can all appreciate that @hexakopter took all the equipment and made a trip to a place where are no other interfering devices. And did this test and documented all the results

[Trampa]

Sure, you can always try and measure things.
The question is: Is it relevant or do you draw the correct conclusion?

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.

That is exactly the kind of conclusion you can’t draw from such a test.

A good and strong signal is always a good thing, no doubt about that. However, if the use case is 1-2m away from the receiver, you are good with any of the tested options and there is zero difference in real life, even riding through high RF environments like London city centre. There is always enough link budget overhead in any case. That is why we can actually ride there and do not see any cutouts happening.

The conclusion is: All of the tested devices perform just the same in our use case. There is enough overhead in signal strength to not worry about connection.

RF reliability is a multi factor thing and you can draw little to no conclusions from taking only one factor into account (eg. range). What really counts is: Real life testing in all sort of environments, actually riding and visiting places that are problematic RF wise. No testing in this world will ever be as good as actually putting your device through real life testing.

@Lee_Wright Lee might have the experience and equipment to do some more scientific testing in the near future. He is quite a busy man though…