Do radios need a minimum range to function properly?

Question

I'm going to use some well-known radios (nRF24L01+) to wirelessly read some voltage/current sensor data over little distances on a quad-copter since the drone is quite populated and we don't want to wire them up. The distance is rather short, less than 50 cm from the master radio.

The radio models are shielded ones, but since the drone has lots of high-current cables going to big motors, I'm wondering if the short distance causes any problems or not since I had experience with some 868 MHz radios that I had to keep at least 1 meter apart to be able to link them properly; not sure If it was my misunderstanding.

For shorter on-air travel time, the air-rate is set to 2Mbps.

Answer 1

Generally, the issue that mainly occurs with very short distances is overloading the receiver with too high input power.

According to the linked datasheet, the receiver at hand has a Maximum Input Power of 0 dBm for <0.1% BER. If you have a transmitter of say 20 dBm output power in very close proximity, you could possibly exceed this input power.
My personal experience with generic WiFi devices is that you typically want to keep receiver input power below say -40 dBm or so.

Although, this could easily be solved by decreasing the output power of the transmitter if need be.

You also mention high-current cables, those could possibly be another issue which is more difficult to answer without much more detail.

Answer 2

CARE & FEEDING OF 2.4 GHZ LOW POWER TELEMETRY MODULES.

NRF24L01 and the "+" variant and the superset Si24R1 are a VERY variable feast. There is some utter junk out there and some reasonably good modules.

I have no connection with the company in any way, except as a reasonably happy customer, and would recommend you consider offerings from EBYTE and compare them with alternatives. They sell some equipment directly from their web site and all products on AliExpress.

I have MUCH bad experience from starting at the cheapest bottom end no name brands that LOOKED OK and hoping it could be knocked into shape. Some could.

There are many tales of woe and quite a few "what I did to make it work well" accounts. From memory,

• Some/many/... modules tend to self interfere at higher power output settings in some cases. This MAY relate to poor antenna matching and much reflected power, or may not. Many recommend wrapping the modules in metal foil shielding.

• Random lengths of wire on the end of coax neither transmit as well as they should. Quarter wave dipoles ALWAYS need a ground plane or electrical equivalent (eg sleeve balun etc) to work properly. Without such they will return current down the coax outer and also not work as well as they could..

• On board antenna designs should in all cases work OK over your range. Beyond mere metres the fact that SOME makers have cloned antennas visually and used "matching networks" of suspect "design" and ... doesn't help. different PCB materials or thicknesses or orientations or matching components or ground plane positions or ... all often matter. Papers are available on web to explain how the onboard antenna designs work. Their main value to most people is showing why you should buy modules from companies who know what tyey are doing and who explain on their website why you should believe they 'know their stuff'.

• Some say that operating at low TX powers is a good idea. This should not be necessary usually with a designed module. In your case you should need minimal power.

• If you MUST use genuine NRF... ICs (which may not be genuine unless you buy them for eg EBYTE who will sell them to you if you insist, be SURE to get the + suffix ones (also = P suffix_) I think which allow the 250 kbps rate which gives better results. Over your range there MAY be a good reason to use radio, but I'd avoid it unless a direct wired connection is not viable.

Motor and other electrical noise definitely could be an issue but may not be. The devices can be set up to invisibly automatically repeat messages until confirmation is received.

Operate at the lowest air data rate (250 kbps) for best results.

If you do have problems consider LORA. This should be utter extreme overkill in your situation, but allows using settings with increasingly robust noise immunity as data rate reduces.

IR optical transmission MAY be an option IF noise proves a major problem. Cost is not vastly greater than the Si... NRF... modules. The SPI interface versions give better access to all IC commands and lower DC power than the UART interface ones.

There are many libraries on github to assist with their application.

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For interest - why are you using wireless over such a short range? I realise that eg rotor-head to body may make wiring problematic :-).

30 years ago I made radio data links to transmit signals from load cells in the sails of Americas Cup yachts to a central data processing point mid deck. Wires were not an option :-).