Update #1 - Antenna Research

Laws and regulations

The first thing I thought of before beginning my controller research was whether our project had any legal constraints. After some digging, I found this federal regulation in the FCC:

§ 15.23 Home-built devices.

(a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use.

(b) It is recognized that the individual builder of home-built equipment may not possess the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of § 15.5 apply to this equipment.[1]

Fortunately, since we're not marketing this, we don't have to jump any large legal hoops to get our RC plane approved. It looks like we're expected to at least try to meet the regulations in this legal document, but not necessarily expected to exactly. Now, what are the technical standards?

The first and foremost guideline is found in 15.5:

15.5 General conditions of operation.

(a) Persons operating intentional or unintentional radiators shall not be deemed to have any vested or recognizable right to continued use of any given frequency by virtue of prior registration or certification of equipment, or, for power line carrier systems, on the basis of prior notification of use pursuant to § 90.35(g) of this chapter.

(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.

(c) The operator of a radio frequency device shall be required to cease operating the device upon notification by a Commission representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected.

(d) Intentional radiators that produce Class B emissions (damped wave) are prohibited. [1]

Since we'll be buying an RF module with its own modulation scheme, we don't have to worry about (d) (otherwise, why would it be on the market?). Although we'll still check to ensure that the modulation isn't Class B (damped wave). (a) and (c) are if we mess something up. (b) will be my primary concern.

Ensuring that my controller does not cause "harmful interference", we want to check:

Harmful interference. Any emission, radiation or induction that endangers the functioning of a radio navigation service or of other safety services or seriously degrades, obstructs or repeatedly interrupts a radiocommunications service operating in accordance with this chapter.[1]

So we'll want to ensure that my controller isn't so powerful that it obstructs other ISM-band services. We'll want to limit both its range and power output. And follow the regulations for whatever ISM band we choose (thinking 2.4GHz as of now).

In addition, we must "accept" interference from other ISM band devices. I'm not entirely sure what this means, but from some Google searches it seems to mean that the device must be able to "handle' interference from other devices, and I must legally be okay with interference from other devices. Technically speaking, I should probably choose a modulation scheme that can handle interference well or look into filtering. AM modulation, from the outset, seems like a bad idea, and ensuring my modulation scheme has some kind of header and error detection is a must.

Now, since I'm purchasing an RF module, many of the regulations surrounding modulation and scheme will be handled for me. However, the biggest question now becomes a matter of the other components of wireless communication: namely, antenna usage, data rate, and transmit power.

Many RF modules are FCC-approved and likely have firmware/power-limiting circuits to limit transmit power. My main concern now, is which RF module to use and what antenna to use. 

Frequency band and picking an RF module

I'll want to pick an ISM band that has both a good range and allows for a good data rate. 

However, before I do that, I'll need to investigate my desired abstraction level.

My options are:

• Build an RF module or circuit from scratch.
• Purchase a raw RF module and design my own breakout board
• Purchase an RF module with a breakout board

Something I've noticed is that the lower the abstraction level, the greater the engineering challenge and the more "engineering" it actually is. Buying a pre-built remote isn't engineering. It's being a consumer. So I'll need to go "low" enough to be an engineer and learn what I want, but not so low that I'm mining copper and producing my own wires, so to speak. Since my group and I want to finish this project this summer, I'll want to get some estimates for how long each of these might take. Since I don't know any RF engineers personally, the best I can do (for now) is Google "how long does ____ take" and simply examine the requirements. 

Building my own RF module from scratch: This seems like it'll take too long. A Google search suggests it might take "months," which extends beyond the scope of our summer-long project. By the time I finish my custom RF module and design it beneath the pages and pages of FCC regulations, I'll have a module. Not a controller.

Building a raw RF module and designing my own breakout board: The lovely thing about this is KiCAD (and similar software) seems like a great, marketable skill to have. How long does it take? A Google search... isn't helpful. It seems like it might take an uncomfortably long time. 

Perhaps my first option is to purchase an RF module that already has a pre-built breakout board, and then, if I finish early or want to learn, I can build my own breakout board and compare its performance with the pre-built one. Especially because an RF module is only one component of my controller. I'll still need to construct a protocol to send a signal with an RF controller and have everything working in harmony with joysticks, buttons, etc. 

Let's go with an RF module with a breakout board. It's still complex enough that I'll need to write a lot of code to interact with it, but not so deep that we never finish making our plane this summer.

I decided to go with the NRF24L01+. Seemed like a reputable chip:

I'm going to make a safe assumption (for now) that this reputable seller with 421k items sold has likely followed FCC guidelines for me. Otherwise, the data sheet seems to allow variable power output/input, and I can always purchase another antenna. In addition, the data sheet shows that the max output power is 0 dBm, which is far from the .125 W power limit in FCC part 15 for 2.4 GHz. However, FCC part 15 does limit power output for directional antenna gains greater than 6 dBi [1]. I've messaged the seller asking exactly what type of antenna that is. A quick search (Chat GPT and Google) shows that these external antennas are typically omnidirectional and likely up to 5 dBi. This shouldn't be an issue then. 

However, once I make my own breakout board, I'll need to be more careful with regard to power output and antenna usage. 

While I wait for what gain this antenna has, I won't be able to calculate the exact range. And before our first test flight, I won't be able to determine how the Doppler effect will alter the system. For now, we're going to assume it'll be fine. 

Other considerations: I may opt to purchase an NRF24L01+ with a patch antenna so it can more easily fit inside the plane's shaft. I want to use this chip for other projects in mind anyway (and they're fairly cheap), so I don't mind purchasing another chip if need be. 

Link budgeting calculations will come at a later update!

Finally, you may be thinking: why antenna first? Why not other things? Why not a high-level diagram? I'm getting there. The reason I chose this first was for our first "milestone" test: a 10 second flight. My computer can act as the controller (connected to an Arduino). I just wanted to get this out of the way first. 

Sources:

[1] https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15