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Assume a middle school or home hobby lab with a bunch of breadboards, jumper wires, and 3.3V microcontrollers fast enough to toggle IO at RF frequencies (if not at the fundamental, at lower harmonics of the toggle frequency). Assume a digital IO toggle frequency right in the middle of some random HF or VHF band. Assume that the school or hobbyist are unlikely to have access to a calibrated spectrum analyzer.

So, to help encourage best practices:

Are there any good jumper wiring guidelines to reduce the chances of a breadboard (or set of breadboards) radiating EM outside various limits (FCC Part 15 in the U.S., etc.)?

e.g. maximum length of jumper wires, maximum loop area with respect to the nearest ground return, minimum termination resistance of a circuit loop, etc.?

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    \$\begingroup\$ You can do things in better or worse ways, but not really, no. The only real choice would be PCB submodules with filtered connections, or putting the whole thing in a shielded box and treating the connections that pierce that. What is your actual goal here? \$\endgroup\$ – Chris Stratton Jan 9 at 16:48
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    \$\begingroup\$ Is there any particular reason why you think a prototype needs to meet FCC requirements? \$\endgroup\$ – Scott Seidman Jan 9 at 17:32
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    \$\begingroup\$ (1) It is good that you're thinking responsibly. (2) Don't worry. Chances are low that you'll interfere with anything. (3) Search on "breadboard best practices" and you'll find information. You do want to use good assembly practices, but it's more because you want your stuff to work than because you might radiate enough RF to irritate anyone. \$\endgroup\$ – TimWescott Jan 9 at 18:15
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    \$\begingroup\$ you could use an AM/FM radio to see if there is any annoying signal being radiated \$\endgroup\$ – jsotola Jan 9 at 18:34
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    \$\begingroup\$ I believe that all experimental physics research would grind to a halt if Part 15 limits were enforced at universities ツ \$\endgroup\$ – John Doty Jan 10 at 15:35
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There isn't a reason to worry about exceeding limits with a breadboard, the FCC allows for building of single devices with no testing:

What does this mean for a hobbyist? Very little, actually, depending on what you’re doing. The FCC allows a hobbyist to build up to five devices of a single design for personal use with no testing whatsoever. If you are contacted by the FCC (or anyone else) about a matter of spectrum interference, immediately stop using the device, don't use it again, and you should be okay. Stick to the ISM bands (13.56MHz, 27.12MHz, 40.68MHz, 915MHz, 2.45GHz, and 5.8GHz, +/- a bit for each) for added comfort.
Source: https://www.sparkfun.com/tutorials/398

The point at which you may get in trouble if you are unintentionally radiating large amounts of RF on bands that are in use by people that care. For example: A electronic billboard was leaking RF on a cellular band, the phone company saw the leak with their equipment and complained to the billboard company. If the company didn't fix the leak they then could complain to the FCC, and they could take action with fines, ect.

It's unlikely that you'll build an unintentional radiator with a breadboard that would get someones attention. You might however build an intentional radiator (like an FM radio) that could disrupt a radio a few 10's of meters away. If you're building an intentional radiator that's a different story.

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    \$\begingroup\$ To cite that FCC article: In this case, the builder is expected to em- ploy good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of § 15.5 apply to this equip- ment. \$\endgroup\$ – Marcus Müller Jan 9 at 18:04
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    \$\begingroup\$ In other words: it's expected of hotpaw2 to build the device to the best of his knowledge of good engineering practice. That's not too high a requirement, but does indeed mean that it's not OK to say, run a long leaky cable and amplify the RF signal put in just because it was hard to measure at the other end of the cable. \$\endgroup\$ – Marcus Müller Jan 9 at 18:06
  • \$\begingroup\$ Right, and you also need to be careful if using a signal generator that works in the MHz range. It's not good (and in some cases not lawful) to cause a disruption to someone else, but it's also unlikely that you'd get someones attention. On some bands you might have someone show up at your door very soon. \$\endgroup\$ – Voltage Spike Jan 9 at 18:09
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    \$\begingroup\$ e.g. the point may be moot, as the school, to save on energy costs, may have converted a bunch of light fixtures to inexpensive LEDs, whose cheap power supplies create an HF noise floor well above whatever gets radiated out of the physics/electronics lab. \$\endgroup\$ – hotpaw2 Jan 10 at 2:23
  • \$\begingroup\$ @VoltageSpike Yea, stay out of the aircraft bands for example. \$\endgroup\$ – Mast Jan 10 at 10:31
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Are there any good jumper wiring guidelines to reduce the chances of a breadboard (or set of breadboards) radiating EM outside various limits (FCC Part 15 in the U.S., etc.)?

No. Breadboard is a nightmare, RF-wise; jumper wires simply look like antennas for sufficiently high frequencies, and the connecting "bars" are long enough to become monopoles themselves...

e.g. maximum length of jumper wires,

Rule of thumb: a wire smaller than 1/10 of the wavelength is not going to be a great antenna. So keep things short!

maximum loop area with respect to the nearest ground return,

That would assume you can be sure any of the ground returns would have low impedance compared to a loop. That's usually not the case for breadboard!

minimum termination resistance of a circuit loop, etc.?

Obviously, you're absolutely right to think about this: when you know where the energy is sunk, there's less chance it can radiate! So:

  1. Terminate, if possible
  2. keep wires short relative to wavelength
  3. don't use maximum drive strength at high switching frequencies
  4. Add local decoupling capacitors – that might both improve performance and supply stability (and that has the potential of reducing production of harmonics to begin with!) as well as it will shorten out RF to supply/ground.
  5. adding e.g. a 10 nF capacitor can trade slew rate (which often is desirable) for EMI immunity and emission
  6. Especially with microcontrollers, it might make sense if an experienced electronics expert or enthusiast (cough that'd be you! :D ) simply designs (or uses an existing design, see e.g. "sharing" on oshpark.com) PCB that carries the microcontroller (hint: sometimes, especially when ordering really cheap PCBs, e.g. from JLCPCB, the difference in cost between the DIP and an SMD variant of an MCU can already amortize the board), decoupling capacitors and the oscillator; keeping the capacitors close reduces supply/ground loops. Having a PCB also makes the project more robust, gives a bit of space to add a label etc.
  7. Don't fret too much – typical school experiments will not have multiple amperes switched at 10s of MHz es; if you take care to not intentionally build antennas ("I added a 1,5m cable so I can measure this 100 MHz clock on that scope over there"), you'll probably be fine, even with the low-power modes
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    \$\begingroup\$ The first breadboard kit I had as a kid had a bunch of schematics for building radios, but they worked in the kHz range. \$\endgroup\$ – Voltage Spike Jan 9 at 18:04
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    \$\begingroup\$ I think Ronald is more worried about building unintentional emitters than intentional ones and is looking for best practices :) \$\endgroup\$ – Marcus Müller Jan 9 at 18:07
  • \$\begingroup\$ Even if poorly grounded (no ground plane), I might minimize the loop area made by the shortest or closest ground wire to the signal jumper. I've used two jumpers to make a pseudo-semi-twisted-pair, adding an additional chip-to-chip ground next to the flying wire signal, if the components aren't adjacent. \$\endgroup\$ – hotpaw2 Jan 9 at 19:45

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