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Should one always identify ones station (say via CW keying) when building/testing an HF oscillator and/or anything that could amplify that oscillator, when powering the circuit on a breadboard and into a dummy load? Or is there a lower power limit (U.S. FCC regulation and/or worldwide best practice) before this is required?

Is the answer different on any other non-HF bands?

(I am specifically not assuming that dummy loads radiate zero power, since that can be (and likely is) false.)

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Part 15 sets limits on unlicensed RF emissions. The limits vary by frequency, the type of device, the nature of the emissions, and so on. It's a pretty huge document and you need a lawyer and an EMC engineer to approach it. However, you can get the intent of it from this blurb that is printed on just about every piece of consumer electronics sold in the US:

This device complies with part 15 of the FCC rules. Operation is subject to the following two conditions: (1) this device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.

It never hurts to ID. However, if you are testing an oscillator that isn't even connected to an antenna or an amplifier, or if you are using a dummy load and modest power (less than 10 watts, say), your effective radiated power is going to be less than a microwatt.

If you are in doubt, listen with another receiver. Can you hear your device under test above your switching power supplies, the power lines, touchlamps, and ambient atmospheric noise? If not, then it's unlikely you are "causing harmful interference". If you don't have a second receiver, it's not terribly difficult to construct a field strength meter.

This doesn't per se mean that you are complying with part 15 (that could require filing with the FCC, passing a compliance test, ...) but at some point, a little common sense is sufficient. You aren't mass producing a device that will be used by millions of people for years and years. You are performing a temporary test as a licensed amateur radio operator. Take reasonable steps to avoid causing interference and you will be fine.

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It is up to you, as a licensed amateur radio operator to determine what your emissions actually are, and to ID as appropriate.

Whether your dummy load is radiating enough to count depends on too many variables to give you a nice, easy answer. If in doubt, ID anyway.

Understand your emissions, understand your license, and operate accordingly.

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    $\begingroup$ That's a good legal answer. What do most operators and circuit builders do in practice to attempt to meet this standard? (given that the typical shack and garage workbench lacks a calibrated spectrum analyzer.) $\endgroup$ – hotpaw2 Feb 18 '14 at 2:05
  • $\begingroup$ If in doubt, ID anyway. $\endgroup$ – Adam Davis Feb 18 '14 at 2:07
  • $\begingroup$ A typical Arduino runs at a frequency that is not licensed for amateur radio. I wonder if most of the people breadboarding (unshielded) with such are aware of this fact. $\endgroup$ – hotpaw2 Feb 18 '14 at 2:17
  • $\begingroup$ @hotpaw2 all electronic devices radiate. Even your brain radiates. I think the thing you are looking for is part 15 $\endgroup$ – Phil Frost - W8II Feb 18 '14 at 12:50
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    $\begingroup$ I think this is a non-answer. Saying "Understand your emissions, understand your license" isn't really helpful, because a desire to possess that understanding is already implicit in the question. $\endgroup$ – Phil Frost - W8II Feb 18 '14 at 13:41

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