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The only 'free radio frequency' that I could think of are the HAM radio bands. But they really aren't free, for they are regulated by laws that require you to hold a license, restrict what purpose you can use them for, and are governed by some organization or body.

Are there any useable radio frequencies that are 'undefined'? i.e. I could use them any way for any reason without breaking any laws or be unethical.

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    \$\begingroup\$ Look up ISM bands, they are available for unlicensed use. They are regulated, as far as maximum power transmitted is concerned, but otherwise can be utilized. This is why WiFi, Bluetooth, Zigbee, are all in the 2.4 GHz ISM band. \$\endgroup\$ – Captainj2001 May 31 '16 at 23:29
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Sorry, no, at least not in the US. The FCC has allocated everything between 9 kHz and 275 GHz.

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    \$\begingroup\$ Apologizing for this is the wrong message. The rules are there for good reasons. I'm not sorry the OP can't do whatever he wants. \$\endgroup\$ – Olin Lathrop May 31 '16 at 23:15
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    \$\begingroup\$ @OlinLathrop -Oh, come on. The poor guy will be so disappointed. \$\endgroup\$ – WhatRoughBeast May 31 '16 at 23:38
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    \$\begingroup\$ 9 kHz?? like to see the antenna for that. \$\endgroup\$ – robert bristow-johnson Jun 1 '16 at 3:40
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    \$\begingroup\$ @robertbristow-johnson They basically look like power lines. [pictured antenna is actually for 17.2 kHz] - See also Project Sanguine, which transmited at 76 Hz. \$\endgroup\$ – Random832 Jun 1 '16 at 4:54
  • \$\begingroup\$ i knew about Project Sanguine, except neither the name nor the current status (shut down). \$\endgroup\$ – robert bristow-johnson Jun 1 '16 at 5:12
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There's the ISM bands that you can use for whatever you want, as long as you stay withing certain restrictions, mainly power level.

The 2.4Ghz band is one such ISM band, which is why there is so much traffic on it (WiFi, Bluetooth, Analog video cameras, ZigBee). For that band, I believe that the power limit is 1 milliwatt for continuous transmission, and 10 milliwatt for low duty-cycle transmission. No license or certification is required, but the FCC would come after you if you exceed the limits.

Wikipedia has a list of ISM frequencies: https://en.wikipedia.org/wiki/ISM_band

The two bands listed as "Amateur" are usable, but you need to know what you are doing.

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  • \$\begingroup\$ Of course, if you use the ISM bands, you might need to be able to prove you're operating legally within that band, its not good enough to claim you tried to build a license exempt transmitter... \$\endgroup\$ – Sean Houlihane May 31 '16 at 23:43
  • \$\begingroup\$ @Sean Houlihane - Yes, if the FCC came after you, you would need to make a case that you are within bounds. But you would need to cause a lot of interference for them to bother you. With the chips that are available today, it's hard to screw that up. Look at all of the NetGear/OpenWrt users who boost their WiFi much higher than the limits with no repercussions (except for their neighbors). \$\endgroup\$ – Mark May 31 '16 at 23:55
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    \$\begingroup\$ Mark, I thought that even unlicensed "intentional radiators" had to be approved and certified by the FCC under Part 15, even if it falls into the ISM category. The user doesn't need to be licensed, but the transmitter does. Am I mistaken? \$\endgroup\$ – bitsmack Jun 1 '16 at 0:45
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    \$\begingroup\$ @bismack - You might be right. The devices that I have designed were for ZigBee, so I was using "ZigBee certified platforms", which likely already had FCC approval. However, someone else in my industry rolled their own spread-spectrum 2.4 GHz radio, and did not seek any FCC approvals (although maybe they should have). \$\endgroup\$ – Mark Jun 1 '16 at 0:55
  • \$\begingroup\$ @bitsmack, Mark to my understanding you don't even need a certification to sell something comercially if it's within ISM. You only need to declare that your device is compliant. Most companies still test this before going to market though and the results of those tests can be (mistakenly?) called a certificate. I'm in Europe though. \$\endgroup\$ – Jan Dorniak Jun 1 '16 at 4:46
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No, not for normal interpretations of "radio". Think about it. If there were such a frequency range, lots of people would be abusing it, thereby making it unusable. That's why we have a central authority that allocates space. Doing so is in everyone's interest, even if not in individual interests.

It's a lot like the rules we have for driving on a specific side of the road. If there is a traffic jam one way and little traffic the other, you as one person would be better off driving on the other side of the road. However, when everyone does this, as they would if there weren't any rules, we'd have a dangerous mess, and everyone would be worse off.

You can use much shorter wavelength, like IR and visible light, pretty much any way you want for communication. Even then, you can't go lighting your neighbor's house on fire with a stron IR beam or point a laser in the eyes of airplane pilots.

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    \$\begingroup\$ Not sure the road analogy holds. I've definitely seen roads that had only room for 1 car for both directions combined, and people go both ways anyway without getting into accidents... \$\endgroup\$ – Mehrdad Jun 1 '16 at 3:33

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