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I am somewhat confused about the regulations on the radio frequencies.

  • I am designing a system using an antenna, which is not used for communications. It is used for sensing based on the reflection coefficient of the antenna; therefore, there are no modulations used.
  • The system operates on a single frequency around the range of 500MHz to 800MHz. It will be working continuously on that frequency (or can be designed to be turned on and off in short periods of milliseconds to preserve power or reduce exposure).
  • It might periodically readjust to select another frequency from the same range around every 15 to 30 seconds.

In a nutshell, it will be a continuous signal on a single frequency that might hop to a new frequency in 15-30sec periods.

According to these specifications, do I have to stay in ISM bands?

If yes, can I reduce my signal power to an adequately low level to be able to use any frequency in my desired 500-800MHz range? And if yes, how much is the maximum power that can an antenna have while operating outside the ISM bands? (specifically in terms of frequency regulations, not to be confused with SAR.)

North American regulations are the immediate priority, but I also wish to know about European Union regulations.

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    \$\begingroup\$ If you are transmitting, whether or not the transmission is intended to be used for communications, you will be subject to the relevant radio regulations to ensure that your transmissions do not interfere with authorized users of the frequencies you select. \$\endgroup\$ – Peter Bennett Feb 13 at 20:12
  • \$\begingroup\$ Did you look on the US Government's FCC web site? \$\endgroup\$ – SteveSh Feb 13 at 20:12
  • \$\begingroup\$ @SteveSh Yes, but could not understand correctly and got more confused. Done some extra research and found e2echina.ti.com/cfs-file/__key/… , which I am not sure if it applies to my case or not. \$\endgroup\$ – zxcmehran Feb 13 at 20:53
  • \$\begingroup\$ @SteveSh The document states "Outside the restricted bands, one can transmit at any frequency as long as the radiated output power is below the spurious emission limits in Table 1." on page 4. \$\endgroup\$ – zxcmehran Feb 13 at 20:54
  • \$\begingroup\$ @PeterBennett Thank you. But what if I use a very low power? can I be exempted? \$\endgroup\$ – zxcmehran Feb 13 at 21:04
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Of course it depends on where you are, there are FCC standard and EN standard and whatever standard in other countries.

The main denominator would be field strength. If you stay in the relevant EN 61000 emission curve you don't really need to use the ISM bands, it's simply considered a 'tolerable unwanted emission'. I can do a 4MHz capacitive sensor, for example, without issues if it doesn't go over the limit.

If you need more signal however you need to use the ISM band (the textbook example is an RF heater) and probably comply with the relevant standards (like the electroheating ones, in this case). Similar to your case could be a radar level detector for silos, for example. It's actively transmitting with some significant power but it works inside a standard framework (the ISM one)

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