# Why are many FHSS / DSSS transcievers limited to 63 mW?

While researching a DIY digital video streaming solution for an outdoor quad-rotor project, I've noticed that a lot of the popular digital transceivers seem to be limited to 63mW transmit power:

(sorry, newbs can't post too many links)

My original guess was that it was a legislation issue, but the last module (that operates on 900MHz unlike the other 2.4GHz ones) has the following message in the datasheet:

Selectable 1, 10, 63, 250, 500 or 1000 mW transmit power with a firmware interlock of 63 mW maximum for 500 kb/s operation

That would suggest that something in the underlying physics breaks down above 63 mW for high bitrates. Is that the case, or is it just a combination of coincidence and poorly written datasheets (the third module is rated for 63 mW on its datasheet, but 100 mW on its specs page)?.

I'm an established programmer with a decent understanding of physics (albeit I'll admit to skipping quite a few electrical engineering classes) so as long as you keep it below line integrals and wave functions (the quantum type), I should be able to follow.

• It's extremely implausible that this is a limit of physics rather than regulation. I don't have info on where this particular limit actually comes from, though. – Kevin Reid AG6YO May 24 '15 at 15:23
• The devices you posted all use the ISM band. Regulations are created to protect persons against health risks and interference to other pre-existing services. The main concern about microwave frequencies, is non-ionizing radiation which can cause localized heating (arrl.org/rf-radiation-and-electromagnetic-field-safety). This is the FCC's rules explained by the ARRL: arrl.org/the-fcc-s-new-rf-exposure-regulations – sessyargc.jp May 24 '15 at 21:48
• So why is the 63mW limit imposed only if operating at 500kb/s? Or am I reading the specs wrong and they're supposed to read: "it can do 1000 mW transmission, but you have to change the firmware" ? – Radu Dan May 24 '15 at 23:55
• You've got to ask your technical contacts at the supplier for that information. That is probably proprietary information. – sessyargc.jp May 25 '15 at 2:23