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Say I have a consumer device that uses a pre-certified WiFi module. As I understand it, radio modules are certified with a fixed set of antenna choices that were included as part of the module's testing.

I understand that using an antenna outside of the approved list is a no-no. I'd like to know specifically why it is a no-no. I have two theories:

TECHNICAL RISK: Using an antenna that was not on the approved list is a technical risk, because you are outside the scope of what has been previously confirmed to pass testing. So you may happen to fail compliance testing because the module performs poorly with this new antenna type. However, you could also get lucky, and end up passing, if the new antenna performs well with the module.

LEGAL ISSUE: Using an antenna that is not on the approved list INVALIDATES the compliance certification of the module. Due to this, the overall consumer device will now be subjected to more vigorous FCC testing, which would have been avoided by using a compliant antenna.

Which of these two theories is the problem with using a non-compliant antenna? Or, is it something else entirely?

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    \$\begingroup\$ The latter, because of the former. \$\endgroup\$ – TimWescott Jul 19 at 22:35
  • \$\begingroup\$ ^This was my assumption going in. If it was expanded into an answer with some additional detail, I'd accept it. \$\endgroup\$ – Chris Fernandez Jul 22 at 18:53
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It is both. Basically it is legal issue, because it can be a technical issue. Like you guessed, an untested antenna can cause undesired radiation.

FCC is a bit strict on this matter and you can't just do the required tests that prove it works and send them the report. Unless you forget about the pre cert and do all the radio tests yourself. You have to make so called class II permissive change, where you apply for a new FCC number for the module. In FCC's eyes it is then a completely new module that is allowed to be used with the new antenna

The required tests for the change are not that bad, though they still cost several thousands of dollars. But there is a catch, to avoid doing all the radio tests, you need a permission from the manufacturer to use their old test reports for the tests that are still valid after the antenna is changed (mostly the conductive measurements). And it depends on the manufacturer how helpful they are.

So all in all, if you choose a reasonable antenna the technical part is quite easy, but the legal stuff and paper work is awful.

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  • \$\begingroup\$ I started thinking that maybe the class II permissive change doesn't come with a new FCC number. It might have been some other matter (or some other country) where a new number was needed. Still the c2pd alone is quite laborious. If someone remembers this better they can correct me. \$\endgroup\$ – TemeV Jul 22 at 19:05
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If you modify a pre-certified module, it then become (in my mind) a non-certified module, and you get to re-test it. That may or may not fit into your budget.

Under FCC rules you cannot modify the pre-certified module.

A different antenna changes the gain and radiation patterns, which could also make it non-compliant to FCC standards. The FCC is kind enough to let people use pre-certified modules if they desire. It's the FCC's job to make sure devices don't interfere with each other, if you modify an intentional radiator you can bet that they'd want to find out if it's compliant or not.

enter image description here
Source: https://emcfastpass.com/rf-modules/

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