Final answer:
The FCC rules that you must immediately stop operating on Amateur Radio Service frequencies if your license has expired, even if it is within the grace period.
Step-by-step explanation:
According to the Federal Communications Commission (FCC), which regulates all domestic interstate communications, an amateur radio service operator is not allowed to operate a transmitter on Amateur Radio Service frequencies if the license has expired, even if it's within the grace period. You must immediately cease all operation. The FCC carries out various functions including issuing licenses, designating sign-on/sign-off times, and defining operating power, and for Amateur Radio Services, the adherence to validity of the license is mandatory. Any operation without a valid license or within the grace period is considered illegal.
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