[AMRadio] FCC's Definition of Bandwidth

Donald Chester k4kyv at charter.net
Thu Oct 6 16:47:05 EDT 2016

FCC Part 2 (General Rules and Regulations) thus defines occupied bandwidth:

§ 2.202 Bandwidths.

(a) Occupied bandwidth. The frequency bandwidth such that, below its lower
and above its upper frequency limits, the mean powers radiated are EACH
equal to 0.5 percent of the total mean power radiated by a given emission. 

Part 97 defines bandwidth differently:

§97.3 Definitions.

(a) The definitions of terms used in Part 97 are:

(8) Bandwidth. The width of a frequency band outside of which the mean power
of the transmitted signal is attenuated at least 26 dB below the mean power
of the transmitted signal within the band

I see a 6 dB discrepancy between the Part 2 and Part 97 definitions. 0.5
percent is a ratio of 1/200, or minus 23 dB. According to the Part 2
definition, it appears that the power radiated above the defined frequency
band is 0.5 percent, and the power radiated below the defined frequency band
is another 0.5 percent; adding the two together makes the total power
radiated outside the defined band 1 percent, which would equal minus 20 dB.

According to the Part 97 definition above, the total mean power radiated
above and below the defined frequency band is minus 26 dB. This means that,
according to the amateur rules, the definition of occupied bandwidth is more
severe by 6 dB, than the definition under the general rules and regulations.

Am I missing something, or are amateur signals under a stricter standard
than the general definition? §2.202 (a) goes on to state: "In some cases,
for example multichannel frequency-division systems, the percentage of 0.5
percent may lead to certain difficulties in the practical application of the
definitions of occupied and necessary bandwidth; **in such cases a different
percentage may prove useful**". So perhaps the FCC has intentionally applied
a stricter bandwidth standard for amateur radio than for other services? Or
is this an error where someone added 6 dB instead of subtracting it. Or
maybe I’m the one reading it wrong.

I’d be interested in other opinions.

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