|[AMRadio] Frequency Response|
k4kyv at hotmail.com
Sun Sep 26 14:02:32 EDT 2004
>What about bandwidth? +/- 5kc would be a 10 kc band width. I thought we
>were supposed to limit our band width to 6kc. Please correct me if I am
That is a popular urban myth. There is NOTHING in the US regulations that
specifically limits bandwidth. The regulations specify "good engineering
and amateur practice", and deliberately leaves the specific bandwidth
vague. In Canada, there is a rule on the books limiting bandwidth to 6 kHz,
but I have never heard of them enforcing it against AM signals that may
exceed that figure. Besides, accurate measurement of bandwidth would
require a visit to the station with test instruments. Over-the-air
measurement leaves too many possibilities for error due to propagation, QRM,
The US regulations could be interpreted to mean a reasonable bandwidth for
the mode being used, considering band occupancy . If you had a cw signal
with so much noise, hum or FM on the carrier that it was 3 kHz wide, the FCC
probably could interpret that as a violation of good engineering practice.
If the band is empty, as for example, 10 m. most of the time nowadays, or
160m in the middle of the day, you could run hi-fi AM with audio all the way
up to 15 kHz and that would probably be ok as long as you made sure you
were not causing any harmful interference to anyone. On the othre hand if
you were limiting the audio response to 3000~ and generating the same wide
bandwidth due to splatter (overmodulation or distortion), that would be
considered not to be "good engineering practice." If you operated the full
hi-fi audio at high power on 75m at night when the band was crowded, that
could be interpreted as violation of good amateur practice.
The bottom line seems to be, use common sense and adjust bandwidth according
to conditions, and make sure your transmitter's spurious distortion products
fall within the FCC's specifications, which are listed in the rules.
There was a flare-up regarding bandwidth a year or so ago, with "hi-fi SSB".
This resulted in petitions to specifically limit bandwidth. The FCC
apparently turned them down. Now the ARRL is proposing to change the
definitions of subbands to be defined by bandwidth instead of emission mode,
to promote "digital" experimentation. The proposed bandwidth limit for AM
is 9 khz. The League has received so much mail questioning the wisdom of
such a change, that the League seems to be rethinking the idea. They still
have an open invitiation to the amateur community to send them comments and
opinions on this subject.
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