[AMRadio] Frequency Response

Donald Chester 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, 
QSB, etc.

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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