[AMRadio] Frequency Response

Edward B Richards zuu6k at juno.com
Sun Sep 26 16:00:49 EDT 2004

Hi Don;

Thanks for your input. A discussion of bandwidth is meaningless unless
you tie it to how far down from peak voltage. The standard is 1/2 voltage
or -6 dB down. However, as I remember, the 9 kc the FCC is talking about
is further down. -26 dB if I remember right. We cannot compare apples to
oranges. That may be 6 kc at -6 dB. The Canadian regulation is probably
at the standard -6 db. So they may be the same. I think the implication
is 6 kc @ -6 db and I am going to comply with that.

73, Ed Richards K6UUZ

On Sun, 26 Sep 2004 18:02:32 +0000 "Donald Chester" <k4kyv at hotmail.com>
> >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
> >wrong.
> >
> 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.
> Don K4KYV
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