|[AMRadio] Anonymous QST Author suggests that Part 97 requires 6
w4bws at comcast.net
Fri Apr 18 01:38:40 EDT 2008
Adding up the bell labs recommendation is 3300 KHz plus 3300 KHz or 6.6 KHZ
Just because you roll off the 0 to 300 range doesn't make the signal
So 6KHz is narrower and 7 KHz is more normaL. Of Course, AM transmitters do
have 2.7 or 3.1 KHz filters to limit bandwidth as on SSB transmitters. So 7
to 8 KHz is
normal AM transmission bandwidth.
Of course the ARRL would not be agreeing to the 6 KHZ suggested by the
organization because the Directors have told us so.
Those using clipping circuits should have an audio filter following the
clipper to reduce
the clipping "noise" not needed for communications.
I find that most of the broad signals I hear on "modern" solid state
receivers is generated
with in the receiver in the noise limiters and other limiting circuits as I
don't copy the same
broadness of the signal on my 75S3.
The Amers got kicked in the pants with the 1500 watt peak power limit
probably due to
the same faulty technical understanding by those making the decisions. Like
having a Extra
Class license these days doesn't mean you have an understanding of theory
an engineering degree today doesn't either. With AM the average power due to
characteristics is only about 20 to 30 percent of the total output power of
added to the carrier. Not 4 times the carrier output which is only on
peaks.So a 375 watt
carrier on average is only 490 to 500 watts output.
What would be effective perhaps is to limit the audio bandwidth with
filters, use compression
of 10 to 20 percent and boost the averge output in the final or with a
driver then amplifing
with a linear that can boost the signal to a higher average output. Or use
a properly adjusted
controlled carrier modulated driver for the linear so the resting carrier is
say 125 to 150 watts and on
average will put out 450 to 500 watts and sound like it is much more power.
----- Original Message -----
From: "D. Chester" <k4kyv at charter.net>
To: <amradio at mailman.qth.net>
Sent: Thursday, April 17, 2008 1:22 PM
Subject: [AMRadio] Anonymous QST Author suggests that Part 97 requires 6
kHzAM Bandwidth Limit
> On p. 64 of the May issue, the anonymous writer of the monthly Q-A
> "Workbench" column in QST entitled "The Doctor is In", responds to a
> report by the coordinator of the ARRL OO program regarding "wide AM
> signals with bandwidths of up to 30 kHz on 75 m."
> He briefly explains the relationship between signal bandwidth and the
> frequency response of the audio that modulates SSB and AM transmitters,
> then states that Bell Labs concluded many years ago that high quality
> voice transmission (toll quality in telco terminology) can be carried over
> a 300-3300 Hz frequency response, but that "This worked better for the
> grey-haired Bell scientists who were likely to have lost some of their
> high frequency hearing". He goes on to allege that an AM signal occupies
> "a bit more spectrum" than two SSB signals, based on the notion that there
> is no need to transmit audio frequency components of the voice that fall
> below 300 Hz, and that the "usual approach" with SSB is to transmit
> frequencies from 300 to 2700 Hz.
> He then cites §97.307 of the FCC rules that states "No amateur station
> transmission shall occupy more bandwidth than necessary for the
> information rate and emission type being transmitted, in accordance with
> good amateur practice." Since voice is what is being transmitted, he
> suggests that the rules require AM to occupy a bandwidth of "no more than
> about 6 kHz.
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