Frank Dresser wrote:

"Paul Sherwin" wrote in message
...

[snip]

Modern AM transmitters have a very sharp rolloff above a certain
frequency. Broadcasting above this would just waste transmitter power,
since (almost all) radios wouldn't be able to receive it because of
their IF selectivity characteristics. The 9kHz or 10kHz AM channel
width is just a convention, but once it has been adopted there's no
point in trying to receive a wider bandwidth - you'll just get
interference from adjacent stations.

If the received signal is very strong, the tuner's gain will have to be very
low. This will supress the adjacent channel interference quite well.


In the US and Canada, AM stations are allocated 10kHz bandwidth,
giving a theoretical 5kHz treble cutoff. In most other place that's
9kHz/4.5kHz. Stations transmit a more restricted frequency range than
this though, for a number of technical reasons. That's where my rough
and ready 3.5kHz figure came from.

RDH4 says most AM BCB radio makers tried for a final IF bandwidth response of
3.5 kHz
That was in 1955/
Since then, the BW has shrunk in many sets to even less than 2 kHz, especially
in solid
state gear, giving horrid state AM listening.
No good turning up the treble control knob, there is no treble there to boost.


Best regards, Paul
--
Paul Sherwin Consulting http://paulsherwin.co.uk

The FCC requires US AM radio stations to have an audio bandwidth between 4
and 10 kHz or a total bandwidth from 8 to 20 kHz. Typical radios with IF
transformers, rather than crystal or ceramic IF filters, don't have very
sharp skirt selectivity. Few radios will be able to block out a strong
adjecent channel 10 kHz off channel. Many can't block out a strong adjacent
20 kHz away. Some can't even block out a strong adjacent channel 30 kHz
away.

Oz local stations are rarely closer than 45 kHz, which is 5 x 9 kHz spaces.

In Canberra, we used to have 2XX community station of 300 watts on 1,008 kHz,
with 2CA of 5 Kw at 1,053 kHz, and it was a good test of any AM radio if
2CA couldn't be heard when tuned to 2XX.
My own radio allows me to pick up a weak signal at 27 kHz away from 2CA
without 2CA being heard.

Most simple transistor based tuners fail this test.
They have high Q single tuned IF coils.

7AD on 1008 kHz sometimes drifted in late at night all the way from Tasmania,
if conditions were freaky. Antenna type and location/direction minimised this
effect.



The FCC limits interference only partly by bandwidth restrictions. Mostly,
it uses geographic seperation and power restrictions.

By ear, I think most stations go to about 7 or 8 kHz audio. Many of the AM
stations are talkers, but the ads can really sparkle. There's one I hear
which sounds like it goes to the 10 kHz audio max.

Much AM is talkback from mobile telephones, and its pretty dreadful....



Patrick Turner.