[Elecraft] Broadcast AM bandwidth

[Alan Bloom]

One of my pet peeves is people who refuse to admit they are wrong even
when presented with irrefutable evidence.  Well, I've recently seen some
irrefutable evidence so it's time to admit I was wrong.  :=)

I had claimed that US AM broadcast stations must not have an audio
bandwidth greater than 5-6 kHz or so because FCC regulations
<http://www.fcc.gov/mb/audio/bickel/amfmrule.html#AM> require that the
emissions be attenuated a minimum of 25 dB at +/-10.2 kHz from the
carrier.  It would be difficult to design a reasonable analog filter
that is -25 dB at 10.2 kHz and has a flat passband much more than 5 or 6
kHz.

I've recently spent some time with a spectrum analyzer looking at
on-the-air signals on the AM band.  For example, my local station (KSRO
1350 kHz) clearly has modulation sidebands out to almost 10 kHz from the
carrier.  It looks to me like the passband starts to roll off about 9.7
kHz.  They must be using a "brick-wall" digital low-pass filter to
obtain the 25 dB attenuation at 10.2 kHz while maintaining the passband
as wide as possible.

Such filters were not available until relatively recently.  But
apparently many AM stations are using them now, although I'm not sure
why since the additional bandwidth is only important for music and most
AM stations are talk-only format these days.  (The one exception being
the Spanish-language stations, for some reason.)

So if your goal is to listen to music, an IF filter with a bandwidth of
up to 19-20 kHz might be advantageous.  The K3's "FM" filter (13 kHz
bandwidth) would work well if you detune plus or minus 5 kHz to tune in
only one sideband.  For DXing or for non-music formats, a narrower
filter would be better to improve the signal-to-noise level.

Al N1AL