Re: [HCDX]: Medium wave bandwidth + popularity

[Radiomatt@xxxxxxx]

In a message dated 98-10-18 11:47:47 EDT, you write:

<< I Europe the bandwidth for medium wave stations is supposed to be 4.5 kHz,
 but in the UK they get away with using 6 kHz. In the US and elsewhere in
 the Americas I believe the bandwidth is 9 kHz and in Japan 10 kHz.
 Australia 9 kHz I think. South Africa ? >>


I think you're confusing bandwidth, with bandpass and channel spacing.
The channels are spaced every 9 kHz, and theoretically the station stays
inside the + or - 4.5 kHz with its sidebands.
BUT, there's an allocation policy that generally limits stations in the same
city (or area) to being no less than 40 kHz apart, and therefore stations can
occupy more of a bandwidth than just 4.5 kHz each side of the carrier.
In AM the sidebands are directly proportional to the modulating frequencies...
pump in a 5 kHz signal and your side bands are + or - 5 kHz.
However, the info carried in EACH of the sidebands is identical, so in some
respects one sideband is wasted. The carrier is also useless, especially if
you can recreate it in the receiver. Enter SINGLE SIDEBAND (SUPPRESSED
CARRIER) or SSB.
Now you can take all the energy you were using for transmitting the other
sideband and the carrier and use it to transmit one sideband (which contains
all the information anyway). You pick up 9 dB of modulation efficiency which
is a big number in communications.
But I digress.
Here in the States, I'm told stations can feed up to a 7500 hZ signal into the
modulator of a transmitter, so that the occupied bandwidth is 15 kHz, not the
10 kHz that's allocated. No interference (except to DXers) results. And even
here, the sidebands being generated are greatly reduced in strength since most
audio intelligence is under 5 kHz anyway.
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