Hi Doc,
That's quite a collection of questions...and an even bigger list of
reflectors it's posted to...but I'll try to answer what I can here.
Longwave broadcasters do have to limit the maximum modulating audio
frequency. Generally, it's 4.5kHz to accomodate the 9kHz channel spacing.
Even so, it's no small feat to get a total 9kHz bandwidth through practical
antenna systems at longwave frequencies.
The result is rather dulled audio, even when heavy compression and multiband
signal processing are used. There is an audio file in the LWCA library of
Alvin and the Chipmunks singing "Let It Snow" in Icelandic, sent to us by
Jacques d'Avignon. Since we are temporarily without a page with links to the
sound library at this time, I'll e-mail you the file separately to give you
an idea of the sound quality.
(Now, there is a misimpression among some non-broadcaster technical types in
this country that we limit AM broadcasters to 5kHz audio because of the 10kHz
channel spacing here. Not so! There was no explicit regulatory limit at one
time. The FCC allocation rules were originally based on the idea that 15kHz
and even 20kHz audio components might get through the system. That's why our
American allocation rules have so many different values of permitted overlap
between signal strength contours, depending on the number of channels between
the two frequencies of interest. But that was also back when aggressive
audio processing was not the norm, and high audio frequencies were not
artificially emphasized. These days, the FCC prescribes a very rigid
emissions mask, limiting nearly all modulation energy to less than 10kHz, for
an occupied bandwidth of 20kHz.)
The wide-open American model doesn't apply to the LF broadcasters, though,
because there are so many of them on such a relatively small number of
channels. There, emission limits are a necessity right up front.
Also, directional antenna systems are more common among longwave broadcasters
than they were among European mediumwave AM broadcasters, also because of the
crowded conditions on the limited number of channels. (A high-power
international longwave station is such a big capital investment anyway, that
a more elaborate antenna array wasn't really that much extra cost for many
broadcasters.) You may have seen the Donnebach two-tower array in The
LOWDOWN recently, for instance. And you may have noticed on this reflector,
back in mid-August, a quoted excerpt from the Europe 1 web site describing
their 4-tower directional array.
All major longwave broadcasters, directional or not, use very substantial
vertical masts, mostly on the order of 200 - 350 metres in height, along with
buried radial systems and ground mattes. Except for their sometimes larger
size, these radiating systems look very much like their counterparts at any
high-power US or Canadian mediumwave broadcast station.
73,
John
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