EXACTLY! I've brought this up obliquely before, but in more detail here:
BW limit means "occupied BW" defined as less than 0.5% of power is below and
less than 0.5% power is above the bandwidth. There is also necessary BW. See the
regs, see
47 CFR 2.202 (a) and (b).
In the case of two tone amateur RTTY (or using Chen's affectation "steam RTTY"),
that means the
BW=B+1.2S where B is the baud rate and S is the shift. The common 45.45B 170S
works
out to 249.45 = 250 Hz necessary BW.
So 1000 S at 300 B works out to be 1500 Hz necessary BW. That is a nice possible
limit since it
parrots the current regs for RTTY as 300 baud limit and max 1000 Hz shift. I
won't debate whether that
is needed or even used, but it is currently permitted.
The lowest order PACTOR-III SL1 mode has 100+1.2(840) = 1108 Hz necessary BW,
the highest order
PACTOR-III SL6 is 100+1.2(2040)= 2448 Hz. There are four levels in between. The
lowest order
PACTOR fits in the 1500 BW.
The FCC/NTIA and ITU-R publish guide lines on computing required BWs. See
US 47 CFR 2.201-2.202-emission designators, modulations and necessary BW.
Our 47 CFR 97 points to that.
We indeed need to be careful about what we ask for!
Additionally, modes like PSK31 (63 Hz necessary BW on TX) and JT65
(170 Hz BW on TX) are actually practiced by hams as multiple users in a
2-4 kHz BW subband. Individual TX BWs are small, but general usage is
for multiple simultaneous decodes in a contiguous BW. I guess that this is
the kind of innovation and usage that we don't want to shut ourselves out of.
73
Kai, KE4PT
On 11/22/2013 2:51 PM, Bill Turner wrote:
I am a little surprised that no one has brought up the question of measuring
bandwidth. We need to be careful what we ask for - we just might get it.
If the FCC should establish a bandwidth limit of 500 Hz, what exactly does
that mean? Does that mean that all tones AND SIDEBANDS must be within the
500 Hz? Or does it mean that the shift of a signal must be within 500 Hz but
the sidebands can be outside 500 Hz? And if the latter, how many dB down
must they be?
You may recall that the "real" bandwidth of a 170 Hz shift RTTY signal is
approximately 300 Hz because of the sidebands. Given that, what is the real
bandwidth of a mode that claims to occupy 500 Hz, such as Olivia 500/16?
This needs to be settled before the rule is made by the FCC, otherwise chaos
will surely follow.
73, Bill W6WRT
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