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[aprssig] APRS Bandwidth

Chris Moulding chrism at crosscountrywireless.net
Tue Jun 23 12:08:07 UTC 2009

Hi Jim,

I've "made" some time to work this out. I've changed some of the symbols 
to fit with the email format.

Carson's Rule states that 98% of the power of a frequency modulated 
signal lies within a bandwidth Bt where:

Bt = 2 ( pF + Fm)


Bt = bandwidth

pF = peak deviation of the instantaneous frequency F(t) from the centre 
frequency Fc

Fm = modulating frequency

Entering numbers for 300 bd gives:

pF = 100 Hz (half 200 Hz frequency shift)

Fm = 300 Hz (300 baud data rate)

This gives:

Bt = 2 ( 100 + 300)

= 800 Hz

That doesn't look too good if the RSGB want 500 Hz maximum! If anyone 
has more time than I have at present please feel free to pick holes in
my calcs!


Chris, G4HYG

Chris Moulding
Cross Country Wireless (2009) Ltd
7 Thirlmere Grove, Bolton, Lancs, BL4 0QB, UK
Tel/fax: +44(0)1204 410626
Mobile:  +44(0)7752 391908
Website: http://www.crosscountrywireless.net
Company number 6780346 registered in England and Wales  

G0JXN Jim wrote:
> Hi Guys
> The UK Amateur Radio license no longer allows the operation of an 
> APRS digipeater without special permission (Notice of Variation) from 
> the regulator (Ofcom). It being filtered first through the RSGB.
> I wish to run a digipeater on 10.151MHz (KAM) but am advised the it 
> would probably not be recommended by the RSGB because the bandwidth of 
> 300bd would exceed the 500Hz laid down in the IARU bandplan for 30m. 
> The implication being that we should not operate APRS on 30m at all.
> Monitoring signals on 30m I find that they all seem to be within 400Hz 
> but I will need to prove that on paper.
> Can anyone please tell me what the bandwidth will be for a properly 
> adjusted 300bd station and how this might be calculated?
> 73
> Jim, G0JXN
> ------------------------------------------------------------------------
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