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[aprssig] APRS Bandwidth

Stephen H. Smith wa8lmf2 at aol.com
Wed Jun 24 01:10:21 UTC 2009


Frank or Barbara Rossi wrote:
> Looks like IARU-Region 1 Band plan is way to restrictive Click 
> <http://www.ari.rc.it/Download%20vario/bandplan.pdf>   
> http://www.ari.rc.it/Download%20vario/bandplan.pdf
> With all the restrictions, there will be no APRS on 30 Mtrs.
> So much for a world wide band plan.
> N3FLR - Frank
> On 6/23/2009 6:37 AM, 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

See my previous response to this thread under the heading:
    "Re: [aprssig] APRS Bandwidth - Carson's Rule Recalculated"

300 baud / 200 Hz shift AX.25 packet is perfectly capable of fitting 
into a 500 Hz occupied bandwidth.   Aside from the issue of "unattended 
operation" mentioned in the bandplan document mentioned above, I don't 
see any problems. HF APRS is certainly far less disruptive than the 
Pactor III being used with the AirMail mail box system on HF. 

Keith VE7GDH wrote:
> Regarding 500 Hz bandwidth... aren't 300 baud tones just 200 Hz apart?
> I viewed the reply from Chris G4HYG about Carson's rule...
> http://en.wikipedia.org/wiki/Carson_bandwidth_rule. Doesn't that apply
> to FM? See

You are partly correct.  Carson's rule applies to modulating audio or 
data frequencies applied to an FM transmitter.
> For SSB, I would have thought the 200 Hz difference between
> the two tones would set the bandwidth.

Sending FSK data with an SSB transceiver is more like FM than SSB.
Applying a single audio tone to the mic jack of an SSB rig creates a 
single RF frequency offset from the (suppressed) carrier frequency by 
the frequency of the audio tone.

Applying an audio tone 200 Hz higher to to the SSB rig will create a 
single RF frequency 200 Hz farther away from the (suppressed) carrier 

The net effect is exactly the same as if you have a single key-down CW 
carrier and shift it 200 Hz periodically (i.e. direct FSK with a 200 Hz 
     [Some, mostly higher end, HF transceivers actually offer this 
direct FSK mode.  Rather than having feed alternating audio tones into 
the mic jack, you feed a TTL-logic-level 300 baud (for packet) or 45 
baud (for RTTY)  data stream directly into a special FSK input on the 

The bottom line is that you essentially have a FM (i.e. constant 
envelope power) transmitter modulated by 150 bps square waves rather 
than rather than higher-frequency audio sine waves.  

[Note that "300 baud" (300 bits / second) corresponds to only 150 Hz 
square waves.  Assuming the worst case of alternating "1"s and "0"s, 
each square wave cycle will require TWO bits to complete -- not one 
since one bit has to be the high half of the square and next bit the low 
half. Any bit pattern that  has adjacent "1"s or adjacent "0"s will have 
a lower effective "squarewave frequency".  ]

In either case, the occupied bandwidth is greater than just the the 
spacing between the two tones (a.k.a. "mark" and "space" frequencies).  
This is due to the transients created during the instantaneous shift 
from one frequency to the other.    [Even CW transmission has a 
bandwidth greater than a single frequency when you start keying at a 
rate higher than zero!]

The original poster that applied Carson's rule for approximating 
occupied FM bandwidth forgot that the base band modulating "frequency" 
would be 150, not 300 Hz.  Recalculating the occupied bandwidth,

     2 (100 Hz deviation + 150 bps) =  500 Hz.   

Considering that  the actual data "frequency" is actually lower than 150 
Hz since runs of 2 or 3  ones or zeros DO frequently occur, the actual 
average occupied bandwidth is actually lower. 

[Note that the occupied bandwidth created by true square-wave modulation 
would be greater than for sine waves. However, the finite bandwidth of 
the SSB modulator and IF filters "rounds off" the corners of the square 
waves and increases the rise/fall time to be more trapezoidal than 
square, resulting in a bandwidth nearly the same as with sine waves.]

I can attest to the fact that this is "REAL" since I have no difficulty 
receiving 200 Hz HF packet through an 500 Hz bandwidth CW filter on HF 



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

JavAPRS Filter Port 14580 Guide

"APRS 101"  Explanation of APRS Path Selection & Digipeating

Updated "Rev H" APRS            http://wa8lmf.net/aprs
Symbols Set for UI-View,
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