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[aprssig] APRS Bandwidth

G0JXN Jim g0jxn.jim at ntlworld.com
Thu Jun 25 06:47:18 UTC 2009


Hi Guys

Seemed to have opened a can of worms here. My thanks to all those who have contributed to the discussion. I'm not generally responding to individual inputs because there are many common points.

It has been suggested that the 300bd APRS signals can be considered both as FM or AM signals. Dealing with the FM approach first using Carson's Rule we have that 98% of the power will lie within the bandwidth:

Bt=2(pF+Fm)

Where Bt = bandwidth, pF = deviation & Fm = modulating frequency

pF = 100Hz (being half the tone spacing) & Fm = 150Hz (being half the baud rate)

Bt = 2(100+150) = 500Hz

This would sit neatly in the requirement of the 500Hz bandwidth requirement but FM signals have sidebands that go on ad infinitum, albeit declining rapidly. While the first sideband at + & -150Hz would be within the 500Hz bandwidth the second at + & - 300Hz would not and it would be only some 20% of the carrier. This would not fit in with the UK license requirement 'that not more than 1% of the mean power of the transmission falls outside the nominal modulated carrier bandwidth'. In any event the argument is flawed in that an FM signal only has one carrier that moves between the deviation limits not two as suggested.

Alternatively we have the logical approach suggested by Steve, K9DCI, considering that the two tones are two separate carriers modulated by the 150Hz signals. If the 150Hz signals were pure sine waves we would have side bands at -250 & +50Hz for the lower tone and -50 & +250Hz for the higher tone. Again this would fit neatly into the 500Hz bandwidth requirement but with the 150Hz modulation not being pure sinewave there will be sidebands generated by the harmonics. Bearing in mind that the main sidebands will already be 25% down on the carriers and any sidebands generated by harmonics, that with square waves will be odd harmonics, will be further reduced.

If I'm talking crap no doubt someone will put me straight.

Those of you who sit States side of the pond should not try and relate to the situation here in Europe. Mobile APRS operation and the tracking of in the UK and the rest of Europe is not the predominant activity. At the time of writing I have the following stations displayed:-

             Mobiles        Others

2m           16                23

20m           0                  4

30m           6                19

Many of us use APRS as a straight communications tool. Among the advantages the ability to see who is QRV without the need to trawl the band and messages can be left if the operator is not at the keyboard.

Prior to his premature silent key, a ham in Scotland and I, near London, used to have a regular exchange of messages, usually via a digi in Switzerland. I have recently had a email from a ham in Germany expressing his regret that I no longer ran a digi on 30m because it provided him with links during his mobile excursions through the length and breadth of Europe.

With regards to propagation on HF, yes if conditions are right of course signals will be heard. Unless you sit in the skip distance that is. My friend G0IQK and I are 13 miles apart. We cannot always communicate directly on 2m and never on 30m. For that we have to rely on digies on mainland Europe. Also bear in mind that we do not have a KW to play with. As a full license I am limited to 400W PEP, Intermediate licenses 50W PEP and Novice license 10W PEP. Not that I could even run at the maximum I'm allowed because I am surrounded by domestic TVs and radios of crap design the owners of which would have the Regulator around my ears.

With regard to Igates, putting aside my own negative views for the moment, internet is not available throughout Europe. Even in the UK there are countless places where it is still not available or only as a slow dial up. Our government is suggesting that they may levy about $10 on our TV license, yes we pay about $200 a year for the privilege of owning a TV set even if it's off air, to pay for a broadband roll out. In the UK again we have to apply for a Notice of Variation if we wish to run an Igate. And phone hook ups forget it, not allowed at all. While I'm talking Igates, I dread the thought of two way Igates surely worse than HF digies.

I was pleased to note Keith's (VE7GDH) comments about my article on www.apritch.myby.co.uk/hf.htm I would be interested to hear comments on my proposal for a world wide APRS net on 20m. See www.bandscommunications.co.uk under the 'aprs hf net' tab.

I think that covers everything for now.

73

Jim, G0JXN


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