[OZAPRS] APRS Databusrt

Darryl Smith Darryl at radio-active.net.au
Sun Aug 12 14:47:19 EST 2007

Hi All...

100 KHz is a heap too wide for APRS data. 

Every time I look at this, I remember that there are two conditions needed
to use this type of hardware in any practical way.
	a) Increase transmission power
	b) Increase the number of reception sites

Increasing the number of reception sites is not all that easy. This is
particularly so since we need to have connectivity since there are no
proposals to run in a mesh network.

Increasing the transmission power leads to some issues. Since we are
about transmit and receive, we need dual oscillators. The best way to do
this, price wise, is to have a single oscillator that will operate on
different Rx and Tx frequencies (RX needs a 45 MHz offset commonly for
So, we need to use a PLL or two sets of crystals. Given that silicon is
cheaper than other hardware, PLL it is. 

So, now we need to turn the PLL on and get it up to frequency. Then we
to turn an amplifier on, and get it stable too. This commonly will take
50-200 mSec. This is a heap of time. With a good design and 50W, lets
that it will take 150 mSec to become stable. And then at the end we need
dump the transmitter, and turn it off. This might take another 25 mSec,
where we are transmitting useless rubbish onto the channel. 

Now, the payload. 40 bytes. 320 bits. At 19200 this becomes about 17 mSec.
With stuffing this might become 25 mSec.

So, we have just transmitted one APRS packet in 200 mSec, rather than 1000
mSec. But once again, the energy per bit is much lower, meaning that we
a much higher error rate. There is 1/16th the power being transmitted.
Therefore we may as well be transmitting at 3W. Only 3 %$^&* watts! Or to
get the same performance as an 50W 1200 bps transmitter, we would need to
transmit 800W

This just does not make sense, at least for a low bandwidth application
APRS. Higher bandwidth is different...


P.S. Wide bandwidth modes on 70cm... Why not run Wifi on 440 MHz?
come to think of it, we almost are. Much of Wifi was 'borrowed' from Ham
Radio. What is the public name of a WiFi station called? The SSID? This is
the same as the -1 in VK2TDS-1. The part that I can legally change. 

-----Original Message-----
From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
Behalf Of Marcus B
Sent: Sunday, 12 August 2007 12:24 PM
To: vk4tec at people.net.au; VK / ZL APRS Users
Subject: Re: [OZAPRS] APRS Databusrt

On 11/08/07, Andrew Rich <vk4tec at people.net.au> wrote:
> The databurst is true FSK at 100 kHz.
> I am now curious as to the licence implications.
> Comments ?

Licence implications, none.  You can have (really) wideband modes on
70cm and above (eg. ATV).

This may be the perfect opportunity to take advantage of some of these
reserved segments that aren't in use yet while the bandplan is still
changing, and it could open up a wideband data a segment on 70cm
before it was squeezed out by bandplans.

Maybe an option is to email the bandplan people at the WIA and ask for
500kHz (or more, 1MHz?) of 70cm for "wideband data", including spread
spectrum, databurst & highspeed data.  Maybe two segments (431 &
441MHz, see below)?

The UK UHF bandplan has 430.990-431.900 reserved for data, and there
are only a handful of 426MHz ATV repeaters left in the country (7
licensed out of about 20 according to the WIA), if the right frequency
is chosen, this could be good as we could play with European data
equipment, and we could sell them our stuff.  Aussie reserved is
431.275 - 431.975MHz.

> Where would you run a digital signal around 100 kHz wide on 70 cm ?

There is also a reserved segment at 441.025 - 441.975MHz.  I have been
using this band for all kinds of playing for years.

For $60 a TRX I could be interested in this, especially if we could
get the WIA to reserve some wide data segments (so it doesn't get
re-allocated and make the equipment useless), although it does seem a
little wide (100kHz) for only 19k2 data...  Something like this could
do digital voice and/or mid speed data.

"Anyone who tells you different is selling something." -- Kent Beck
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