[aprssig] APRS for a "ocean buoy"

[Ben Jackson]

On Mon, Jan 08, 2007 at 08:39:06PM -0800, Jim Lux wrote:
> An acquaintance asked if there was decent APRS coverage along the 
> coast of California, say out to 10-20 mi from the shoreline.  The 
> transmitter would be pretty close to sea level (on a kayak, for 
> instance).  30m HF is an option, but there was some uncertainty about 
> what sort of antenna would be possible for that.

I was interested in the same thing.  I'd still like to build a buoy,
but my findings suggested I needed a new design and a dedicated listening
station.

VHF is line of sight.  10-20mi out is not really practical for a buoy
which is practically "on the ground".  30m HF has its own problems.
There aren't many listening stations.  I built a 5w transmitter and
ran it from my home in Beaverton, OR (near Portland).  I was able to
hit two HF listeners, one I think is in south central Washington and
one in the LA area.  That required a second story dipole.  Packets only
got through during band openings a few times a day.

The main things I need to change are:

1)  More powerful transmitter.  5w was marginal on a much better antenna
than a buoy would have.  I've done tons of antenna simulations looking
at various configurations, and even with fairly tall (3+ meters)
configurations and assuming saltwater was a fabulous ground, the buoy
antenna is pretty lossy.  15w seems reasonable for a buoy, given the
low duty cycle.  My notes show that 32lb of D batteries cost ~$100 and
have around 2500wh which is about 3000 powerup/transmit cycles assuming
a 15w final.

2)  A more stable transmitter.  20ppm doesn't cut it for HF packet.  1ppm
is 10Hz at 30m, sub-ppm would be better.  The minimum standard would be
a TXCO.  Better would be a TXCO with long-term compensation using the
GPS timebase.

3)  A dedicated listening station.  It's very hard to locate HF stations,
but I'm fairly sure there are only about 3 on the west coast.  Even if
I'm off by a lot, it's still far, far less than the number of VHF
listeners.  And if they have any directional gain, it's probably not out
to sea.  If I had a buoy out there, I'd want to set up a big beam or
maybe a rhombic pointing at the Pacific.  I have a friend with a farm
closer to the coast, so I could put something up there.

-- 
Ben Jackson AD7GD
<ben at ben.com>
http://www.ben.com/