[aprssig] APRS for a "ocean buoy"
ben at ben.com
Tue Jan 9 00:31:46 CST 2007
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
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
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>
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