Serial
Radio Test at Oyster
Following
the October 9 meeting with the VCR/LTER PI's (see Diary 37) we
conducted our first radio tests. On Wednesday, October 10, when
I arrived at the Oyster LTER a little before noon, the crew was
well prepared. During the previous week they had purchased equipment,
ladders, safety gear, tomato stakes, and everything else they
could think of for work on an 80-foot tall unused fire Tower on
Hog Island, where we hope to create a radio relay/hub. The crew
had also quietly re-invented the gin pole, a temporary pulley-holder
used by ham radio folks for decades.
Gin
Pole (upper left) on Broadwater Tower
(The
tomato stake idea was for plotting a line-of-sight path from the
Tower to the rented farmhouse that currently serves as the LTER
headquarters. If we were to set up a directional antenna on the
tower we would need to find the compass bearing to point to the
farmhouse. Unfortunately, you can't use a magnetic compass on
an iron tower. So you take the tomato stake a few hundred feet
away in the approximate direction where the farmhouse ought to
be, get your compass bearing, and set up the stake in the "middle"
as a line-of-sight pointer for the fellow in the tower.)
We
gave a one-hour workshop for the staff, discussing radios, antennas,
serial ports, and the like. We also bench-tested the radios one
more time, so they could see FreeWave green lights (indicating
that the radios consider themselves to have a good link) and a
loopback test.
The Loopback Test
For
serial radios like the FreeWave, a simple link quality test can
be executed by plugging a laptop computer into one radio, running
a serial port terminal program -- this writer prefers Kermit --
and a loopback plug on the other radio. The simple serial loopback
plug is made by connecting pin 2 to pin 3 in a DB-9 plug, obtainable
from Radio Shack for about $5.
DB-9
Loopback Adaptors
The
trick here is to set the terminal emulator to use the PC's serial
port (versus some network connection), in "no echo" mode. Then,
to test the link you just start typing. If the characters you
type appear on your screen immediately, you have a decent link.
If there is a noticeable lag between pressing the keys and seeing
the characters echo (or you don't see them at all) your link is
probably faulty.
The
link we attempted is some 14 miles, mostly over Hog Island Bay.
At that distance there is no guarantee of success, especially
with the unknowns of reflection and absorption presented by saltwater,
sandbars, and a small stand of trees.
Diagram
of Distances
Broadwater
Tower, much taller than the trees
The
LTER staff didn't like the idea of this writer climbing a rusty
old fire tower, so I stayed at the farmhouse and operated a radio
with laptop and a 9 dB Yagi antenna, pointed towards the tower.
Due to the steep pitch of the roof we also decided to make the
first test from the lower-but-flatter porch roof/balcony (some
20 feet shy of the farmhouse's pinnacle). We kept an extension
ladder handy in case we could not get a connection at that height.
While
we wanted to use a Yagi -- directional antenna -- at the farmhouse,
the preference for Broadwater Tower was/is to use an omnidirectional
antenna, make the tower accessible from anywhere on the island
as needed to extend the link. Phil Smith thus took both a 9 dB
Yagi and a 6 dB omni to the top of the tower, as well as a radio,
battery, and loopback plug.
PI
Hughes with 6 dBi Omni above 9 dBi Yagi
Around
5 PM,during final preparation to start the test, we noticed that
the farmhouse radio already had its green light on, indicating
more-or-less workable connection -- and that with the Yagi lying
on the porch roof/balcony floor. It was intermittent, but it was
green. A more deliberate aiming of the farmhouse Yagi yielded
a solid connection to the omnidirectional antenna on Broadwater
Tower. After we established a more accurate bearing, we repeated
the loopback test, first a character or two at a time, and then
by cutting-and-pasting several screens' worth of text. The link
worked well whether we used the Yagi or the Omni at the tower.
We
thus determined that we could easily establish a reliable fourteen
mile link between the VCR/LTER headquarters and Broadwater Tower,
using one watt, 900 MHz, 115,000 bit per second serial-port-only
FreeWave radios and a 9 dBi Yagi antenna at the farmhouse and
either another Yagi or a 6 dBi omni on Broadwater Tower.
The
researchers at the VCR/LTER have a far-reaching vision both for
research and for science education. Mr. Carlson reiterated the
desire for live video as had previously been stated by the PI's.
In addition to predation studies (mentioned in Diary 37) he mentioned
a need for videoconferencing between the island and the local
high school, where he operates an ongoing program. Due to logistical
and insurance concerns, they simply cannot get high school kids
over to the island for field study, and so he would like to use
live video plus other real-time data to aid in teaching. As long
as we are appropriately supporting the research efforts, we're
glad to be able to help the education effort as well.
Tom
Williams
Special
Assistant
NSF
Wireless Field Tests
