One of the problems occasioned by the severely restrictive FCC
rules for spread spectrum radios, is that small, economical
radios that can be associated with just one sensor do not have
the effective range to reach centralized access point. That
is especially true through masking trees.
An illustrative requirement for this problem is the '40 wells'
in the Trout Lake research area.
The
research project at Trout Lake includes monitoring 40 separate
'wells' each of which is a narrow - 2 inches in diameter - plastic
pipe inserted into the ground down below the water table. Then
the water depth is measured on a recurring basis. It would be
desirable to have the levels measured to 1cm accuracy, and recorded,
at least once a week, or more frequently if feasible. But given
the labor effort required - slowed down even more in the winter
when the snow is deep - the set is surveyed only once a month.
It is obvious that a wireless device at each well, connected
to a depth sensor which can be either timed to read and store
the depth until polled through the radio, or read the depth
only at the time called, can be set up with suitable rechargeable
or replaceable radios. But if the radios are large enough to
produce the necessary wattage, such as a full 1 watt, like Freewaves,
they will be costly - currently between $1,000 and $1,800 each.
Lower power - like 100mw - can be found in small radios such
as World Wireless Microhoppers, which can be purchased around
$100 in volume. But these radios have very limited range in
treed areas - less than ¼ mile. That is insufficient to reach
central relay radio points covering a large number of radios.
We
decided to try an approach which could minimize the linear feet
that a 100mw radio signal would have to travel through masking
trees before reaching a relay radio. We decided to use a small
helium filled balloon which, tethered could reach to 400 feet
of elevation carrying up to 10 lbs of radio/battery payload,
and 'look down' on the radios at each well through less foliage.
Then
the researcher can drive to within 1 or more miles of a cluster
of wells, raise the balloon carrying the configured radio and
a battery with at least 4 hours of life, to the altitude necessary
to clear the majority of trees in between, and using a laptop
computer connected to a base radio, 'poll' by number the well
radios, capturing the data.
Before
doing a field test at Trout Lake, we needed to assemble the
balloon and harness, configure test radios, and weigh the package,
run up the balloon and try the radio links. Just to be sure
all the parts would work together.
We
did this near Port Ludlow at Dan Withers home/lab on the Olympic
Peninsula.
Dan
Withers assembles while Dave Hughes watches
In this first instance we ordered an 8 foot balloon from Balloonworks
of Florida---a black balloon, with name of Trout Lake Research
Station in yellow. And a harness that could hold with Velcro
items suspended beneath the balloon.
We
considered several 'packages' to be suspended, and batteries
and weighed them.
Possible
batteries. Picked one at left.
Then,
with a Microhopper radio at the heart of it, we assembled an
antenna cable, so the antenna could hang 'straight down' below
the balloon, a power connector, a pack of 6 small batteries
which produces 9+ volts. Then we used an aluminum case in which
to stuff the components, surrounded for now with plastic bubble
packing.
Components
of Payload
The
straps on the balloon base had to be extended around the aluminum
carrier.
Harness
with balloon and carrier
Weighing
all the above came to only 1.25 lbs of payload!
Payload
weighed
With
the 400 foot length of light but strong tether cord, I expect
to use a 6 foot balloon next time. It can easily carry the load,
and requires much less helium to fill it. One standard tank,
costing from $60 to $90 each, could fill the balloon twice,
and 'top off' weekly if the smaller balloon is stored inside.
We
tried to fill the balloon with the purchased tank. We were shorted
some gas, since the tank should completely fill out an 8 foot
balloon. So the resultant balloon did not properly hold the
straps in place, and the rig at the bottom had a tendency to
tilt.
Tom
Williams holding balloon and payload
We
took too much time getting the three Microhoppers to configure
properly - one at base Laptop connected by serial cable, one
relaying, on the balloon, and one to be connected to a CR10X
Data Logger - so we could 'read' a proper connection of the
links.
In order to be sure the radios were working properly we attached
a clever device I had Dan make, which, when connected to the
serial port of a radio which does not have - as the Freewave
does, a 'connect' indicator light, will light up at both ends
if one is attached also to the other radio's serial port.
The
Microhoppers have no connect indicator lights, as do not some
much more costly radios like the Novaroam. This handy little
device can be used in many ways.
Connect
Indicator connected to Microhopper
Using
the Connect Indicator device, we first flew the balloon with
relay radio attached, to insure we had communications between
ground and air. We did. The green LCD on the Connect Indicator
Box continually blinked as the balloon went aloft, indicating
a continuous connection to radio on the ground.
Dan
Withers with the completed balloon system being launched
We
then set up the total experiment, with the Data Logger, connected
to the end-point Microhopper and placed away from the house.
The base station Microhopper was then connected to a laptop
running PCW208 software so we could observe the connection and
gauge its reliability in reaching the distant CR10X where the
data would be stored. Later, if this worked, we would substitute
a single depth-measurement sensor at the distant radio end for
the data logger. That has yet to be fabricated.
It was near dusk with the wind coming up before the Balloon
flew and let us test it end to end.
Controlling
the balloon up 200 feet
The
balloon fighting the evening breeze
As
we hoped, the connection was completed.
The
PCW208 software reported that it was connected to the Data Logger,
via the relay radio on the balloon.
The
'Connected' signal
Learning
much about the details of what it would take, we took the balloon
down, and returned to Colorado to prepare it to be taken to
Trout Lake for a real-distance test in the field.
But
we did not leave before Dan showed us his progress on designing
a depth gauge that (1) would fit down in the very narrow 'well'
pipe (2) would be accurate enough for the researchers and (3)
cheap to make in quantity.
The
interface between the radio input port, and the sensor, which
would have to be analog, would have to be designed also. Dan
was turning to the tiny PIC processor for that task, which is
a complete processor and memory on a tiny chipset.
$5.00
pic processor and $40 pic ready for applications
Two
Tiny Transceivers
