This is an evolving project to create a hidden transmitter
that is self powered, survivable from friend, foe and
weather and will run like a beacon transmitting with a 10%
duty cycle 24/7. This note has been edited from that used
for the GeoTran I and also includes the considerations of
the solar charging note.
Introduction:
First, some considerations
for just what you are doing and why. This transmitter is a
training aid for transmitter hunting and as it is
transmitting in the amateur band it requires the person
responsible to have a valid Amateur Radio License, although
one is not required to hunt and find the unit. Another
consideration is that the operating frequency should be one
coordinated or at least well know for transmitter hunting
and not really too popular with other users that might
consider it's operation to be interference to what ever use
they have for the frequency. Also, this unit will excite
Muggles or "civilians" who might consider the hidden unit to
be a bomb or something dangerous and report it to
authorities who are also ignorant about the whole
transmitter hiding thing and will destroy your new toy. A
third issue is those "friends" that find the hidden
transmitter had a tendency to be cute and either move the
unit or change the recorded message or both. This is to be
avoided as it doesn't help the purpose or anything.
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Two things I have found will help a lot in reducing these
issues is to lock the unit securely and lock it to something
immovable and to label the unit with its purpose and your
phone number in case it is a problem to someone or
something. This locking mechanism is described below. |
Description:
The unit consists of several parts. A transmitter, an
antenna, a battery a box and a solar panel. The
transmitter is a
Squawkbox
that puts out 50 milliwatts for 60 milliamps of DC input
7-18V. It will idle at 5 mA so for a 10% duty cycle
(transmit to total repeat time) it will draw an average
of 10.5 mA or about 0.252 AH/day. At 20% duty this will
be 16 ma and 0.384 AH/day. I think 10% is fine for
hunting and would allow other activities on the
frequency at the same time. More on the battery/solar
system later. |
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Electronics case:
The squawkbox, charger controller and batteries are
located in a weather proof box (Pelican 1060) available
at Fry's and other large electronics or scuba stores.
This box is dry when 30 ft under water and so rugged
that it is guaranteed against breakage from any cause
for life.
I installed a BNC feed through and soldered the antenna
output of the squawkbox to the inside part of the
connector using 2 inches of #18 twisted pair. The
batteries have a power pole connector (PP) as does the
charger controller output so there are two PPs on a
short leads from the squawkbox to allow battery changes
and some measurements test points.
The power plug from the solar panel is intended to be
installed into a cigar lighter socket in a car. I
removed the little solar controller board that is inside
this connector and it is in the case with the batteries
and squawkbox. The wire from the solar panel to the
control board passes through a small hole in the
electronics case that is fully sealed with hot glue.
I drilled a 1/2 inch hole through the top and out the
bottom of the Pelican box so a trailer hitch lock
(Harbor Freight) could be fitted to keep the box locked
and to anchor a steel cable that was to be attached to
something difficult to move. |
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Power source: |
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Now we need 250 mAH for 7 days or 1750 mAH of battery. I
chose 3000 mAH made up with a 10% duty cycle, the transmitter needs 252 mAH/day.
For all practical purposes the transmitter is a
constant-current load for input voltages from 7 to 18
volts. This will come from a solar panel during the day
and from a battery at night and on cloudy days. The
solar panel will also charge the battery. |
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The solar power folk have some "rules" that they use in
designing solar systems. Solar panels are rated in
watts, the watts they will provide in direct very bright
sunlight. In other words the maximum possible power the
panel will make. The power will fall off of this maximum
for the angle the sun makes with the surface of the
panel and the amount of clouds and blockage you have to
the sunlight. The power variation is cosine shaped and
the industry simplifies the whole calculation by
addressing the square wave that has the same included
area and peak wattage. This is about 5 hrs per day at
the peak power. This hours/day for your area is
published somewhere on the internet, I remember seeing
it, but I don't know where. I measured it here in
Fontana at 5 hrs on Dec 6 about the worse time of the
year. |
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On a cloudy day it is less than 0.2 hours.
Another rule is that the solar panel will charge
a lead acid battery at about 80% efficiency.
However, a NiCd battery
charging at 0.05C is only 70% efficient. I don't know
about NiMH. Another rule is you should have at least 7
days of battery with no charge to account for cloudy
days. |
This gives about one cloudy day for every sunny day. Two
considerations here. One is if the battery is open for
some reason and the squawkbox is only drawing 5 mA then
the solar panel voltage will be around 20 volts, in
excess of the 18 volts rating. I have not had any
problems with this (yet). The second problem is in over
charging the battery with several weeks of sunny days.
The sub C cell can take an over charge current of 0.05
C where C is the cell capacity in AH or for the 3000 mAH
battery is 150mA. This is more than the solar can make
so the battery will last a good long time (800 cycles or
3 years). |
Antenna:
The antenna is an omni directional J-pole
made from 1/2 inch copper water pipe. Take a
10 foot piece and cut it into a piece 57
inches for the 1/2 wave section, 19 inches for
the 1/4 wave section, 18 inches for the ground
pole, 1 3/4 inches for the element offset and
6 inches for the foot rest. With three Tee
fittings one elbow and two end caps,
assemble the antenna with the spacing
between the 1/2 half and quarter wave pieces
equal to the width of a 2x4. Just wipe the
places to be soldered with rosin flux and
solder with a butane torch and rosin solder.
The foot rest is near the bottom and gives
you a place to put your foot when shoving
the antenna into the ground. The third Tee
is at the end of the foot rest and is used
for driving a hunk of rebar or something
into the ground to keep the antenna from
rotating. The coax is soldered to the pipe
at an inside dimension of 1.25 inches up
from the offset part with the center
conductor to the 1/2 wave part and the shield
to the 1/4 wave part. |
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The top 15 inches of the antenna 1/2 wave part
is bent over at an angle equal to 90 degrees
minus your latitude (in my case it is bent
56 degrees)...This is the angle for the sun
when it is over the equator. Mount the solar
panel to this end of the antenna using two
or more 1/2 inch cable clamps. Run the wire
down the outside of the pipe. Paint the
whole thing some nice camo color (after
masking off the solar part of the solar
panel). Point the solar panel due south or
in the middle of the clear sky if it is
blocked by trees buildings or whatever.
Drive a post threw the Tee at the end of the
foot rest to keep the antenna from rotating.
The bending of the 1/2 wave part and adding
the solar panel moved the center of the
antenna VSWR curve down about a half a
megahertz but it is still under 1.5:1 over
the 145-148 MHz band. |
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Security:
To
keep the thing from wandering off or to keep meddlesome
hands out of the electronics, I bolted the Pelican box
together with a 1/2 inch trailer-hitch lock from Harbor
Freight. I also ran a 6 foot length of 3/32 steel cable
with loops on each end between the two ends of the hitch
lock and wound around the antenna post above the foot
rest and if nothing nearby was stationary under duress,
I would install a dog-screw (used to tie out dogs in the
park). Running the cable through the big handle prevents
anyone from unscrewing it without removing the cable. I
added a label to the Pelican box that says:
"This
is a Search and Rescue training transmitter. It is
licensed under Part 97 of the FCC Rules. Please do not
disturb or move this unit. If you have any issue or need
more information please contact Robert Thornburg at
(909) xxx-xxxx". |
