The "Bird House
Antenna"
(Earl
Schlenk-W0ES)
I
recently moved into a Condo. As I read the restrictions of the
Association, there were 15 pages of rules, 8 of these were concerning
antennas!
I wasn't going to let this stop me from having my station up
and operating.
Also, this location is in the bottom of a bowl
geographically with hills on all side of me.
A flyer came from the association suggesting the residents
put up bird feeders with the provision, that they must be off the
ground and be "squirrel proof", again, their quotes. Boy what a bit of
synchronicity!
This is the perfect ploy for using
theirsuggestions to get up a 2 meter antenna.
I enjoy
experimenting with Magnetic loop antennas and I have a design that would
fit inside a bird house/feeder, and I would build a bird house to enclose
it. The dimensions are sized to enclose the loop and I added a roof for
appearance purposes and decorative windows and a door.
This
article isn't a step by step instruction, but a general concept one.
I
will describe the design and control systems, but will leave the
construction processes to the reader.
Figure
1.
Figure 1 above shows the basic
loop I built using 3 in. wide copper strapping .022 in. in thickness and 6
in. in diameter. This was mounted on a dc motor I bought from
"Allelectronics", http://www.allelectronics.com , for about 7 bucks.
I mounted limit switches,
using lever switches, to stop the turn motor at its maximum rotation in
the CW and CCW directions. I also mounted a home position indicator (for
initial loop orientation reference) using an infrared opto-coupler and a
LED, see Figure 2 below.
Figure 2. Note: on left side that it says "Turn Motor
Voltage"
The capacitor I used was from my junk box and was a 2 to 10pf. The
stepper motor was purchased from "Motion King Motor
Industry" http://www.motionking.com,
and was mailed directly from China. This was the
smallest stepper motor I could find with enough torque to turn the
capacitor and have enough resolution (1.8 degree) for the very fine tuning
this loop requires. The stepper motor controller was bought as a Kit. I
purchased it from "Cana Kit",
http://www.canakit.com.
I
used a LM350 regulator adjusted to turn the loop at a slow speed.
See
Figure 3 below:
Figure
3
The bird house
was made from scrap material I had on hand, and I gave the "house" a coat
of paint to match the color of the facade of the complex and coated it
with 2 coats of Polyethylene. I purchased the "door and windows" from
"Doll House Heaven". http://www.dollhouseheaven.com .
The stepper motor is used to adjust the capacitor for
resonance on the frequency that you want to use, you can use your SWR
meter to tune for resonance by tuning for a 1.1 swr.
The loop is
bi- directional and has good nulls off its sides. I mounted the "house" on
1.5 inch diameter PVC pipe 11 foot in the air. It is supported by a 6 foot
tall 2x4, attached to landscape timbers that were in the planting area
around my patio. I used Belden 8214 coax to feed the loop, running the
coax inside the PVC pipe. I used a 10 conductor shielded cable purchased
from Allelectronics for the control of the loop and it too, was run inside
of the PVC pipe.
The
coax and control cable were buried under the mulch in the planting area
and ran into the shack through a board mounted in the bottom of the shack
window. I am very satisfied with this antenna.
I have received
compliments from my neighbors about the "Bird House" and no one knows it
is an antenna. I was quizzed as to what the electrical wires were for and
I replied that I am going to install a "weather station" inside in the
future. This ruse was to explain any future changes I may make or if
repairs are necessary in the future. I could also have said it has a
camera inside to observe the Birds!
Figure 4 below, shows the loop
and it's associated components. Figure 5 shows the finished
project!
Figure
4.
Figure 5. Finished
"Bird House Antenna:
You could just mount a
vertical coax dipole inside of the PVC pipe instead of making a
complicated Antenna, such as a loop. I went with the loop antenna because
of the low elevation that I live at and I like building loop antennas. You
could purchase a Bird House instead of building one and simply mount it on
the PVC.
If anyone wants more particulars on building this loop
antenna and controls, feel free to e-mail me at: w0es at hotmail.com
I am now in the process
of designing a stealth HF Magnetic loop antenna, using some "Fractal"
antenna concepts. If this works out I will share the design with
you. 73 W0ES
Editors note: Drawings
have been compressed. Contact author if more detail is
needed.
