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Antenna Elements
"Thank you so much for visiting our class on
Friday! The kids loved it...they thought it was pretty cool to meet a
"real" Astronomer! Thanks again, Janine"
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antenna antenna design consists of wire mounted on pieces of 3/4 inch plywood
drilled out to slide onto a galvanized metal pipe that serves as the boom. The
elements on the setting blocks are then set in position with set screws
(ordinary wood screws). The original antenna was actually designed to work at 108 MHz rather than
the 92.1 MHz it actually detected meteors at. Not great, but it shows that even
a poorly made antenna can be used to detect meteors. There
are six elements that need to be cut to the proper length and spaced at
specified distances from one another for the antenna to be "tuned" to
the observing frequency.
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See the table at this link
to see the length of the antenna elements
and the spacing between them for given frequencies. Lengths and spacings
are given for the FM radio band. Or you can use the formulas on the How
to Build an Antenna page to
calculate the dimensions for your antenna. |
See the instructions on how to make the antenna
elements. You will need these to fasten to the top of the adjusting blocks.
Once you have cut and labeled all the elements, you need to mount them to the
metal boom using setting blocks.
| The setting block can be made from any sturdy
wood such as 3/4 inch plywood. It should be at least 3/4 inch thick. Cut 6
pieces approximately 20 cm square. Drill a hole slightly larger than the
outside diameter of the boom you are using so that the block can slip over
the boom easily. Don't make the hole too large otherwise the block can
move too easily on the boom. Use an exterior grade stain or paint to
finish the blocks to protect them from the weather. You won't need much
and suitable left over stain or paint can probably be acquired from home. |
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| Each adjusting block needs to have two 2 inch
wood screws added to it as set screws. Pre-drill 2 holes in the bottom of
each adjusting block positioned so the screws enter the centre hole at
either side as shown in the illustration. Make sure the drill holes are
centred on the wood and that the pre-drilled holes are smaller than the
diameter of the screws. This will allow the screws to set snugly.
Screw the wood screws in far enough that they are just about to enter the
centre hole. |
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| Slide the adjusting blocks onto the boom
placing them at the spacings that you marked on the boom from the
Dimension Table for your frequency. Mark each block so you know what
element, D1, Reflector, etc., should be attached to which block. Secure the
blocks at the correct spacing by tightening the set screws being careful
not to over-tighten them. Attach the |
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elements to the top of the block using
cable clamps of a suitable size.
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