"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"
Updated April 09, 2004
Tips for building antennas
by Guy Almberg
I chose to build a five element Yagi-Uda type antenna with a folded dipole driven element. The design of the antenna was derived from a program written specifically for Yagis. Because of time limitations I chose materials that were easy to work with and readily available. The boom and mast are built from wood and the elements are made of electrical conduit.
Using wood for the boom and mast kept the requirement for specialized tools to a minimum - a saw to split a 2x4 in half and a drill and bits to drill holes for the elements. When choosing the lumber select straight and clear (no knots) pieces as much as possible. I had to dig through quite a number of pieces to find suitable boards. Conduit is less than ideal, it is heavy, prone to corrosion and conducts less efficiently than copper or aluminum, its advantages are that it is cheap and readily available.
A much better choice would be 3/8 or 1/2 inch aluminum tubing, it costs 40-50 cents a foot depending on the wall thickness, alloy and diameter. You will need approximately thirty feet of tubing plus some extra that will be lost as wastage depending on the length of the pieces. For 92.1mHz the reflector is longer than five feet but the third director element is shorter than five feet so these two pieces can be made from one ten foot piece (conduit came in ten foot lengths). A source for the aluminum tubing might be The Metal Supermarket (in Edmonton).
Prior to securing the elements to the boom I cut a 5x7 inch piece of 1/2 inch plywood with a hole just big enough for the cross section of the boom. I slid this onto the boom to the position of the driven element leaving 1/2 the diameter of the tubing between the driven element position and the plywood position. I secured this in place with glue and let it dry overnight before proceeding. I used PL Premium glue to secure the elements to the boom. It is a polyurethane base glue that is weather proof, expands slightly as it dries and is nearly invincible.
I split a 2x4 and took one 2x2 as the boom and drilled holes perpendicular to the sides at the appropriate places, I then drilled holes 5/16 in diameter from the top of the boom into the existing element holes. Once the elements were appropriately placed I used the 5/16 hole to inject glue into the joint with a tube/glue gun while slowly turning the element insuring that the space between the element and wall was filled with glue.
The mast and base were constructed of wood also. The mast should be a 2x4 approximately eight feet in height. We attached the antenna at about seven feet. I would not go any higher than eight feet to help avoid the possibility of the antenna blowing over. The base can be constructed of the other 2x2 cut into 3 foot lengths for the feet of the base and 1x4 inch wood for support members running from the end of the foot of the base to the mast at a 45 degree angle. Everything should be screwed and glued with PL Premium.
We had problems with the coax. Aluminum was used for the shield braid and we were forced to use connectors rather than soldering the cable to the balun at the antenna. If you can find coax with copper or steel shielding braid it would be preferable because they can be soldered.
The balun will have to be clamped/screwed to the driven element itself because it is aluminum. Once secure use glue to weatherproof the connection. Use plastic ties and/or elecrical tape to secure the balun to the boom and the coax to the mast and base, leave some extra cable to permit the antenna to be moved about, do not coil up the extra cable.
The materials needed and costs are approximately: