3 Element Yagi

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Updated March 14, 2011

How to Build a 3 Element Yagi Antenna

by Guy Almberg

These are the instructions for building a three element yagi tuned for 92.1 MHz (suitable for the Edmonton area) out of commonly available materials. For other locations different lengths may be required depending on the observing frequency. See the dimension chart for other frequencies or contact us at Sky Scan. 

The antenna is constructed of spruce 2X4s that were split on a table saw to the dimensions required. The boom of the antenna is a 2X2, 143cm long (includes a little extra length), the elements are constructed of a 2X2 quartered into equal parts, their dimensions are:

reflector (back of antenna) 162.cm
driven element (centre) 160.3cm
director (front of antenna) 146.cm

The spacing between the reflector and driven element is 89.2cm; driven element and director 48.7cm. The spacing from the driven element is based on the surface that the ladder line is attached to, for the reflector and director they are centred on those measurements. The reflector and

director element 1X1s (after cutting are actually about 3/4X3/4 inches) need an additional groove cut down the centre of one surface about 1/4 inch deep to hold a piece of copper wire.

 

The copper wire is household three wire 14 gauge electrical wire. Strip off the outer plastic case that holds the three internal wires together. Do not strip the insulation off the internal wires. One will be white, one black and one bare. Stuff the insulated wire into the groove of the reflector and director elements with a screw driver and then fill the remainder of the groove with glue to secure the wire in place. I used PL100, but silicone will work just as well, both are waterproof and stick to just about anything.
Trim off the ends of the wire to match the length of the elements and then glue and screw them perpendicular to the boom at the appropriate places. I predrilled the holes for the screws in the elements to prevent them from splitting the element.

The driven element does not need a groove cut into it because we will attach the ladder line to one surface with glue and staples. Ladder line is available from ham radio shops at a very reasonable cost, alternately 300 ohm twin TV lead can be used. Cut a piece 2cm - 4cm longer than required and then strip enough off the ends (be wary of the length of your wire) so that the copper wire exposed can be soldered together. Tape and glue the ends to prevent water from entering the cable. At the centre of one side of the twin lead cut open the wire and expose 1 - 2cm of wire, cut the copper wire in half, this is what we will solder the balun to so that we can attach RG59 coax cable to the antenna. Follow this link for cable instructions.

Attach the driven element at the appropriate place on the boom with glue and screws. Make sure that all the elements are reasonably square with the boom and in the same horizontal plane, it does not have to be perfect.

 

After the glue has set, balance the antenna on your finger and drill a 3/8 hole through the boom at the balance point for a 3/8 bolt to attach it to a mast of some kind. We used another 8 foot 2X2 for a mast and attached four 3 foot cross pieces to the bottom for a platform. 

To keep it steady we used sand bags to hold it down. The sand bags should either be wrapped in black garbage bags or the bag material should be the kind that is resistant to breakdown from UV radiation from the Sun. Please note: The wind is a very powerful force. Make sure you put at least 4, fifty pound sandbags on each of the four legs holding your mount. 

It is a good idea to varnish the wood to help keep the wood dry. We also used a small strip of wood to stabilize and hold the antenna at the correct angle between the mast and the back of the antenna.

 

Copyright 1999-2012 by Sky Scan, Edmonton, Alberta, Canada. 

We gratefully acknowledge the financial support of the 

Edmonton Centre of the Royal Astronomical Society of Canada, Department of Physics (University of Alberta)

and the

Natural Sciences and Engineering Research Council of Canada

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