How to build an FM yagi antenna

Written by j.t. barett
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How to build an FM yagi antenna
You can build a simple Yagi antenna. (Jupiterimages/Photos.com/Getty Images)

The Yagi radio antenna is relatively simple to build, provided you learn a few design concepts. The Yagi, also known as a beam antenna, has a directional geometry that works best when pointed at a radio source. It consists of a main antenna, called the driven element, and a set of auxiliary antennas, known as reflectors and directors, laid out in a flat plane. Each element is a thin metal pole whose length and spacing depends on the antenna's main frequency. While simpler Yagis follow a predictable formula, more complex ones are hand-tuned by trial and error.

Skill level:
Moderate

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Things you need

  • Calculator
  • Metric measuring tape
  • 1/4-inch aluminium rod
  • 1-1/2-inch x 1-1/2-inch slotted angle iron, painted
  • Hacksaw
  • Marker pen
  • Drill
  • #29 drill bit
  • Epoxy adhesive for metals
  • #8 crimp-on ring connectors
  • #8 machine screws 1-inch, at least 2 each
  • #8 washers and nuts, at least 2 each
  • Crimping tool or pliers
  • Flat 300-Ohm television cable
  • Wire strippers
  • U-bolt and nuts
  • Antenna mast

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Instructions

  1. 1

    Select an FM (frequency modulation) radio frequency for which you want the Yagi antenna to receive. FM radio stations fall on odd multiples of 100 kilohertz between 88.1 and 108.1 megahertz. Calculate the radio wavelength in meters by dividing 300 by the frequency in megahertz. For example, 300 / 88.1 = 3.4 meters.

  2. 2

    Compute the driven element, director and reflector lengths by multiplying the wavelength by .473, .440 and .495 respectively. Taking the above example gives you 1.6, 1.5 and 1.68 meters.

  3. 3

    Compute the spacing between elements by multiplying the wavelength by .125. Using the example of a 3.4-meter wavelength gives you a spacing of 42.5cm. Multiply the spacing by 2 and add an extra 10cm to get a total length.

  4. 4

    Cut the slotted angle iron to the overall length you calculated in Step 3. Find the middle of the piece and mark the metal with the marker pen. Use the measuring tape to find the two points from the middle having the spacing length you calculated in Step 3. Make marks at these points.

  5. 5

    Saw three pieces of aluminium bar to the lengths you calculated in Step 2. Set the longest and shortest aside. The longest is the reflector and the shortest is the director. Find the centre of the middle piece and cut it in half. Near one end of each half, drill a hole to accommodate a #8 screw.

  6. 6

    Slide the director rod through the holes in the angle iron near the mark you made at one end. You will pass the rod at an angle through two holes so that the iron will support the rod. Make sure the rod is centred. Slide the reflector rod near the mark at the opposite end of the angle iron, centring it. Glue the rods in place with epoxy.

  7. 7

    Slide one of the two remaining half-rods near mark you made in the middle of the angle iron. The end of the rod with the drilled hole should protrude about an inch from the iron. Slide the other half-rod through an adjacent pair of holes near the first rod. The drilled end should protrude about an inch.

  8. 8

    Slip a machine screw through a ring connector. Slip the screw through the drilled hole in one of the rods. Put a washer and nut on the screw and tighten. In the same way, attach a machine screw and ring connector to the other rod. Once you have the rods connected, glue them in place with epoxy.

  9. 9

    Strip about an inch of insulation from one end of the 300-ohm antenna wire and slit the remaining insulation about ten inches. Place one end of the bare wire in an ring crimp connector and crimp the wire. Crimp the second bare wire in the other connector.

  10. 10

    Select a U-bolt that will slide sideways through the holes in the angle iron as the rods did. Place the U-bolt near the middle of the angle iron's length to keep the antenna balanced. Mount the antenna to the antenna mast with the U-bolt. Connect the opposite end of the 300-ohm cable to a radio receiver.

Tips and warnings

  • Since the antenna is directional, you may need to turn it to pick up the best signal.

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