This design is for a standard GSM 900MHz Yagi antenna. Its small size allows it to be mounted in a room or used as a hand held. The antennas gain (directional focus) is between 9 and 10 Db. It can pick up frequencies that range from 876MHz to 960MHz. This makes it a perfect antenna that covers the three main GSM-900 bands. The simple design makes it easy to build and use.
- Skill level:
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Things you need
- Ruler and pencil
- Drill and drill bits
- Small round file
- Straight metal file
- Soldering iron and solder
- One-inch paintbrush
- Epoxy resin adhesive
- Cable ties
- 50-ohm coax cable with connector to match your device
- 3mm diameter brass rod 33 1/2 inches long
- 1-by-2-by-15 ¾-inch strip of wood (2)
Cut the brass rod into seven pieces, one "Reflector" 171mm, two "Driven Elements" 74.5mm and four "Directors." The directors will be different lengths, which are 132mm, 127mm, 120mm and 114mm. All the lengths need to be precise.
Prepare the two pieces of wood that will form the top and bottom of the boom holding the rods in place. Place the two pieces of wood together on a table to make a 4-by-15 3/4-inch rectangle. Run the ruler down the 15 3/4-inch length. Make a mark at these points, 20, 74, 134, 199, 237 and 278mm and label them one through six. Place your T-Square at the marks and draw a line across the wood at each mark. This is where the rods will go across the wood to make a "T" shape.
Use a small rounded file to make a 1.5mm deep groove along each of the lines on both pieces of wood. Do not make the grove too deep or the rod will not stay in place. Test this by placing a rod in one of the grooves and put the second piece of wood over it lining up the grooves. The two pieces of wood should hold the rod in place.
Drill a hole in the middle of the fifth groove that will hold the two driven elements. Make the hole just large enough to fit the coax cable. Center the hole from the edges or the driven elements will stick out different lengths on each side.
Feed the coax cable through the hole to come out in the groove, giving enough length of wire for flexibility. Trim 15mm of insulation covering the end of the cable with your knife. Unwrap the braided metal and pull it to one side. Twist it together between your thumb and forefinger to keep the wires together. Scrape away the metal foil leaving the Teflon intact. Cut off approximately 12mm of Teflon covering the centre copper wire. Gently bend the copper wire to the opposite side of the twisted side to form a small "T" shape.
Prepare the Driven Element for soldering. File a 5mm section to a flat surface at one end of each of the two rods that are 74.5mm long. Trim the twisted and sold wire from the cable end to fit the flattened sections of rod. Solder the twisted braid to one of the driven elements. Solder the solid wire to the second driven element.
Follow manufacturer's instructions to mix up a small amount of epoxy resin adhesive. Paint a layer of resin onto the grooves of the piece of wood with the cable in it. Pull the cable to bring the driven elements into position in the groove without breaking the connections. Press the rods into the groove making sure each side does not contact the other.
Place the smallest of the directors into the first groove at 20mm and press down. Place the remaining three directors into their grooves in order of size from smallest to largest. Place the reflector (longest rod) in the last groove. The complete unit goes from smallest to largest.
Paint a thin layer of resin on the top surface with the rods in it. Adjust the rods to centre them on the antenna. Paint the grooves and flat surface of the second piece of wood and carefully place over the rods lining up the edges of both pieces of wood. Clamp the wood together until fully set.
Remove clamp and secure the coax cable to the wood using cable ties to ensure the cable does not move and weaken the contacts. Mount the antenna where you wish to use it and connect it to your device.
Tips and warnings
- Be precise on all measurements.
- Use quality parts to get the best signal.
- Figure it all out first before buying parts. Some Yagi's are very large and 1-inch wood in high winds will break.
- Double-check your calculations. One wrong spacing and your antenna may not function correctly.
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