How to Build a Dipole Antenna

A dipole antenna was developed by a German physicist called Heinrich Rudolph Hertz in 1886. It is one of the most common and simplest types of RF antenna, fed with a balanced, parallel-wire RF transmission line.

What is a Dipole Antenna?

As the name says, the antenna consists of two poles or terminals through which the RF current flows. A RF transformer known as 'balun' is inserted in the system where the feed line connects the antenna, for optimizing the RF current distribution on the antenna element and in the feed line. The current amplitude consistently decreases from being maximum at the center to zero at the ends. Whereas the RF voltage is maximum at the ends and minimum at the center. This current and corresponding voltages cause the electromagnetic or radio signals to radiate. The antenna can be oriented horizontally, vertically or slantingly. The polarization of the electromagnetic field (EM) radiated by the transmitting antenna correlates to the orientation of the element. While receiving the RF signals, the antenna is extremely sensitive to EM fields whose polarization is analogous to the orientation of the element. Different types of simple dipole antenna include folded dipole antennas, ideal half wavelength antennas and hertzian dipole antennas.

How to Build a Dipole Antenna?

Before you start, first select its working frequency. A dipole antenna works well on frequencies that are an odd multiple of ½ wavelength frequency the antenna is cut for. For example, if the dipole is cut for 7.0 MHz it will function well on 21 MHz.

Things Required:

RG-8X Coax, 30-foot aluminum push-up pole, antenna tuner, center insulator/end insulators, 100-feet lightweight nylon rope, electric drill, screwdrivers, ground wires, pulley hooks and pulley, 135 feet of copper wire, cable ties, electrical sealants, wire nippers and wire-insulation strippers.

Steps:

• Design the layout of the dipole and extend each dipole leg about 65 feet in a straight line.
• Use a 30-foot TV mast (sturdy upright pole) fixed to house eaves to support the center insulator.
• Connect a small pulley to the mast top. Pass a light rope through the pulley and erect the mast.
• Scrap the copper wire, either bare or insulated and cut the wire in half.
• Connect one end of each end insulator to the center insulator, covering the connection with the sealant.
• Use cable ties to fix the coax to the house and between the mast and the entrance point.
• Connect the center insulator to the rope end and pull it towards the top of the mast.
• Feed the coaxial cable from the antenna support mast to the point closest to the radio room.
• With the help of an electric drill, drill a hole in the floor or wall and feed the cable through it.
• Connect the cable to the antenna tuner and pass the ground wire from the tuner to the ground rod.
• Finally, using a coaxial cable attach the tuner to the radio.

While building it, the dipole antenna should be more than ½ wavelength above the ground or any conducting medium like a sheet metal roofing. Also keep in mind that the element should be few wavelengths away from the electrically conducting obstructions like towers, utility wires, guy wires and other antennas.