The dipole is one of the most important and popular antennas, it forms the basis of many other different types of aerial which are directional and have gain in certain directions.
The dipole in its basic form consists of two identical lengths of wire with the feeder connected in the middle as in the diagram above. It can be any number of electrical wavelengths long. The most popular is the half wave dipole, normally thought of as a single band aerial. Although it has little gain, it can be easy and cheap to construct, and they often prove to be the ideal solution with amateur radio operators who are only interested in one particular band .
The wire dipole is normally mounted horizontal, vertical or sloping between two supports, but it can be erected in many different shapes such as the inverted-V or a Z shape. On the lower HF bands the the lengths become rather long so it is sometimes necessary to bend the antenna to fit the average garden.
The formula to calculate a half-wave dipole (in feet) on any HF frequency is 468/frequency in MHz. This is total wire length tip to tip, or 234/frequency in MHz for each element length. For example = If you want the antenna for 40 meters, choose the centre part of the band or centre part of the frequencies you want to work, 468/7.05 MHZ = 66.3 feet. It is better to make the wire slightly longer and then trim the wire to resonance. Make sure you trim the same length of each end.
In free space the feed point impedance is 78W so it is a good match to 75W coax. However when erected in the garden or over the house the impedance changes and it is acceptable to use 50W coax cable.
Although it is not essential, it is best to use a 1:1balun. This is connected between the two elements and the feeder cable. This ensures the correct operation of the antenna. It does not provide any impedance match, but it will balance the load by causing equal but opposite phase currents to flow in the conductors reducing radiation on the transmission line.