There are only a few theoretical things we need to know about dipoles. The total length of the antenna should be exactly half wavelength (there are other types of dipole, but here they are of no interest to us at the moment), with some corrections (we are looking for the term electrical length)More about electrical length can be seen on Wikipedia). That length is to be reduced when building practical antenna. If pipe is being used, or multi-threaded conductor, or angle between radials is changed, all those things affect wave propagation and we need to further reduce the length of radials.
If the dipole is oriented vertically like on images above, it will radiate what is called vertically polarized EM waves. If it is horizontally oriented it will radiate horizontally polarized waves.
If the angle between radials is not 180° like on image, but we shift it to 90°, impedance will be 50Ω instead of 72Ω and we call that antenna Inverted V.
If we make folded dipole, impedance will be 300Ω which is a good match for Twin Lead cable and some devices. We will leave folded dipoles for later.
Half-wave dipole has gain 2.15dBi, Inverted V 1dBd (in practice it is 1.9dBi), Delta Loop 2.78dBi.
Any dipole consists of two radiating elements, insulated ad the feed point. Regular dipole, with the angle of 180°, has feed point impedance of 72Ω. It can be made with a wire or with rods or pipes.
Half-ware dipole is the one most commonly used. The entire length of the antenna is equal to the one half of the wavelength.
Typical equation for dipole: , , f - frequency in MHz, l - length or the entire antenna in m.
The exact equation is as follows: where . k is the adjustment factor, for thin wires (compared to the wavelength) it is equal to 0.98. If conductor is thick (about 1/100 of a wavelength) the adjustment factor is:
The construction is the same as with dipole, but the radials are at angle between 90° and 120°. Since Inverted V is being used for low frequencies, it is reasonable to assume that the wire would be used as radials. We must therefore have a mast to which two insulators are attached. On those insulators we attach the wires, like on the image. It is desirable that the feed line be as close as possible, i.e. that the wires’ ends are as close as possible.
As with any antenna, it will give the best results if placed high above the ground.
Delta loop is the wire triangle with equal angles of 60° and a feed point at one corner. The measurements are different than for the dipole, and are given in the table below.