The length of a dipole is determined by the frequency of the electromagnetic wave it is intended to transmit or receive.
The formula to calculate the length of a dipole antenna in free space is:
length (in meters) = (0.5 x wavelength) / (number of half-waves)
where wavelength is the distance of one complete cycle of the wave, and number of half-waves is the number of half-cycles in the length of the antenna.
The formula can be simplified to:
length (in meters) = 143 / frequency (in MHz)
where frequency is in megahertz (MHz).
For example, if the frequency is 100 MHz, then the length of the dipole antenna would be approximately 1.43 meters using the simplified formula.
This length assumes that the dipole is in free space. In practice, the length may need to be adjusted to account for the effect of nearby objects or the environment. Also the velocity factor of the antenna wire contributes to the overal length of the dipole.
Velocity Factor of antenna wire
The velocity factor of a wire is the ratio of the velocity of electromagnetic waves in the wire's insulation to the speed of light in a vacuum. This factor takes into account the delay in the propagation of signals along the wire due to the wire's insulation.
The velocity factor is usually expressed as a decimal fraction or a percentage. For example, a wire with a velocity factor of 0.8 will transmit signals at 80% of the speed of light in a vacuum.
The velocity factor of a wire depends on the type of insulation used, as well as the wire's diameter and the frequency of the signal being transmitted. Different types of insulation have different dielectric constants, which affect the velocity of electromagnetic waves in the wire.
It's important to take the velocity factor into account when designing or analyzing transmission lines, as it affects the wavelength and impedance of the signals transmitted along the wire.
Insulated wire typically has a velocity factor of .96 to .90, therefore the overal length of a dipole will be shortened from a standard calculation if you take into account the wire velocity factor.
Feed impeedance of a Dipole
In free space, a half-wavelength dipole has an impedance of approximately 73 ohms at the feed point. However, the impedance will be different if the dipole is not resonant at the operating frequency or if it is not in free space. If in free space with and at resonance the impeedance is 73 ohms which combined with 50 ohm feed line will give an SWR of around 1.5:1, not 1:1.
How efficient are Dipoles
At frequencies above about 30 MHz, dipoles can be very efficient, with efficiency levels of around 90% or higher being common. However, at lower frequencies, the efficiency of dipoles can decrease, particularly if they are not properly matched to the transmission line and the impedance of the transmitter and receiver. In these cases, other types of antennas, such as vertical antennas or loops, may be more efficient.
A Dipole is current feed
The reason for using a dipole current feed is that it provides a balanced transmission line, which is important for achieving efficient radiation from the antenna. By applying the current at the midpoint of the two conductive elements, the voltage at each end of the dipole is equal but opposite in phase.
This results in a balanced current flow along the two conductive elements, which reduces the likelihood of unwanted radiation from the transmission line and improves the antenna's performance.
In addition, a dipole current feed is easy to implement, as it can be achieved by simply connecting the feed line to the midpoint of the two conductive elements. This makes it a popular choice for many applications, including radio and television broadcasting, wireless communications, and amateur radio.