AVTE 123
Batang, Nathanael
Antenna Proposal
Marcelo, Camille
Folded Dipole
Mendoza, Amir Garvida, Ian Doctor, Vien Carlo
Channel Target: Channel 7
Reflector = .8 x 5% = .04 + .8 m = .84
Frequency: 175.25 MHZ to 180.75 MHZ
Director = .8 x 5% = .04- .8 m = .76
Formula: 0.96 x 300/178 = 1.6 meters 1.6/2 = .8 meters or 80cm.
A dipole antenna is a radio antenna that can be made of a simple wire, with a centerfed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other (in line with each other), with a small space between them. The radio frequency voltage is applied to the antenna at the center, between the two conductors. These antennas are the simplest practical antennas from a theoretical point of view. They are used alone as antennas, notably in traditional "rabbit ears" television antennas, and as the driven element in many other types of antennas, such as the Yagi. Dipole antennas were invented by German physicist Heinrich Hertz around 1886 in his pioneering experiments with radio waves.
Folded dipole basics
In its basic form the folded dipole consists of a basic dipole with an added conductor connecting the two ends together to make a complete loop of wire or other conductor. As the ends appear to be folded back, the antenna is called a folded dipole. The basic format for the dipole is shown below. As can be seen from this it is a balanced antenna, like the standard dipole, although it can be fed with unbalanced feeder provided that a balan of some form is used to transform from an unbalanced to balanced feed structure.
Simple half-wave folded dipole antenna
One of the main reasons for using the folded dipole is the increase in feed impedance that it provides. If the conductors in the main dipole and the second or "fold" conductor are the same diameter, then it is found that there is a fourfold increase in the feed impedance. In free space, this gives an increase in feed impedance from 73Ω to around 300Ω ohms. Additionally the RF antenna has a wider bandwidth.
Folded dipole impedance rationale
In a standard dipole the currents flowing along the conductors are in phase and as a result there is no cancellation of the fields and radiation occurs. When the second conductor is added this can be considered as an extension to the standard dipole with the ends folded back to meet each other. As a result the currents in the new section flow in the same direction as those in the original dipole. The currents along both the half-waves are therefore in phase and the antenna will radiate with the same radiation patterns etc as a simple half -wave dipole. The impedance increase can be deduced from the fact that the power supplied to a folded dipole is evenly shared between the two sections which make up the antenna. This means that when compared to a standard dipole the current in each conductor is reduced to a half. As the same power is applied, the impedance has to be raised by a factor of four to retain balance in the equation Watts = I^2 x R.
Folded dipole advantages There are a number of advantages or reasons for using a f olded dipole:
Increase in impedance: When higher impedance feeders need to be used, or when the
impedance of the dipole is reduced by factors such as parasitic elements, a folded dipole provides a significant increase in impedance level that enables the antenna to be matched more easily to the feeder available.
Wide bandwidth: The folded dipole has a flatter frequency response - this enables it to
be used over a wider bandwidth. Folded dipole applications
Folded dipoles are sometimes used on their own, but they must be fed with a high impedance feeder, typically 300 ohms. However they find more uses when a dipole is incorporated in another RF antenna design with other elements nearby. This has the effect of reducing the dipole impedance. To ensure that it can be fed conveniently, a folded dipole may be used to raise the impedance again to a suitable value.
The folded dipole is the perfect antenna for stations that require a truly professional antenna for their broadcasting. No tuning or adjustment is needed for any frequency on the band which makes this antenna the only one to use i f you plan to move your transmitter’s frequency often. Completely hermetically sealed with a full all weather warranty - you can be sure this antenna won't let you down, making it our choice for commercial sites where access is difficult.
