Invented by Heinrich Hertz in around 1886, the half wave dipole is still one of the most simple and practical of antennas and still very much in use today. It consists of a half wavelength long centre fed conductor. Since these early days, improvements have been made to the initial design by adding a second, and often a third conductor, joined at the two ends to form a loop. This antenna, known as the Folded Dipole, can provide improved performance over the single wire antenna when well designed, constructed and installed…
A good knowledge of the characteristics of the folded dipole will ensure that you are matching the right antenna to the job in hand in the best location. The length of the antenna in respect to frequency and the height it is erected above ground as well as how well it is matched, the type of ground and the nearby environment will all have an impact on performance
Both standard and folded half wave dipole antennas are fed at the centre where the current is a maximum and the voltage at minimum. This provides a low impedance feed point. 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. In the folded dipole, the second conductor acts as an extension to the first conductor, with the result that currents flow in phase and in the same direction in both sections of the wire. As a result, both antennas have the same radiation pattern.
The act of folding the antenna transforms the input impedance to a higher value providing an easier match in some cases. Power is shared evenly between the two sections resulting in an increase in impedance, and current in each is reduced to half. With the same power applied to the antenna, the impedance is increased by a factor of 4.
A single wire dipole, which has a characteristic impedance of about 70 ohms, can be fed directly with 75 ohm line. However, assuming the conductors have the same diameter, a two wire folded dipole can be fed with 300 ohm line (factor of 4) and a 3 wire folded dipole can be fed with 450 ohms or 600 ohm line.
When a balun of the correct ratio is used to bring the impedance down to 50 or 75 ohms, coaxial cable can be used. Essentially there are two types of balun, the Choke balun and the Transformer balun.
The Choke balun aims to choke the current off at the feed point by creating a large series impedance on the outside of the cable thus preventing currents from flowing. It can be as simple as a few turns of cable in a loop, cable wound around a toroid or ferrite or fed through ferrite tubes or beads to form a sleeve.
If the installation is asymmetrical, however, the choke balun may not be able to prevent surface currents from electromagnetic coupling further down the cable, especially if the feedline length is resonant. Therefore if surface current flow persists, more chokes are required at intervals down the feedline. Adding a choke will not normally affect the VSWR, as it only affects the outside of the cable and cannot influence impedance inside the cable. However unbalance will, as the feeder becomes part of the antenna system and load impedance is altered. The choke merely suppresses surface currents and makes no attempt to create balance.
The Transformer balun, on the other hand, acts to create balance by forcing equal and opposite currents at either side of the feedpoint, through closely coupled windings within the transformer. With the transformer balun it is also possible to alter impedance by a factor, either up or down. As with the choke balun quality and efficiency are all important. Unwanted resonances due to distributed capacitance between the turns of the transformer can induce impedance mismatch and unbalance, as can leakage inductance from loose coupling between windings.
For total balance to be achieved on either side of the feedpoint, installation and environmental factors must also be taken into account. If any unbalance remains, the transformer balun will not suppress any resulting surface currents. However, these can be negated by the use of choke baluns further down the line.
In the folded dipole, the reactance varies less rapidly as you move away from the resonant frequency than it does with a single wire antenna, resulting in a somewhat “flatter” impedance v. frequency and a wider bandwidth. This can be significant at lower frequencies.
However, in order to operate efficiently over a range of frequencies a matching device is necessary to balance the antenna with the feed line. Transformers, resistors, inductors and capacitors are used to provide impedance matching in baluns, transformers, matching networks and automatic antenna tuning units (ATUs) or couplers. Without efficient impedance matching, high VSWR will result giving poor performance. Moreover, an unbalanced feed line can radiate and, as a result, RF can be induced into other nearby electronic equipment causing interference More significantly for you, this will also distort the radiation characteristics.
Matching Networks made up of a network of series and parallel impedances that combine to perform the transformation can be used. The parallel type represents a load as a resistor (high value) and capacitor in parallel. The tune capacitor adds to the parallel equivalent capacitance, as in PI networks. The series type represents a load as a resistor (low value) and capacitor in series. The tune capacitor appears in series with the series equivalent capacitance, as in L networks, or the series capacitance is resonated out by the inductor leaving just enough inductance to tune, as in T networks.
An ATU can also be used for matching over a wide range of frequencies and can also assist in reduction of RF interference due to its inherent selectivity, as interference on one frequency will also affect other frequencies with broadband wire types.
Antenna length is an important factor too, especially at HF and below where lengths vary greatly from frequency to frequency. Choosing an antenna that is less than a half wave length at your lowest frequency will mean that the VSWR, and therefore performance, will not be as good as it could be. This is because the radiation resistance will be low and reactance high. Of course this may be a necessary evil where site restrictions cannot be overcome, especially at very long wavelengths. Consulting an SWR sweep will indicate antenna performance at your specific frequencies.
Radiation characteristic of the folded dipole will depend upon the height of the antenna above ground and how it is erected. Normally for practical reasons it is erected horizontally or in inverted V configuration. In the azimuth plain the horizontal antenna erected at recommended height starts to become less omnidirectional at around 8 MHz.
website originally available at http://www.moonraker.com.au/techni/foldeddipole.htm