Understanding the Folded Dipole Antenna: Debunking Myths and Clarifying Facts

The folded dipole antenna is a type of antenna that has been the subject of much discussion, often leading to confusion due to a lack of accurate information. Let's delve into the true nature of this antenna and dispel some common misconceptions.

What is a Folded Dipole Antenna?

A folded dipole antenna is a simple modification of a standard half-wave dipole antenna, involving a second parallel conductor that is fed at the ends by the same feed as the original dipole, but with a split center as a feed point. Despite its name, it is not a full-wave loop, nor does it act like one. The folded dipole remains 1/2 wave long, and the two conductors are so close together that there is insignificant spatial distance and phase delay between the radiated fields.

Key Characteristics of a Folded Dipole Antenna

Several key properties of a folded dipole antenna are noteworthy, distinguishing it from other types of antennas:

Increased Feed Impedance

One of the primary changes in a folded dipole is the increase in feed impedance. With two identical conductors, half the current necessary to radiate a given electromagnetic field intensity flows in each conductor. Thus, the current halves and the voltage doubles, resulting in a feed impedance that is four times that of a standard dipole. This change can be tailored by adjusting the size ratios or the number of parallel conductors, allowing for almost any desired value higher than that of a standard dipole, which is typically 30 to 80 ohms depending on height, length to diameter ratio, and other factors.

Physical Size and End Effects

Physically, a folded dipole can be thicker and have blunter ends, which might reduce electric field gradients at the antenna ends. This could make the antenna less susceptible to corona noises. However, this is a result of the physical size change, not the folded nature of the conductor.

No Advantages in Radiating or Noise Rejection

Contrary to some claims, a folded dipole does not match space better, radiate better, or reject propagated noise better than a standard dipole. It is simply a dipole with a different feed impedance. The only advantage it offers is a higher feed impedance, which can be tailored to meet specific requirements. If it had magical properties, they would be used almost everywhere, but they are not.

Conclusion

Understanding the folded dipole antenna requires dispelling common myths and focusing on its fundamental characteristics. Its increased feed impedance is its primary feature, and this can be advantageous in certain applications. However, the primary roles of radiating and rejecting noise are not significantly improved by the folded design. In summary, the folded dipole is a simple, effective modification to the standard dipole, offering tailored impedance for specific needs.