Folded DipoleEdit
Folded dipole is a practical and well-established variant of the classic half-wave dipole antenna. It consists of two parallel conductors connected at their ends by a conducting strap, forming a folded loop. This simple modification preserves the familiar broadside radiation pattern of a dipole while dramatically increasing the feedpoint impedance, making it easier to match to common transmission lines. In typical conditions, a folded dipole presents around 280–320 ohms at resonance, roughly four times the impedance of a single straight half-wave dipole, and remains broadly resonant over a respectable bandpass when constructed with reasonable conductor sizes. The geometry keeps the same basic current distribution as a standard dipole, so the main radiation lobe sits broadside to the axis of the wires, with a gain near that of a conventional half-wave dipole in free space.
Because of its higher input impedance, the folded dipole is a natural partner for impedance-matching schemes that use ladder line or wide-band baluns. For example, a 4:1 balun or an impedance transformer can translate the roughly 300 ohms of the folded loop down to 50 ohms for coax feeding, while a 1:1 balun can help suppress common-mode currents on the feedline. These matching approaches are commonly discussed in the context of balun design and impedance matching in antenna systems. The folded form also tends to be less sensitive to slight variations in wire diameter and spacing than some other high-impedance configurations, which can be an advantage in field deployments.
Folded dipoles are widely used across HF and VHF bands, as well as in some UHF deployments, because they deliver a straightforward construction with predictable performance. They are often built as simple two-wire elements, but variants exist that use a single wire folded back on itself, or that employ a rectangular frame to form a compact loop. In amateur radio practice, folded configurations are common for weekend projects and field stations where reliable matching to 50-ohm lines is desired without resorting to complex adjustable networks. They are also seen in antenna arrays and in some broadcast and receiving installations where a robust, balanced feed is beneficial.
History and theory
The folded dipole emerged from early investigations into how altering the feed geometry of a half-wave radiator affects impedance while preserving radiation characteristics. The core idea is that folding the conductor back on itself creates a parallel path for current, which increases the effective feedpoint impedance without dramatically changing the current distribution along the radiating elements. The resulting impedance is approximately four times that of a single leg of the equivalent straight dipole, with the pattern remaining broadly similar to the original. This relationship makes folded dipoles a natural choice when a higher feed impedance is desirable to suit common transmission-line impedances.
Key theoretical concepts relevant to folded dipoles include the behavior of standing waves on the element, the influence of conductor thickness and spacing on loss and bandwidth, and the role of balance in the feed. The fold itself is not a source of radiation; the radiation originates from the alternating currents in the two legs driven at their center, and the folded connection simply alters the current distribution and impedance seen at the feed. The electrodynamics are typically discussed in the broader context of antenna theory and the study of wire antenna performance.
Design and construction
A folded dipole is constructed from two parallel conductors of approximately equal length, joined at the ends by a strap or shorting bar. The overall length is chosen to be close to a half-wavelength (lambda/2) at the target frequency, similar to a standard dipole, ensuring resonance. The separation between the two legs is a design parameter that moderately affects impedance and bandwidth; very tight spacing yields impedance closer to the ideal fourfold increase, while larger spacing can shift some reactive components but generally keeps the same resonance. Practical builds often use light aluminum or copper wire, with careful attention to mechanical stability and corrosion resistance.
Feeding a folded dipole typically requires a balun or a matching transformer to connect to a coaxial feedline. A common choice is a 4:1 balun to convert the approximately 300-ohm folded impedance down to the 50-ohm impedance of coax, enabling efficient power transfer with minimal feedline radiation. In some installations, a 1:1 balun is used to preserve a balanced feed and minimize common-mode currents, especially when the feedline may pick up interference or contribute to pattern distortion. Designers and builders also consider the mechanical footprint, wind loading, and the potential for proximity effects when deploying folded dipoles in arrays or in restricted spaces.
Performance and comparisons
Compared with a straight half-wave dipole of the same total length, the folded dipole offers a higher input impedance without a fundamental change in the radiation pattern. The bandwidth is governed more by conductor size and environment than by the fold itself, so practical folds behave similarly to their straight counterparts in terms of usable frequency range for a given wire gauge. The gain remains in the same general range as a dipole (about 2.1 dBi in free space for an isolated element), and the front-to-back ratio depends on element orientation and surroundings.
Many operators value folded dipoles for their impedance compatibility, especially in systems where ladder-line or wide-band feeding is standard. In field experiments and in temporary installations, they can deliver reliable performance with relatively simple matching networks. The design also supports a degree of tolerance in construction, since small variations in spacing or wire thickness have a bounded impact on overall performance when the element is kept close to a resonant lambda/2 length.
Controversies in the practical sphere tend to focus on matching strategy and feedline management rather than fundamental physics. Some practitioners emphasize keeping everything truly balanced and using specialized baluns to mitigate common-mode currents; others argue that well-chosen coax-fed configurations with robust shielding and proper installation can perform just as well with simpler gear. The debates often center on cost, ease of deployment, and maintenance rather than on core efficiency, with the folded dipole remaining a dependable option in many standard antenna portfolios.
Applications
Amateur radio: folded dipoles are popular on multiple bands where a balance between impedance and ease of matching matters, including several HF and VHF segments. Their predictable impedance makes them suitable for field-day style deployments and portable operations where quick setup without complex tuning is preferred. See amateur radio for related design considerations and operating practices.
Ground-mounted and rooftop installations: the folded configuration can be advantageous when space constraints favor a compact, robust element with a high feed impedance suitable for ladder-line or matching networks.
Antenna arrays and loops: folded dipoles can serve as elements in simple or modestly complex arrays where uniform impedance and balanced feeding support straightforward phasing and combining.
Receivers and monitoring: in some surveillance or monitoring stations, folded dipoles offer stable performance with modest environmental sensitivity, contributing to reliable reception on target bands.