What is the tightly coupled dipole array concept and how does it achieve very wide bandwidth?

The tightly coupled dipole array (TCDA) achieves very wide bandwidth (3:1 to 5:1 or more) by exploiting strong mutual coupling between closely spaced dipole elements, rather than trying to minimize it as in conventional arrays. The concept is based on Wheeler's ideal current sheet antenna: an infinitely thin, uniform current sheet over a ground plane radiates with constant impedance (377/2 = 188.5 ohms for free-space backed, or approximately 150-200 ohms with a ground plane) over infinite bandwidth. A TCDA approximates this current sheet by: using thin printed dipoles on a substrate with spacing much less than lambda/2 at the midband frequency (typically 0.3-0.4 lambda at mid-band), adding strong capacitive coupling between adjacent dipole tips (the capacitive coupling bridges the gap between elements, creating a nearly continuous current flow across the array surface at low frequencies where each element would otherwise be too short to radiate), placing the array approximately lambda/4 above a ground plane at mid-band (which provides a constructive reflection that enhances radiation), and feeding each element through a wideband balun (the balun must be as wideband as the array to avoid limiting the overall bandwidth). The TCDA achieves its wide bandwidth because: at low frequencies, the inter-element capacitive coupling creates the continuous current sheet behavior (the individual elements are too short to resonate, but the coupled array radiates efficiently), and at high frequencies, each element approaches lambda/2 and radiates individually. The transition between these regimes is smooth, providing continuous wideband operation.