The Rooftop Classic
The Yagi-Uda antenna is perhaps the most recognizable antenna in the world — the array of parallel elements on a boom that once crowned nearly every rooftop for television reception. Invented by Shintaro Uda and popularized by Hidetsugu Yagi in the late 1920s, this elegant design achieves high directional gain using mostly parasitic (unfed) elements that shape the radiation pattern through electromagnetic coupling.
How Parasitic Elements Work
Only the driven element connects to the transmission line. The reflector, slightly longer than a half wavelength, has an inductive impedance that causes it to re-radiate with a phase that constructively adds in the forward direction and destructively cancels backward. Directors, slightly shorter, have capacitive impedance and re-radiate to guide energy forward. The cumulative effect of multiple directors creates a progressively narrower beam with higher gain.
Gain vs. Size Tradeoff
Each additional director extends the boom and adds gain, but with diminishing returns. The first director adds about 3 dB (doubling the gain), while the tenth might add only 0.3 dB. The spacing between elements (typically 0.15-0.35λ) affects both gain and bandwidth. Closer spacing increases gain per unit length but narrows bandwidth and makes the design more sensitive to manufacturing tolerances. This simulation lets you find the sweet spot for your application.
Modern Applications
While rooftop TV antennas are fading, Yagi-Uda designs remain vital for amateur radio, point-to-point communication links, radio astronomy, wildlife tracking, and EMC testing. Printed Yagi designs on PCB substrates serve millimeter-wave 5G applications. The principles of parasitic coupling discovered by Uda and Yagi underpin all modern phased array and MIMO antenna systems.