How Does a Yagi Antenna Work?
It seems like a simple enough question, right?
Three weeks ago at the McARCS meeting in Mendocino, a few of us built cheap Yagi-Uda antennas for 2M. I wrote about the experience in Snipping the Yagi. There’s nothing like building an antenna to make you curious as to how and why they actually work. Yagi-Uda antennas provide excellent gain and directiveness, and the more directors you add, the more they provide!
I remember once reading that the Yagi design was invented back in 1929, but that their theory of operation was not understood until 1975. That being said, good luck finding a decent description of how they work! I’ve searched online and also looked through a number of radio textbooks, but so far I’ve come up empty for an adequate description of the Yagi’s mechanism. There are plenty of descriptions covering its design and properties, but very little about the theory behind it.
While over on the coast, I chatted with Derek, KE6EBZ, who uses a nice 2M Yagi beam that he built from an old TV antenna. We agreed that the function of the reflector seemed like that of a mirror — not difficult to imagine. But, the directors were more of a puzzle. Derek suggested that the shorter lengths of the director elements helped focus the wave in the forward direction.
As Cindy and I drove home later that afternoon… I thought more about Derek’s suggestion. I’d seen that kind of focus pattern before — 30 years ago while building a laser diffraction lab at UC Davis. Essentially, if you paint a big dark circle on a window, light diffracts around it much like a lens. It’s called a zone plate, and… if you continue the pattern of dark and clear in concentric circles around it, it will become even more focused. It becomes a Fresnel lens.
So, is that how Yagi directors works? They form a sort of Fresnel lens? I don’t know. Seems like something to study a bit deeper.
Such a lens might account for the directivity of a Yagi, but where does its high gain come from? Its gain can go beyond 18 dBd. Producing that much gain from parasitic (non-driven) elements seems difficult to comprehend, especially if you think of the directors as focusing only the radiative EM field (the far field) and only in a single dimension.
Recently it dawned on me that many of a Yagi’s directors fall within the near field of the driven element. That means they have mutually inductive and capacitive coupling to the driven element, more than just an effect due to re-radiation of the far field. So, in terms of magnetic induction, the first director works a lot like a transformer. That means that the director can actually pull more energy from the driven element — energy that cannot be tapped from the radiative field alone. Something similar could also be said about its capacitive coupling of the E field to pull energy. So, the directors can pull more power from the feed point… which really helps explain where the high gain comes from. Very cool.
Of course, this also means that the phasing of the whole mechanism must be very precise. The induced and coupled energies must perfectly match the phase of the radiated wave as it passes by. Wow, isn’t that elegant. You could almost think of a Yagi as a medium of propagation… with the wave travelling through it. (Of course, that means the wave velocity is changed while it does so… interesting to consider… hmmm, could be something to this.)
Well, I suppose everyone already knows this, and I’m just the last guy on the block to figure it out. The elmers reading this article are yawning and commenting, “yes of course that’s how it works, dummy.” But, it does get me thinking… what happens in the subsequent directors as the near and far fields pass by… and how far backwards can the induction and coupling have an effect?
Obviously, the mechanism of a Yagi is fairly complex. It sure would be nice to learn more about it. You’ve really got to wonder why such a elegant antenna has so little published about how it actually works.
If you’ve come across in-depth descriptions of Yagi theory, please post a comment here for all to read! Many thanks.