Home > Antennas, Sharing Information, Theory > How Does a Yagi Antenna Work?

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.

  1. sktravis
    2011/11/24 at 6:29 PM

  2. 2011/12/09 at 7:33 AM

    Thanks for posting that video. Using a simple field detection method, Diana Eng shows how the fields of a yagi become more directed as elements are added. Quite dramatic really.

    Of course, the video shows the results of the element interactions. It does not explain how they occur. I can give a good example. When Diana inserts the first element, the reflector, you can easily see that the original dipole field is lost, or more correctly, it becomes directed forward. The reflector element couples with the driven element to produce this result, but it’s not at all clear how. We have to ask if that original field is still there but is being canceled by an opposing field, or if somehow the field is magically warped into the new shape that makes it directive.

    This question becomes even more problematic the closer we look. EM radiation begins immediately upon the first change in the current that drives the antenna. It does not “wait” until the full current cycle is complete and then launch off radiation. So that means for the “how” asked above, the correct answer must be cancellation, not magic field warping. (And, of course, we also know that fields themselves cannot directly interact.)

    That answer helps illustrate the difference between the micro and the macro. Nearly all radio wave theory is macro; almost none is micro. However, what’s really interesting is that in the last 40 years the amazing advances in antenna modeling have come from more of a micro method (method of moments). However, it’s a computational theory, and I’ve never seen it expanded into a true physical theory. In other words, it gives great results, but we’re not really sure why. There is a huge missing piece in EM theory.

  3. Ben Hardless
    2013/09/09 at 6:15 AM

    The reflector is not providing cancellation. The reflector is not connected to the transmission line, however the driven element is. The reflector must be the appropriate length to resonate at the frequency being used.

    The driven element lobe in the unwanted direction induces EMF in the reflector that is 180 degrees out of phase with that of the driven element – this is because we know that induced EMF opposes the inducing effect.

    The reflector then re-radiates, now inducing EMF in the driven element and another 180 degree phase shift occurs. Since there have been two 180 degree phase shifts, we are back in phase again and this action reinforces the radiated field of the driven element.

    Long story short – this processes effectively means the reflector takes energy that comes it’s way and then puts the energy back into the driven element. This enables us to maximise gain via focus in the opposite direction.

    Obviously the opposite is true for the directors which experience induction also, but resonate in phase with the driven element. They then re-radiate also. This has the effect of narrowing the beam width which explains the directivity of the yagi, resulting in more EM energy in a smaller area (think of a polar coordinate graph) which obviously means greater power density and therefore gain.

    Remembering that the reflector length and spacing from the driven element is critical to ensure the appropriate phase shift and resonance at the desired frequency, it is worth noting that these characteristics are just as important for the other parasitic elements (directors). This explains the very narrow bandwidth as a change in frequency will mean the lengths and spacing of the parasitic elements (reflector & directors) no longer correspond to the appropriate wavelength fraction, as the wavelength has changed.

    It is an accepted fact that trial and error plays a significant role in obtaining optimal radiation patterns.

    This explains why it is common to use a folded dipole with parasitic arrays, because the folded dipole reduces frequency sensitivity (increases bandwidth) and also compensates for the impedance changes caused by the parasitic elements.

    Hope these explanations help a little.

  4. Tuan Tran
    2013/10/10 at 10:59 AM

    To shorten the cable length, you need to make a loop instead to flat the end and zip them.
    That way the cable will not have any kinky end.
    You have a kinky cable with your zip ties.

  1. No trackbacks yet.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: