Posts Tagged ‘electromagnetics’

How Does a Yagi Antenna Work?

2011/04/25 4 comments

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.


Antenna Puzzle #2

2011/04/09 2 comments

Here’s a simple but interesting antenna puzzle to think about…

Let’s say you use a wire dipole for the 20M band. You decide you want to boost your gain by adding a second dipole (end-to-end, i.e. colinear) and combine their feeds (in phase, of course.)

The two antennas receive the incoming signal (depicted in green below) which gets combined to double its strength at the receiver. Here’s the diagram:

Simple dipole array (receive)

So, this simple array provides 3 dBd of gain. It makes sense because each antenna is receiving an independent field of energy from the distant station.

Now, you use the same setup to transmit 20 watts on CW. Of course, your power gets divided in half between the two dipoles. Each antenna gets 10 watts.

Simple dipole array (transmit)

But, wait… if you simply put that 20 watts into a single dipole wouldn’t that be the same thing as two antennas at 10 watts?

The principle of antenna reciprocity dictates that an antenna must behave the same for both receive and transmit. It has the same gain, directivity, and pattern. So where’s your 3 dB of gain on transmit?

Perhaps you’ll answer that the two antenna fields significantly overlap. But, even if they add together perfectly, won’t the distant station still see the same signal as one antenna at 20 watts? Where’s the 3 dB? (The output should be equivalent to 40 watts.)

So, there you go, a puzzle. What’s your solution? Post it here as a comment.

Answer now posted here: Power vs Signal Strength 

Diving Back In

It’s 4AM, and as is often the case these days, I lie in bed wide awake… my thoughts returning to EM wave theory, my little obsession.

This morning in particular I ponder how I ended up in this state of mind. My memories drift back to the days of building a small 20 watt ham transmitter for my fifth grade science fair project, but that was not the beginning either. Perhaps plugging the rabbit ears into an electrical outlet at the age of four damaged my brain more than was evident. The memory is clearly etched, the antenna in my hands as if it were yesterday, just like discovering those razor blades on my dad’s bathroom sink at that same age.

In order to type this blog without waking Cindy, I’ve snuck down to the ham shack in the basement. Of course, if you’re going to be in the radio room, you are obliged to flip on the HF rig, and it comes up on 7.000 MHz, as it always does because I’ve yet to solder in a new battery to keep its memories stored.

Unless something similar happens for you, you may not realize that 7.000 MHz is full of multiple overlapping signals, voices of people worldwide generating a cacophony of sounds and songs, nothing logical — quite like the old haydays of CB when skip was in. I often wonder if what I’m hearing are odd intermodulation results from SW broadcasters, or just dozens of people who found HF radios in their basements and aren’t quite sure how they work or what to do with them.

When I started this blog, I promised myself that I’d dive into the theory a bit more… which is one of my secret agendas for doing this site in the first place. But, here I am several months later, and I’ve barely scratched the surface.

For many of you electromagnetic theory may seem dull or boring, a subject best left to nutty physicists who have nothing better to do with their time. However, the topic is a lot more interesting than it seems and doesn’t necessarily require that you brush up on your differential calculus. It can be as fun and entertaining as stringing up new antennas, tweaking with your antenna tuner, fooling with your IF bandwidth, DXing some remote island, or messing around with new digital modes.

So, it’s time to dive in, go a little deeper, and maybe even have some fun.

And, of course as always, you are invited to join the discussion, to whatever level or degree that interests you.

The Four Fields of an Antenna

You don’t need to be an electrical engineer or ham radio extra class to understand the basic electro-magnetic (EM) fields of an antenna. Let’s see if I can explain them in one page.

There are four EM fields related to an antenna:

electric An antenna has electric charge; therefore, it projects an electric field outward. 

What’s a charge? It’s any imbalance in electrons, either extra electrons for negative charge, or shortage for positive charge. For example the charge on the ends of a battery or capacitor, the static electricity when you stand-up from a chair on a dry winter day, or the voltage on the output of your ham radio transmitter.

This field decreases rapidly with distance, and it relates directly to voltage, the electric potential (force).

magnetic An antenna has current (a moving electric charge); therefore, it creates a magnetic field around it. 

The field is described in terms of a cylinder that curls around the direction of movement. For example, the current (the movement of charges) in a wire forms a magnetic field around the wire. Even if you take a charged ball, and move it in the air, you’re creating a magnetic field as it moves.  Wonderful, isn’t it?

Like the electric field, this field also decreases rapidly with distance. It relates directly to current flow, amperes (flux).

I probably should note: when you say that a charge “moves” you must ask “relative to what?” In other words, the magnetic field is relativistic. This makes it even more interesting, but let’s save that for dessert later.

radio waves / photons An antenna current changes (electric charges accelerate); therefore, it produces EM radiation that propagates away from it. 

We know of it as radio waves or photons (aka, “light”, every physicist will tell you it’s all the same.) It is described in terms of a tiny pane that detaches from the charge and travels outward. It is composed of both electric and magnetic fields that oscillate back and forth, always at a specific frequency, which also means a specific energy.

These waves can travel great distances — across the room, the country, or the universe. That’s why this field is called the far field whereas the electric and magnetic fields mentioned above are called the near field. Their effects over distance are dramatically different.

When you measure the signal strength of your ham radio, you are actually measuring the quantity of these waves/photons passing by your location.  Also, by the way, as you whip around the above mentioned charged ball, you’re also creating EM radiation. Interesting, eh?

heat waves / photons An antenna has resistance, which produces heat; therefore, it also produces a different EM radiation that propagates away from it.  

When you apply a current to a wire, resistance within the wire will generate heat — meaning atoms “bumping around” into each other. When they do, they accelerate electrons (changing “quantum energy levels”) which as you know from above, produces EM radiation.

This field also travels outward and is composed of photons of many frequencies — most notably infrared radiation: heat waves. In general you don’t want your antenna to generate this kind of field, because it is wasted energy that’s not going into your radio signal.

Ok, did you get all that? It’s the basic theory in a nutshell… really quite simple, which is kind of cool.

So, perhaps now you’ll look at your antenna a bit differently, imagining the forces, flows, and energies that are making your ham radio transmission possible.

PS: As engineers, we express the theory in terms of mathematical equations describing potentials (forces) and fluxes within a volume of space or through surfaces such as a sphere. They were first summarized by James Clerk Maxwell about 150 years ago and reformulated by Oliver Heaviside, the inventor of coaxial cable, about 20 years later into what we call Maxwell’s Equations. (Although many folks prefer they be called Heaviside’s Equations, giving credit to his remarkable simplification through the use of vector notation and operator-based mathematics, the form we still use today.)

The Truth about Radio Waves

2010/10/12 16 comments

I’ve always been fascinated with radio waves, and as much as I’ve dealt with them in various ways, I must admit that I don’t fully understand what they are, nor how they are generated, nor received.

Sure, sure… as an electrical engineer, amateur extra, and all-round scientist/experimenter, I’ve known the various theories of electromagnetics (EM) for a long time. Yes, those are all quite nice and tidy. The theories work extremely well for calculating just about everything…  yet, when all is said and done, I still don’t quite understand how radio waves are generated.

This is my coming out of the closet notice: I’m an Electromagnetics Truther.

And, in saying that, I sincerely hope that I’m the first to define that term — that it’s not already associated with crazy lunatics at the fringes of pseudo-science.  That’s not where I’m coming from. This blog is my way of declaring: “Ok, I get the formulas, I use the formulas, they all seem fine… but there seems to be something missing. Have you noticed that too?”

Over the years, in search of “the answer” I’ve studied quite a few books and queried quite a few people, including various Internet community groups.  But the books gloss over the main point, and no one I’ve every asked seems to really know.  Also, I’ve found that the more someone knows, like a university professor, the more they realize that they don’t really know. (And the opposite is also true: the less someone knows, the more they think they do know.)

Ok, so what am I actually questioning here?  I can summarize it fairly well. Stick with me here…

First, let me give you the layman’s summary of EM theory:

  • Electrons have charge – they project an electric field.
  • A charge in motion (velocity) is defined as an electric current.
  • An electric current produces a magnetic field.
  • A change in electric current (acceleration) produces radiation: waves.
  • All waves across the entire spectrum obey the same set of rules.
  • A single “wavelet” is called a photon.
  • A photon travels along a single path (in a direction) through space.
  • Each photon is of a specific energy (quantized) and is related to its wavelength.
  • Photons are bosons – singular indivisible “particles”.  You never get half a photon.

Ok, now take an antenna, where we are told:

  • An electric current is applied to it via a feed line.
  • The current travels down the length of the antenna (at a velocity factor speed.)
  • The current emits electric and magnetic fields (called near fields).
  • The change in current emits radio waves (called far fields).
  • The change in current is of specific frequencies.
  • The radio waves produced are of those same frequencies.
  • The near fields drop off rapidly with distance.
  • The far fields travel “forever” (e.g. across the universe) along specific paths.

In other words: the frequency of the current in the antenna produces bursts of photons that travel out in various directions.

All of that is accepted theory, and it’s all quite wonderful.  However, what’s the actual mechanism of photon generation? We know that the result is quantized (specific energy and wavelength) and travels in specific directions, but how was that produced?

It gets rather problematic because you can ask:

  1. What actually determines the direction of each photon?
  2. When did each photon actually get split off from the electromagnetic (near) field?
  3. Is the photon’s energy really quantized according to the frequency/wavelength rule?

Yes, these may all seem like silly beginner questions… but, as I mentioned earlier, I can tell you that no textbook on EM answers these, not even advanced graduate books on the topic or top level research papers.  Over the years I’ve gathered quite the collection, including many older textbooks too, but unfortunately they all gloss over the specific mechanism or simply state that magic happens.

I don’t like magic happens explanations. They indicate that we don’t really understand what’s happening. In a recent discussion with a EM physics professor at a major university, he commented “I don’t know, but I think back in 1978, there were a couple guys that formulated a model that showed how it worked.”

Well, I’ve got a lot more to say about this topic, but I’m worried I’ve said too much, and you’re yawning right about now. Whenever someone questions well-rooted theories there’s a tendency to think of them as a wacko. What I can say in my defense is simply: ok, if you know, tell me. Show me the book.

Part of the problem is that I’m drilling down into a very specific part of the theory.  There are a great many EM theoreticians in the world, but each is focused on very small slice of the entire puzzle. Most learned about this specific topic while they were in college, and at that time, they just wanted to pass the test. And, of course, the original researchers who formulated the theories are all long gone. For the vast majority of everyone else, they simply don’t care.

So, this has been my quest, to understand this mechanism… for nearly a decade now. I often go to sleep pondering it, wake up pondering it, and think about it each time I gaze up at dipole, yagi, or log-periodic.

In a future article I’ll describe the main problems with the theory in more detail. I just didn’t want to put you asleep by making this pleasant little introduction far too long.

Anyway, so now you know. I’m a EM Truther: seeking the truth about EM emission. And, just perhaps, I’m not alone?

Categories: Antennas, Theory Tags: ,