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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.)

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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: ,