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.)