Antenna Basics – Charge
Ok, I finally got some time to begin my short series of articles about antenna theory.
So you know… I like the KISS principle. Keep things simple. I’m not going to make this complicated. As an electrical engineer I can tell you that there are hundreds of books on this subject that dive in to a range of details and complexities… often to such an extreme that few readers understand.
I prefer first principles. These are the fundamental ideas upon which a theory is based. For antennas, these principles were studied beginning in the 1880’s by scientists like Heinrich Hertz, and those ideas are still quite relevant today.
A simple experiment provides a great starting point for discussion. You can actually build this and try it out (but that’s not really necessary to understand the theory behind it. )
- A ten inch sphere made of thin copper or other conductive metal.
- Ten feet of copper wire and a few insulators to hang it.
- An AM radio.
- A high voltage DC source, such as a Van de Graaff generator.
Hang the wire and the sphere near each other, using insulators. Here’s a crude diagram (and not drawn to scale):
Using the voltage source, charge the sphere to negative 10,000 volts (or any high voltage, it does not matter.)
Turn on your AM radio, it can be positioned anywhere in the room, and tune it to a clear frequency.
Now, using an insulated rod, touch the wire to the sphere.
You’ll hear a snap or click sound on the radio. Interesting!
Congratulations. You’ve just built the most basic radio transmitter and antenna possible. Experiments of this sort were first performed by Heinrich Hertz (yes, as in megaHertz) in 1887 to prove the existence of electromagnetic waves.
Physics and radio books just love to dive into the details of voltages, currents, fluxes, and fields. However, it’s good to recognize that those are all derivatives of a fundamental concept: charge. If you think in terms of charge, it’s easier to understand how those other terms relate.
- Think of a charge as an electrically unbalanced atom. It has either too many or too few electrons to balance out the protons of its nucleus. When it has too many, it’s negatively charged. When too few, it’s positively charged.
- Different materials can become charged using various methods. What that means is that their atoms have either too few or too many electrons.
- Charge can be distributed evenly or unevenly within a material. Part of a material may be negative while another part is positive.
- Charge can flow from one point to another. In some materials, such as metals, charges flow quite freely (they conduct.) In fact, they act a lot like a fluid or gas, flowing and sloshing around with very little friction. This fact is really important.
- Charges repel and attract with tremendous force. How much force? Physicist Richard Feynman provides the best example: if you stand one arm’s length from another person, and both of you are charged with one percent more electrons than protons, the force of repulsion between you would be enough to lift the entire weight of the earth. That’s correct. Just one percent of your electrons. It gives you a sense of the magnitude of the force we’re dealing with here, doesn’t it?
- This extreme force causes charges to move. For example, if we add a few extra electrons to a thin metal sphere, the repulsive force causes all of the electrons to flow and distribute themselves across the sphere. This is true for all metal objects, not just spheres.
In the experiment above, here’s what happened in terms of charge:
- You charged the sphere by adding extra electrons to it.
- The sphere is made of metal, and its electrons move freely.
- Because of their extreme repulsive forces, the electrons spread out evenly over the surface of the sphere. I’ll call it electron pressure. That’s a good way to think of it: a force or tension between all of the extra electrons.
- The wire has no charge and when touched to the sphere the electron pressure begins to push electrons down the wire.
- Electrons flow down the wire until the electron pressure within the wire becomes equal to that of the sphere.
- As the electrons are accelerated (begin to move in a specific direction, down the wire) they produce electromagnetic radiation: radio waves.
- These waves radiate outward, and the AM radio detects these waves, making the click sound.
Here’s the diagram again, showing the charge beginning to flow down the wire:
The charge moves down the wire very quickly. The extra electrons also push the electrons that are already within the wire, just like a hose that’s full of water when pressured from a facet. Yes, it’s a lot like a wave of water pressure moving through a hose.
After a short period of time the charge has moved halfway down the wire.
Then, after another short period of time the electrons reach the end of the wire:
The electrons no longer have anywhere to go and they distribute themselves evenly across the wire, and radio waves are no longer being produced.
Essentially, this experiment produced a single burst of radio waves which are heard on the AM receiver.
(Ok, those of you who are experts will recognize that I’ve simplified this explanation quite a lot… but I think the first order effects far outweigh the other details for the purposes of this description. I’ll cover some of that in articles to follow.)
This experiment provides a launching point to get you thinking about the process. I think it’s quite interesting how very simple this experiment is, showing how fundamental this process is in nature. It’s all around us and is happening all the time.
Notice that I’ve not said much about voltage, current, fluxes, fields, frequencies, or phases. But then, the process shown here isn’t really that useful, it just sends a single click.
In the next article, I’ll cover what happens if we charge and discharge this wire at regular intervals… that is, at a specific frequency. In addition, there’s a deeper paradox lurking here: no matter where you tune your AM radio, you’ll hear the click sound. The wave emitted by this process is extremely broad in frequency. You might say it has no frequency, or more accurately, it has all the frequencies.