Radio communications play a key role in modern electronics, but to a hobbyist, the underlying theory is hard to parse. We get the general idea, of course: we know about frequencies and can probably explain the difference between amplitude modulation and frequency modulation. Yet, most of us find it difficult to articulate what makes a good antenna, or how a receiver can tune in to a specific frequency and ignore everything else.
In today’s article, I’m hoping to provide an introduction to radio that’s free of ham jargon and advanced math. To do so, I’m leaning on the concepts discussed in three earlier articles on this blog:
If you’re rusty on any of the above, I recommend jogging your memory first.
Let’s build an antenna
If you’re familiar with the basics of electronics, a simple way to learn about antennas is to imagine a charged capacitor that’s being pulled apart until its internal electric field spills into the surrounding space. This would be a feat of strength, but bear with me for a while:
Turning a capacitor into a terrible antenna.
Electric fields can be visualized by plotting the paths of hypothetical positively-charged particles placed in the vicinity. For our ex-capacitor, we’d be seeing arc-shaped lines that connect the plates — and strictly speaking, extend on both sides all the way to infinity.
An unchanging electric field isn’t very useful: if it’s just sitting there for all eternity, it doesn’t convey information nor perform meaningful work. If we push a charge against the direction of the field and then let it loose, the particle will fly away – but that’s just a matter of getting back the energy we expended on pushing it into position in the first place.
The situation changes if we start moving charges back and forth between the plates. This produces an interesting effect: a ripple-like pattern of alternating fields that are getting away from the ex-capacitor at the speed of light:
Field pattern around a simple antenna.
... continue reading