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Fundamentals of Radio Communication
13 questions By Tony R. Kuphaldt
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Question 1 of 13
What does the acronym RF stand form, in reference to radio-related electronics?
Reveal answer“RF” means Radio Frequency, implying a frequency of alternating current (AC) much greater than that encountered in AC power or audio circuitry.
Notes:Ask your students to list the frequencies of their favorite broadcast radio stations as an example of some radio frequencies. Show them a typical benchtop signal generator (non-RF) for comparison of frequency range, and they should begin to understand the concept.
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Question 2 of 13
We know at this point that any circuit comprised of inductance (L) and capacitance (C) is capable of resonating: attaining large values of AC voltage and current if “excited” at the proper frequency. The so-called tank circuit is the simplest example of this:
The less resistance (R) such a circuit has, the better its ability to resonate.
We also know that any piece of wire contains both inductance and capacitance, distributed along its length. These properties are not necessarily intentional - they exist whether we would want them to or not:
Given that the electrical resistance of a continuous piece of metal wire is usually quite low, describe what these natural properties of inductance and capacitance mean with regard to that wire’s function as an electrical element.
Reveal answerThe fact that an piece of wire contains both inductance and capacitance means that it has the ability to resonate just like any tank circuit!
Follow-up question: qualitatively estimate the frequency you suppose a length of wire would resonate at. Do you think fr would be a very low value (tens of hertz), a very high value (thousands, millions, or billions of hertz), or somewhere in between? Keep in mind the equation for resonant frequency:
fr = 1 2 π √ LCNotes:If your students have difficulty knowing where to start with the follow-up question, ask them to qualitatively estimate the distributed L and C for a piece of wire, say, 10 feet long. Given the lack of any high-permeability core material and the lack of any high-permittivity dielectric (just air), the answers for both should be “very small.” Then, ask them again how they would qualitatively rate the wire’s resonant frequency.
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Question 3 of 13
Shown here is a simple quarter-wave antenna, comprised of a single wire projecting vertically from one terminal of an RF voltage source, the other terminal connected to earth ground:
Re-draw this illustration, showing the equivalent inductance and capacitance exhibited by this antenna. Show these properties using actual inductor and capacitor symbols.
Reveal answer
Follow-up question: how would you expect the inductance and capacitance of this antenna to relate to its physical length? In other words, as you increase the length of an antenna, would its inductance increase or decrease? As the length increases, would its capacitance increase or decrease? Explain your reasoning.
Notes:Do not be surprised if some of your students ask whether or not an antenna is capable of resonating, since it possesses both inductance and capacitance. In fact, this is the subversive point of this question: to get students to realize that even a simple pair of wires may be considered a resonant system, and then to beg the question of what happens at resonance! The follow-up question suggests a relationship between physical size and resonant frequency, by asking what happens to both L and C as length changes. Explore these ideas with your students, and watch them gain a surprisingly deep understanding of how an antenna works based on their knowledge of LC resonant circuits.
