Module 13: Transmission Lines
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Everything in your amateur radio station depends on getting RF energy from one place to another without loss. A 100-watt transmitter is useless if half its power disappears before it reaches the antenna. The component that connects your radio to your antenna — coaxial cable, ladder line, or any other guided pathway — is a transmission line, and how it behaves at radio frequencies is very different from how a simple wire behaves at audio or DC frequencies. This module explains everything you need to know to choose, use, and troubleshoot transmission lines in your shack.
Transmission line theory sits at the heart of amateur radio practice. SWR, impedance matching, coax loss, stub tuners, baluns — all of these topics make no sense without a solid understanding of what a transmission line actually does to an RF signal. By the time you complete this module you will be able to look at a feedline installation and immediately understand what is happening electrically, diagnose problems, and make intelligent decisions about cable selection and matching.
- Explain how a transmission line guides RF energy as a traveling electromagnetic wave
- Define characteristic impedance and explain why 50 ohms became the standard for amateur radio
- Calculate velocity factor and use it to determine the physical length of an electrical quarter-wave section
- Use electrical length in degrees to analyze transmission line behavior at any frequency
- Select the correct coaxial cable type for a given frequency, power level, and run length
- Describe the advantages and disadvantages of open-wire feeders compared to coaxial cable
- Explain how standing waves form on a transmission line and what a standing wave ratio represents
- Calculate SWR, reflection coefficient, return loss, and reflected power from impedance values
- Predict the effect of SWR on feedline loss and transmitter operation
- Design short-circuit and open-circuit stub matching networks
- Calculate the impedance of a quarter-wave transformer for any impedance transformation ratio
- Choose and correctly install baluns and common-mode chokes for antenna feedpoints
- M13A — What Is a Transmission Line
- M13B — Characteristic Impedance
- M13C — Velocity Factor
- M13D — Electrical Length
- M13E — Coaxial Cable Types and Loss
- M13F — Open Wire and Ladder Line
- M13G — Standing Waves
- M13H — SWR and Reflection Coefficient
- M13I — Matched and Mismatched Loads
- M13J — Stub Matching
- M13K — Quarter-Wave Transformers
- M13L — Baluns and Chokes
Module Overview
At DC and low audio frequencies, a wire is just a wire. Current flows through it, a small amount of resistance causes a tiny voltage drop, and that is about all that happens. At radio frequencies the situation is completely different. A wire that is a significant fraction of a wavelength long starts to behave like a distributed circuit — not a simple resistor, but a complex combination of series inductance and shunt capacitance spread along its entire length. The relationship between these distributed elements determines how the wire handles RF energy, whether it transfers power efficiently or reflects it back toward the source, and whether it radiates energy as an antenna or confines it as a feedline.
A transmission line is a structure specifically designed to guide RF energy from one point to another with minimum radiation and predictable electrical behavior. Coaxial cable, open-wire feeders, microstrip on a PCB, and waveguide are all transmission lines. They differ in their construction and applications, but they all share the same fundamental behavior: they support a transverse electromagnetic (TEM) wave that travels between two conductors, carrying power without intentional radiation.
Understanding transmission lines matters for two practical reasons. First, every feedline has loss, and the amount of loss depends on the cable type, frequency, and SWR. Choosing the wrong cable can mean the difference between a competitive station and one that barely makes contacts. Second, the antenna system as a whole — feedline, connectors, balun, and antenna — must be properly matched if power is to flow freely from the transmitter to the antenna. A mismatch does not destroy power outright, but it does cause reflections that increase feedline loss and can damage transmitters that lack modern protection circuits.
This module builds knowledge progressively. You start with the physical picture of what a transmission line is, work through the mathematics of characteristic impedance and velocity factor, then apply those concepts to real cable selection and SWR calculation. The final lessons cover practical impedance matching techniques — stubs, quarter-wave transformers, and baluns — that you will use throughout your radio career.
Lessons
M13A — Lesson 132
What Is a Transmission Line
How RF energy travels as a wave between two conductors and why a transmission line behaves differently from a simple wire at radio frequencies.
M13B — Lesson 133
Characteristic Impedance
What Z0 means, where it comes from, and why 50 ohms became the standard impedance for amateur radio coaxial cable.
M13C — Lesson 134
Velocity Factor
Why RF signals travel slower in coax than in free space, how to find the velocity factor for any cable, and why it matters for cut-to-length applications.
M13D — Lesson 135
Electrical Length
Expressing transmission line length in electrical degrees and wavelengths instead of feet or meters — and calculators to convert between physical and electrical length.
M13E — Lesson 136
Coaxial Cable Types and Loss
A practical guide to RG-58, RG-8X, RG-213, LMR-400, and other coaxial cables — loss figures, power ratings, and how to choose the right cable for your station.
M13F — Lesson 137
Open Wire and Ladder Line
Why balanced open-wire feeders can outperform coaxial cable on multiband HF antennas, and how to install ladder line correctly in your shack.
M13G — Lesson 138
Standing Waves
What happens when a traveling wave hits a mismatched load and reflects back — and how incident and reflected waves combine to create voltage and current standing wave patterns.
M13H — Lesson 139
SWR and Reflection Coefficient
The mathematics of standing wave ratio, reflection coefficient, return loss, and reflected power — with fully worked calculators for each conversion.
M13I — Lesson 140
Matched and Mismatched Loads
What really happens in a system with high SWR — extra feedline loss, transmitter stress, and how modern transceivers handle mismatched antennas.
M13J — Lesson 141
Stub Matching
Using short-circuit and open-circuit stubs to cancel reactive components and match impedances — with a calculator for stub length at any frequency.
M13K — Lesson 142
Quarter-Wave Transformers
The elegantly simple impedance transformer that uses a quarter-wavelength of transmission line to match any two real impedances — with worked examples and a calculator.
M13L — Lesson 143
Baluns and Chokes
Why every dipole feedpoint needs a current balun, how to wind a choke balun on ferrite, and how to choose the right ferrite mix for your frequency range.