What Is a Transmission Line
When you connect your transceiver to your antenna with a length of coaxial cable, you are not just running a wire from one place to another. You are using a transmission line — a structure that guides radio frequency energy as an electromagnetic wave between two conductors. Understanding what that means, and why it matters, is the foundation of everything else in this module.
In a coaxial cable, RF energy travels as a transverse electromagnetic (TEM) wave in the dielectric space between the inner conductor and the outer shield — not through the metal itself.
View LargerWhy RF Is Different from DC
Connect a 12-volt battery to a light bulb with 10 meters of wire and current flows immediately. The wire simply provides a low-resistance path, and as long as the voltage is high enough and the resistance is low enough, the circuit works. The length of the wire is almost irrelevant — you could use 1 meter or 100 meters and the bulb would glow the same.
Now replace that 12-volt DC source with a 14 MHz RF transmitter running at 100 watts and use the same 10 meters of wire as your feedline to an antenna. Suddenly the length matters enormously. The behavior of that wire at 14 MHz is completely different from its behavior at DC or at 60 Hz. Here is why.
At DC, the only electrical property of a wire that matters is its resistance. At audio frequencies a small amount of inductance becomes noticeable but is still usually negligible. At radio frequencies — anything above roughly a few hundred kilohertz — two additional effects become critically important:
Inductance: Every conductor has inductance distributed along its length. At low frequencies this inductance has negligible reactance, but at 14 MHz the inductive reactance of even a short length of wire is significant. A 10-meter wire at 14 MHz has an inductive reactance of many ohms — enough to impede current flow and cause reflections.
Capacitance: Any two conductors separated by an insulator form a capacitor. In a two-conductor feedline, the two conductors are separated by air, foam, or solid dielectric, and there is distributed capacitance between them along the entire length. At radio frequencies this capacitance allows current to flow transversely between the conductors — current that would simply not flow at DC.
The combination of distributed series inductance and distributed shunt capacitance is the fundamental reason why a feedline at RF is not just a wire. Instead it behaves as a distributed circuit — an infinite number of tiny inductors in series and tiny capacitors in shunt, stretching along the entire length. The ratio of inductance to capacitance per unit length determines something called the characteristic impedance, which you will learn about in the next lesson.
There is also a more fundamental reason rooted in the speed of light. At 14 MHz, one complete cycle of the RF waveform takes 1/14,000,000 of a second. In that time, an electromagnetic wave in free space travels about 21.4 meters — one full wavelength. A 10-meter wire is not just a negligible fraction of that wavelength — it is almost half a wavelength long. At half a wavelength, the voltage and current at one end of the wire are not the same as at the other end. The phase of the wave changes continuously along the wire's length. This phase relationship is what creates standing waves and what makes impedance transformation by feedline length possible.
The Two-Conductor Structure
A transmission line always consists of two conductors. This is not a coincidence — it is a fundamental requirement. RF energy can only travel as a wave along a structure that has both an electric field component (requiring two conductors at different potentials) and a magnetic field component (requiring current flowing in opposite directions in the two conductors). A single wire cannot support a complete electromagnetic wave by itself; it radiates energy into space instead of guiding it forward.
The two conductors can be arranged in several ways:
- Coaxial: An inner conductor surrounded by an outer cylindrical conductor (the shield), separated by a dielectric. The fields are completely contained inside the outer conductor, making coax a shielded, unbalanced line.
- Parallel: Two conductors side by side, separated by a fixed spacing. Open-wire feeders and ladder line are parallel lines. The fields extend outward between the conductors and are not shielded.
- Microstrip: A flat conductor above a ground plane on a PCB, separated by the board substrate. Used extensively in VHF, UHF, and microwave circuits.
- Twin-lead: Two parallel conductors embedded in or separated by a plastic insulating ribbon. The 300-ohm twin-lead once used for TV antennas is a familiar example.
In every case, current flows down one conductor and returns through the other. At any instant, current in the two conductors is equal in magnitude but opposite in direction. This opposing current flow is what creates the magnetic field that, combined with the electric field between the conductors, forms the complete electromagnetic wave that carries RF power along the line.
Energy Travels as a Wave, Not Through the Conductor
This is the single most important and most counterintuitive fact about transmission lines: the RF energy does not flow through the metal conductors. It flows in the space between them.
This seems wrong at first. After all, electrons do flow in the conductors — that is measurable. But electrons in a copper conductor move very slowly — on the order of a few millimeters per second in typical RF applications. The RF energy itself travels at close to the speed of light. Clearly, the energy is not being carried by the electrons.
What actually carries the energy is the electromagnetic field: the electric field (E-field) pointing radially between the two conductors, and the magnetic field (H-field) circling around the conductors, perpendicular to the direction of travel. Together these form a transverse electromagnetic wave, or TEM wave. It is called transverse because both the electric and magnetic fields are perpendicular (transverse) to the direction of energy travel.
The conductors do two things: they provide the boundary conditions that confine and guide the fields, and they carry the surface currents that create and are driven by those fields. Without the metal conductors you could not have the fields in a confined, directed form — the energy would simply radiate outward as an antenna. But the conductors themselves are the guide, not the carrier.
This has one important practical consequence: the dielectric material between the conductors affects how fast the wave travels and how much loss it causes. In a vacuum, the TEM wave travels at exactly the speed of light, 300,000,000 meters per second (3 × 10⁸ m/s). In a coaxial cable filled with solid polyethylene dielectric, the wave travels at about 66% of that speed. This is the velocity factor, which becomes important when you need to cut a feedline to a specific electrical length.
The Distributed Circuit Model
To analyze transmission line behavior mathematically, engineers model the line as a series of infinitesimally small lumped circuit elements repeated along its entire length. For a lossless (ideal) transmission line, each tiny section of length Δx contains:
- L·Δx: A small series inductor representing the magnetic energy stored in the conductor current at that point (L is inductance per unit length, in henries per meter)
- C·Δx: A small shunt capacitor representing the electric energy stored in the field between the conductors (C is capacitance per unit length, in farads per meter)
As Δx approaches zero, this model becomes exact. The behavior of the line — characteristic impedance, wave speed, reflection behavior — all emerge from the ratio and product of L and C.
- Characteristic impedance Z0 = √(L/C) — determined by the geometry of the conductors and the dielectric
- Wave velocity v = 1/√(LC) — determines how fast the wave travels along the line
Both are properties of the line's construction, not its length or the frequency in use.
For a real (lossy) line, two additional elements are added: a series resistance R·Δx representing conductor loss (resistive losses in the metal), and a shunt conductance G·Δx representing dielectric loss (current leaking through the insulation). For most practical cables at HF frequencies, conductor loss dominates and dielectric loss is minor.
Understanding the distributed model explains why a transmission line of any length has the same characteristic impedance: Z₀ is determined by L and C per unit length, not by the total length. A 1-meter piece of RG-213 has 50-ohm characteristic impedance. So does a 100-meter piece. Characteristic impedance is a property of the cable's construction, not its length.
Types of Transmission Lines
Amateur radio operators encounter several types of transmission lines, each with different characteristics suited to different applications.
| Type | Typical Z0 | Balanced? | Typical Loss | Best Use |
|---|---|---|---|---|
| RG-58 coax | 50 Ω | No (unbalanced) | High at UHF | HF short runs, test leads |
| RG-213 coax | 50 Ω | No (unbalanced) | Moderate | HF feedlines up to 30 MHz |
| LMR-400 | 50 Ω | No (unbalanced) | Low | HF and VHF/UHF longer runs |
| RG-6 coax | 75 Ω | No (unbalanced) | Low | TV/satellite, some HF antenna balun work |
| 450-Ω ladder line | 450 Ω | Yes (balanced) | Very low | Multiband HF dipole feeders with tuner |
| 600-Ω open wire | ~600 Ω | Yes (balanced) | Extremely low | High-power HF antennas, antenna tuner systems |
| 300-Ω twin-lead | 300 Ω | Yes (balanced) | Moderate (wet) | VHF TV antennas, some portable HF setups |
Balanced vs. unbalanced is an important distinction. Coaxial cable is unbalanced — one conductor (the shield) is at ground potential, and the other (the center conductor) carries the signal. Open-wire and ladder line are balanced — both conductors carry equal and opposite currents, with neither being at ground. Connecting an unbalanced feedline (coax) directly to a balanced antenna (dipole) without a balun forces unequal currents on the antenna elements and causes the feedline shield to radiate, which can introduce interference and distort radiation patterns. Baluns and chokes address this problem, which you will study in detail in lesson M13L.
The four main transmission line types used in amateur radio: coaxial cable (shielded, unbalanced), twin-lead (balanced, compact), ladder line (balanced, low loss), and open-wire feeder (balanced, lowest loss).
View LargerWhy This Matters in Practice
Understanding that a transmission line is not just a wire — that it is a guided-wave structure with specific electrical properties — changes how you think about your antenna system. Several important practical consequences follow directly from the theory above.
Feedline impedance must match the system impedance
Most amateur radio transceivers, SWR meters, and antenna tuners are designed to work with 50-ohm impedances. When you use a 50-ohm transmission line to connect them together, maximum power transfer occurs. If you use a 75-ohm cable instead, there is an impedance mismatch at every connection point, which causes reflections and some power loss. This does not mean 75-ohm cable is wrong for every application — it just needs to be managed. You will learn exactly how to calculate the effects of mismatches in lessons M13G, M13H, and M13I.
Feedline length affects the impedance seen at the transmitter end
If the antenna is perfectly matched to the feedline impedance, the length of the feedline makes no difference — the transmitter sees 50 ohms regardless of feedline length. But if the antenna is mismatched, the impedance seen at the transmitter varies with feedline length in a periodic way, repeating every electrical half-wavelength. This is why adding or removing a few feet of coax can sometimes appear to "improve" SWR — it is not actually fixing the mismatch at the antenna, just changing what the transmitter sees at a particular frequency. This behavior will make complete sense after you study standing waves in lesson M13G.
Feedline loss increases with SWR
A matched feedline transfers all but a small percentage of power to the antenna. A mismatched feedline loses additional power to heat because the standing waves cause higher peak currents and voltages at specific points, increasing the I²R and V²/R dissipation in the cable. High SWR on a long, lossy feedline can waste significant power. The calculation is straightforward once you understand the math of standing waves.
Feedline can radiate like an antenna
When common-mode currents flow on the outside of a coaxial cable's shield — which happens when the feedline is connected to an unbalanced antenna without a proper balun — the feedline becomes part of the antenna system and radiates RF. This distorts the antenna's radiation pattern, can bring RF back into the shack, and causes interference to other equipment. A good balun or common-mode choke at the antenna feedpoint prevents this.
You cannot ignore the feedline below HF either
Even at 3.5 MHz (80 meters), a wavelength is about 85 meters. A 30-meter feedline is roughly one-third of a wavelength — long enough that transmission line effects are significant. Do not assume that because you are operating on a "low" frequency you can treat the feedline as a simple wire. Transmission line theory applies to every amateur band from 160 meters to 23 centimeters.
You are operating on 40 meters (7.1 MHz) with a 15-meter (50-foot) run of RG-213 coax. Is transmission line theory relevant?
One wavelength at 7.1 MHz in free space = 300/7.1 = 42.3 meters. In RG-213 with a velocity factor of 0.66, one wavelength = 42.3 × 0.66 = 27.9 meters.
Your 15-meter feedline is 15/27.9 = 0.54 wavelengths long — more than half a wavelength. Transmission line effects are very much relevant. The impedance seen at the transmitter end of the coax will not equal the impedance at the antenna end unless the antenna is a perfect 50-ohm load.
Frequently Asked Questions
Does the RF current actually flow through the metal conductor, or not?
Both, in a sense. Electrons do move back and forth in the conductor surface at the RF frequency — this is a real current that can be measured. But these surface currents are the result of the electromagnetic wave in the space between the conductors, not the cause of it. The energy is stored and transported in the electromagnetic fields, not in the electron motion. If you could instantly replace the copper conductors with perfect conductors (zero resistance), the wave would travel exactly the same way and at the same speed — but no resistive heat would be generated. The practical implication is that conductor losses (skin effect, resistance) cause feedline attenuation, and these losses occur in the thin surface layer of the conductor, not throughout its bulk.
Can I use any two-wire cable as a transmission line?
Technically yes, but the characteristic impedance and loss will be determined by the geometry and dielectric of whatever cable you use. Speaker wire, for example, has a characteristic impedance of around 100–150 ohms depending on conductor spacing and insulation material — neither 50 ohms nor 450 ohms. It also has higher resistive loss than proper coaxial or ladder line. For short runs at HF frequencies where loss is not critical, improvised cables can work. For any run where loss matters or where impedance matching is important, use purpose-designed transmission line with known, specified characteristics.
Why does adding or removing a short length of coax sometimes change my SWR reading?
This effect occurs when the antenna is mismatched. The impedance at the transmitter end of a mismatched feedline rotates around a circle on the Smith chart as feedline length changes, repeating every half electrical wavelength. At some lengths the impedance looks more capacitive, at others more inductive, and at specific lengths it passes through the resistive axis at different impedances. Your SWR meter reads the mismatch at the point where it is connected, so changing feedline length genuinely does change what the meter reads — but it does not change the mismatch at the antenna. The real fix is always to match the impedance at the antenna, not to hunt for a "magic length" of coax.
My coax is rated for DC resistance of 3 ohms per 100 feet. Why does its feedline impedance read 50 ohms on an antenna analyzer?
The DC resistance and the characteristic impedance are completely different quantities. The DC resistance is the ohmic loss of the conductor itself, measured with a multimeter, and it increases with length. The characteristic impedance is a property of the cable's distributed inductance and capacitance, determined by the physical geometry (conductor diameter, spacing, and dielectric material). At RF, the characteristic impedance determines how the line handles traveling waves and does not change with length. A 1-meter piece and a 1000-meter piece of RG-213 both have a characteristic impedance of 50 ohms, but the longer piece has 1000 times the DC resistance and much more RF loss.
Does it matter which end of the coax connects to the transmitter and which end to the antenna?
Electrically, no — coaxial cable is reciprocal, meaning it works identically in both directions. A traveling wave propagates just as well in either direction. However, for weatherproofing and installation reasons, it is conventional to have the drip loop toward the antenna end and to seal the outdoor connectors. The connectors themselves (PL-259, N-type) are also designed to be used with the male connector on the equipment side and female on the antenna side, though this is a mechanical convention, not an electrical requirement.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.