Transmission Line Calculators

In conventional DC and low-frequency AC circuits, conductors simply provide closed conductive loops. At radio frequencies where physical wire length becomes comparable to the carrier wavelength, phase shifts along the line turn ordinary wiring into distributed resonant elements.

A transmission line possesses distributed series inductance (L) and shunt capacitance (C) along every millimeter of its run. When an RF generator applies a signal, the ratio of the traveling voltage wave to the traveling current wave settles at the characteristic impedance Z0 = sqrt(L / C). For standard coaxial cables, this value is fixed at 50 or 75 Ohms regardless of line length.

In practice, RF currents do not penetrate through the bulk thickness of conductors. Due to the skin effect, magnetic fields inside the conductor force current into a microscopically thin surface layer. At 144 MHz, the skin depth of copper is only 5.4 micrometers, causing AC surface resistance to rise sharply with frequency.

Coaxial line loss is divided into two distinct physical mechanisms. At HF frequencies (below 30 MHz), conductor ohmic resistance accounts for approximately 90% of total attenuation. Copper plating over steel center conductors provides high tensile strength without loss because RF currents remain confined to the copper skin.

As frequencies reach UHF and microwave bands (above 500 MHz), dielectric losses accelerate. Alternating electric fields cause molecular friction within the plastic insulation. Solid polyethylene exhibits moderate loss, while gas-injected foamed polyethylene and expanded PTFE maintain much lower loss factors.

What matters most here is matching cable grade to operating frequency. Running 30 meters of RG-58 at 440 MHz results in 10.5 dB of attenuation, burning 91% of your transmitter power as heat inside the cable jacket. Switching to 1/2-inch hardline reduces this loss to less than 1.4 dB, delivering 72% of your power to the antenna.