I would like to model a piece of cable (less than 5m) over a frequency range of DC to \$1\mathrm{MHz}\$. I am however doubting if I can approximate it by a lumped parameter model (mainly at \$1\mathrm{MHz}\$) or if I need to model it as a continuous transmission line. If I am correct, lumped parameters can be used as long as the physical dimensions of the system (here: cable length) are sufficiently below the wavelength lambda. Since lambda (in free space) can be calculated using the speed of light \$c\$ and \$f\$, for \$1\mathrm{MHz}\$ I get: $$\lambda = \frac{300000000\text{ km/s}}{1000000\text{ Hz}} = 300\mathrm{ m}\text{ (approximately)}. $$ Since my physical system is much smaller, I suppose a lumped parameter representation is sufficiently accurate. However, in transmission line theory, the wavelength is derived through the propagation constant $$ \gamma = \alpha + j\cdot\beta. $$ \$\gamma\$ is calculated through the cable's impedance and admittance and the corresponding wavelength can be calculated as $$ \lambda = 2\cdot\frac{\pi}{\beta}. $$ That would thus mean that the physical properties (\$R,L,C\$) of my cable will determine the wavelength. Does that also mean that two different cables of the same length would possibly need to be modeled as a continuous or a lumped parameter model, just because we would choose thicker wires or have more spacing between the wires? I am a bit confused on the approach that should be followed and I am happy to receive your input.