Why transmission lines are necessary at RF:

In electrical engineering, we typically think of wires as ideal conductors that instantaneously transmit electrical signals. This "lumped element" model works well at low frequencies, such as those found in audio circuits or DC power distribution. In these cases, the wavelength of the electrical signal is enormous compared to the physical dimensions of the circuit. For instance, an audio signal at 1 kHz has a wavelength of approximately 300 kilometers in a vacuum. A typical circuit board, being mere centimeters in size, is tiny by comparison, and any delays in signal propagation along a short wire are effectively zero.

However, as we move into the Radio Frequency (RF) spectrum (generally above 30 kHz, but becoming critical at hundreds of MHz and gigahertz), the situation changes dramatically. At these higher frequencies, the wavelength of the electromagnetic signal becomes comparable to or even smaller than the physical length of the wires or interconnects in our circuits. For example, a 1 GHz signal has a wavelength of about 30 centimeters in a vacuum. On a PCB, where signals travel slower due to the dielectric material, the wavelength would be even shorter. If a 10 cm long trace is carrying a 1 GHz signal, it represents a significant portion of a wavelength (around a third of a wavelength).

When interconnect lengths are a significant fraction of a wavelength, the "lumped element" assumption breaks down entirely. Wires no longer act as simple equipotentials. Instead, they behave as transmission lines, guiding electromagnetic waves. This wave behavior introduces several critical challenges:
- Signal Reflections: When an electromagnetic wave traveling along a wire encounters a change in impedance (e.g., at the end of the wire where it connects to a component, or at a bend), some of its energy is reflected back towards the source.
- Standing Waves: The incident (forward-traveling) wave and the reflected (backward-traveling) wave interfere with each other, creating a stationary pattern of voltage and current along the line, known as standing waves.
- Radiation Losses: An improperly terminated conductor can act like an antenna, radiating electromagnetic energy into the surrounding environment.
- Phase Shifts and Timing Issues: Because signals propagate at a finite speed, there will be noticeable time delays for the signal to travel along the length of the wire.

Transmission lines are fabricated in various physical forms, each optimized for specific applications based on factors like operating frequency, power handling capability, cost, manufacturing feasibility, and shielding requirements.
- Coaxial Cable: This consists of a central conductive wire, surrounded by an insulating dielectric layer, with a tubular outer conductor and an outer insulating jacket. It has excellent shielding properties and is used in RF applications.
- Microstrip Line: A planar transmission line found on PCBs, consisting of a conductive trace on one side of a dielectric substrate.
- Stripline: Designed for superior performance, it sandwiches the conductive trace between two ground planes for better shielding.
- Twin-Lead: A simple transmission line with two parallel insulated wires, historically used for television antennas.

To understand and mathematically model the behavior of transmission lines, we characterize them using two sets of parameters: primary (or distributed) parameters and secondary (or derived) parameters.
- Primary Parameters: These include resistance (R), inductance (L), conductance (G), and capacitance (C). They describe the fundamental electrical properties distributed along the line.
- Secondary Parameters: These are derived from the primary parameters to help model wave propagation, such as characteristic impedance (Z0) and propagation constant (γ).

To overcome the challenges of RF signal transmission and to ensure efficient, low-loss, and controlled transfer of RF energy, we use transmission lines. These lines guide electromagnetic waves and maintain a consistent electrical environment, thereby minimizing reflections and losses.