Coaxial cable is inherently unbalanced because its signal current travels through a single central conductor, while the return current and shielding are handled by an outer braided or foil shield that is typically connected to ground.
Understanding Unbalanced Transmission Lines
An unbalanced transmission line, such as a coaxial cable, is characterized by having one conductor that carries the signal and is isolated from ground, and another conductor (usually a shield) that serves as the return path for the signal current and is connected to a common ground reference. This fundamental design creates the 'unbalanced' nature.
The Structure of Coaxial Cable
To understand why it's unbalanced, let's look at its basic components:
- Inner Conductor (Signal Wire): This solid or stranded wire, usually copper, carries the electrical signal.
- Dielectric Insulator: A non-conductive layer (like plastic or foam) that separates the inner conductor from the outer shield, maintaining a consistent spacing.
- Outer Conductor (Shield): This is typically a braided wire mesh, a foil wrap, or a combination of both. It serves two crucial functions:
- Signal Return Path: It provides the return path for the electrical signal current, completing the circuit with the inner conductor.
- Electromagnetic Shielding: It acts as a Faraday cage, protecting the inner signal conductor from external electromagnetic interference (EMI) and preventing the signal from radiating outward.
- Outer Jacket: An insulating protective layer, often made of PVC, that protects the cable from environmental factors.
Key Insight: The inner signal conductor is protected by this outer layer shield which is connected to ground. When noise currents or interference hit the shield, they are shunted harmlessly to ground, preventing the signal from being degraded.
How the Unbalance Occurs
The unbalance stems from the fact that the two conductors (inner core and outer shield) have different electrical potentials relative to ground, and their physical relationship to ground is asymmetric:
- Ground Reference: The outer shield is almost always connected to an electrical ground reference. This means it's at zero potential relative to ground.
- Signal Potential: The inner conductor carries the actual signal, which varies in voltage (potential) relative to the shield and ground.
- Asymmetry: There is no corresponding "hot" conductor or a symmetrical signal path to balance the ground connection. The signal's path is entirely contained within the shielded structure, with the shield itself acting as both the return path and the primary defense against external noise.
This asymmetry contrasts sharply with a balanced line (like twisted pair cable), where two signal conductors carry equal and opposite signals, and neither is directly tied to ground.
Advantages and Disadvantages of Unbalanced Cables
Coaxial cables offer specific benefits and drawbacks due to their unbalanced design:
| Feature | Unbalanced (Coaxial) | Balanced (Twisted Pair, XLR) |
|---|---|---|
| Grounding | One conductor tied to ground, the other carries the signal. | Neither conductor tied to ground; both carry differential signals. |
| Noise Rejection | Good for rejecting external EMI via shielding; susceptible to ground loops. | Excellent for rejecting common-mode noise; less susceptible to ground loops. |
| Impedance | Controlled characteristic impedance (e.g., 50 Ω, 75 Ω). | Defined differential impedance. |
| Applications | RF, video, digital data (short to medium distances). | Audio, Ethernet, long-distance data. |
Benefits:
- Effective Shielding: The continuous outer shield provides excellent protection against external electromagnetic interference (EMI) and prevents signal leakage. This makes it ideal for radio frequency (RF) applications where signal integrity is paramount.
- Controlled Impedance: Coaxial cables are designed with a precise characteristic impedance (e.g., 50 ohms for data/RF, 75 ohms for video). This is crucial for matching impedances between source and load, minimizing reflections and ensuring efficient power transfer.
- High Bandwidth: They can carry high-frequency signals over significant distances with minimal loss, making them suitable for broadband internet, cable TV, and various communication systems.
Drawbacks:
- Susceptibility to Ground Loops: Because the shield is grounded, if there are different ground potentials at various points in a system, a current can flow through the shield, creating a "ground loop" that can introduce noise into the signal.
- Less Common-Mode Rejection: While good at blocking external EMI, they don't inherently reject noise that couples equally into both the signal and return path (common-mode noise) as effectively as balanced lines do.
- Bulkier: Compared to unshielded twisted pair cables, coaxial cables are generally thicker and less flexible due to their layered construction.
Practical Applications
Coaxial cables are extensively used in various fields where their unique properties are beneficial:
- Television and Satellite: RG-6 and RG-59 coaxial cables are standard for transmitting TV signals from antennas, cable TV providers, and satellite dishes to televisions and receivers.
- Internet Access: Used for broadband internet connections via cable modems, delivering high-speed data.
- Radio Frequency (RF) Systems: Essential for connecting radio transmitters, receivers, and antennas, as well as in test and measurement equipment.
- Video Surveillance: For transmitting video signals from CCTV cameras.
In essence, the unbalanced nature of coaxial cable, with its grounded shield and central signal conductor, is a deliberate design choice that optimizes it for specific signal transmission needs, particularly in RF and video applications where robust shielding and controlled impedance are critical.
