Technical Support for Antennas & Related Gear

Capacitance of Antenna Cable Coax Types: Lower value indicates better signal integrity

George Hardesty
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Capacitance values for various types of coax types

These are typical values for the capacitance per foot or per meter of these coaxial cables used in antenna cables:

  • LMR-100: Approximately 30.8 pF/ft (101.1 pF/m)
  • LMR-195: Approximately 25.4 pF/ft (83.3 pF/m)
  • LMR-200: Approximately 24.5 pF/ft (80.3 pF/m)
  • LMR-400: Approximately 23.9 pF/ft (78.4 pF/m)
  • RG-174: Approximately 30.8 pF/ft (101 pF/m)
  • RG-178: Approximately 29.4 pF/ft (96.4 pF/m).
  • RG-213: Approximately 30 pF/ft (98.4 pF/m).
  • RG-58: Approximately 30 pF/ft (98.4 pF/m).
  • RG-8: Approximately 29 pF/ft (95 pF/m).

These values indicate the capacitance per foot (or per meter) of the cable, which is a key factor in determining the performance of the cable in high-frequency transmission scenarios, such as in wireless communication systems and antenna setups.

LMR-100_ LMR-200_ and LMR-400_Signal-loss _Attenuation

U.FL-cable-coax-1.13mm-RG174-1.32mm-1.37mm-compared

Lower capacitance in an antenna cable indicates better signal integrity for six reasons

The capacitance of a coaxial cable is a significant factor that influences the performance of the cable in high-frequency applications. It affects signal integrity through increased attenuation (signal loss), impedance mismatches, and phase shifts, all of which can degrade the signal quality over distance. Therefore, choosing the right cable with appropriate capacitive properties is essential for maintaining optimal signal integrity in any communication system.

  1. Signal Attenuation: Higher capacitance in a cable can lead to greater attenuation of the signal, especially at higher frequencies, and over longer distances. This is because capacitance can cause more of the signal's energy to be stored in the cable rather than transmitted through it, leading to diminished signal strength over distance.  At higher frequencies, the effect of the cable’s capacitance is more pronounced because it creates a low-pass filter effect, attenuating higher frequency components more than lower ones.
  2. Noise and Interference: Capacitance can affect the cable’s susceptibility to external noise and interference. A well-designed coaxial cable with appropriate capacitance helps in shielding the signal from external electromagnetic interference (EMI), maintaining the purity of the signal.
  3. Bandwidth: Capacitance influences the bandwidth of the cable. Higher capacitance generally limits the bandwidth, affecting the cable's ability to carry high-speed data without distortion. This is particularly relevant in digital communication systems where bandwidth and data rate are closely tied.
  4. Capacitive Reactance: Capacitance in the cable contributes to its overall impedance, specifically capacitive reactance, which is inversely proportional to the frequency of the signal and the capacitance. This means that at higher frequencies, the capacitive reactance decreases, allowing more of the signal to be attenuated. This is especially critical in long cable runs where the cumulative effect of capacitance can significantly weaken the signal by the time it reaches its destination.
  5. Impedance Mismatching: Mismatches can lead to signal loss and degradation. The capacitance of a coaxial cable contributes to its characteristic impedance. Proper impedance matching is essential to minimize signal reflection at connections between the cable and other components like antennas and receivers. Ideally, the impedance of the coaxial cable should match the impedance of the rest of the system (commonly 50 or 75 ohms) to minimize reflection losses. Capacitance can alter the impedance of the cable, potentially leading to mismatches. Impedance mismatches can cause part of the signal to be reflected back towards the source, effectively causing signal loss.
  6. Phase Shift and Delay: The capacitance (capacitive nature of the cable) affects the phase of the signal passing through the cable, because it can introduce a phase shift and delay in the signal. This is because the signal's electric field interacts with the cable's capacitive properties, which can delay the timing of the signal's propagation through the cable. In digital communications, such as digital video or data transmissions, this can lead to errors or degradation in the quality of the received signal.

In antenna and radio frequency applications, selecting a coaxial cable with the appropriate capacitance is essential to ensure effective transmission and reception of signals without significant loss or distortion. This is why the electrical properties, including capacitance, are specified and controlled carefully in the design and selection of coaxial cables for specific applications.

The RG-series cables are often used in various radio frequency applications and have different specifications to meet the needs of different systems, such as amateur radio setups, antenna feeds, and more. Each cable type offers specific characteristics in terms of power handling, attenuation, and flexibility, influencing their choice for certain applications.

The LMR-series coaxial cables are 50-ohm, low-loss, flexible coaxial cables widely used for carrying high-frequency RF signals in various applications, including feeding signals between wireless communication systems and their antennas. The cable features a solid copper-clad aluminum conductor with Foam Polyethylene (Foam PE) insulation, two shielding layers for protection against electromagnetic interference, and a durable Polyethylene (PE) outer jacket

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