Capacitance Values for Various Types of Coaxial Cables Used in Antenna Applications

Capacitance is one of the most important electrical parameters in coaxial cable performance, especially in high-frequency RF and antenna systems. Lower capacitance generally indicates better signal integrity, particularly in long cable runs and high-frequency applications.

Below are typical capacitance values per foot and per meter for commonly used coaxial cables in antenna and RF systems:

• 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 represent the capacitance between the center conductor and the shield per unit length of cable. Capacitance per foot (or per meter) is a critical parameter in determining cable behavior in high-frequency transmission environments such as wireless communication systems, antenna feeds, cellular infrastructure, IoT deployments, and RF test systems.

Why Capacitance Matters in RF and Antenna Systems

The capacitance of a coaxial cable directly influences signal integrity, attenuation, impedance stability, and phase behavior. In RF systems operating at MHz and GHz frequencies, even small electrical differences can significantly affect performance.

Lower capacitance generally improves performance for several reasons:

1. Reduced signal attenuation
2. Improved impedance stability
3. Lower phase distortion
4. Greater bandwidth capability
5. Reduced signal delay
6. Improved high-frequency response

Below is a detailed technical explanation of these effects.

1. Signal Attenuation

Higher capacitance contributes to increased signal attenuation, particularly at higher frequencies and over longer cable distances.

Capacitance causes part of the signal energy to be stored in the electric field between the conductor and shield rather than being transmitted forward. As frequency increases, this effect becomes more pronounced.

At high frequencies, cable capacitance creates a low-pass filter effect, attenuating higher-frequency components more than lower ones. This can reduce overall signal strength and degrade modulation quality in digital systems such as LTE, 5G, WiFi, and GPS.

For long antenna runs—especially in base station, rooftop, or tower installations—lower capacitance cables such as LMR-400 provide measurable improvements in signal preservation compared to smaller, higher-capacitance cables like RG-174.

2. Noise and Electromagnetic Interference (EMI)

Capacitance interacts with shielding effectiveness. While shielding primarily protects against external interference, the internal capacitive characteristics influence how susceptible the signal is to distortion.

Well-designed coaxial cables balance capacitance, shielding density, and dielectric properties to maintain signal purity. Dual-shielded cables like LMR-series products provide enhanced EMI rejection while maintaining lower capacitance compared to many RG-series alternatives.

In environments with high electromagnetic noise—industrial sites, telecom rooms, broadcast facilities—choosing lower capacitance and well-shielded cable improves signal integrity.

3. Bandwidth Performance

Capacitance influences bandwidth capability. Higher capacitance limits the maximum frequency range that can be transmitted without distortion.

In digital communication systems, bandwidth directly impacts data rate. Applications such as:

• LTE / 4G
• 5G Sub-6 GHz
• WiFi 6
• GPS
• IoT gateways
• MIMO systems

require stable high-frequency transmission. Excessive capacitance can reduce signal rise times, distort digital pulses, and degrade overall system throughput.

Lower capacitance supports broader bandwidth and cleaner high-speed data transmission.

4. Capacitive Reactance and Frequency Behavior

Capacitance contributes to capacitive reactance, which is inversely proportional to frequency:

Where:

• Xc = capacitive reactance
• f = frequency
• C = capacitance

As frequency increases, capacitive reactance decreases. This allows more high-frequency energy to be absorbed or attenuated along the cable.

In long cable runs, cumulative capacitance becomes significant. The total capacitance equals capacitance per foot multiplied by total cable length. This is why long antenna installations benefit greatly from lower-capacitance cable types.

5. Impedance Matching

Characteristic impedance (typically 50 ohms for RF systems or 75 ohms for video systems) is determined by conductor geometry and dielectric properties—including capacitance.

Capacitance contributes directly to the cable’s impedance. If capacitance deviates from design expectations, impedance mismatches may occur.

Impedance mismatch leads to:

• Signal reflections
• Standing waves (high VSWR)
• Power loss
• Reduced transmitter efficiency

Proper impedance matching minimizes reflection losses at connections between cable, antenna, connectors, amplifiers, and receivers.

Cables such as LMR-series are tightly controlled in manufacturing to maintain consistent 50-ohm impedance across long runs.

6. Phase Shift and Signal Delay

Capacitance affects signal propagation velocity and phase behavior. The electric field interaction inside the cable causes slight delays in signal transmission.

In analog systems, this may slightly affect phase alignment. In digital systems, particularly:

• Digital video
• Time-sensitive networking
• MIMO antenna systems
• Beamforming applications

Phase stability is critical.

Excess capacitance can introduce timing errors and degrade signal synchronization. Lower capacitance supports more stable phase performance.

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.

Comparison: RG-Series vs LMR-Series Coaxial Cables

RG-Series Cables

RG cables are widely used in general RF applications including:

• Amateur radio
• Short antenna runs
• Test equipment
• Consumer electronics

Examples include RG-58, RG-174, RG-213, and RG-8. These cables vary in diameter, flexibility, shielding, and power handling.

RG cables typically offer:

• Good flexibility
• Moderate shielding
• Higher capacitance compared to premium low-loss cables
• Higher attenuation over long distances

They are suitable for shorter cable runs or cost-sensitive applications.

LMR-Series Cables

LMR-series coaxial cables are 50-ohm, low-loss, flexible coaxial cables widely used in professional RF and wireless systems.

Key construction features include:

• Solid copper-clad aluminum center conductor
• Foam Polyethylene (Foam PE) dielectric
• Dual shielding (foil + braid)
• Durable Polyethylene (PE) outer jacket

Advantages of LMR cables:

• Lower capacitance per foot
• Lower attenuation per 100 feet
• Improved shielding effectiveness
• Better high-frequency performance
• Longer viable cable runs

For example, LMR-400 offers significantly lower signal loss compared to RG-58 over the same distance, making it ideal for:

• Cellular boosters
• Base stations
• IoT gateways
• Industrial antenna systems
• Public safety communications

Practical Cable Selection Guidelines

When selecting coaxial cable for antenna systems, consider:

1. Frequency range of operation
2. Cable length
3. Required signal strength
4. Environmental exposure
5. Flexibility requirements
6. Power handling needs
7. Capacitance per foot

For short jumper cables, RG-174 or RG-58 may be sufficient. For longer antenna feeds, LMR-200 or LMR-400 typically provide superior performance due to lower capacitance and attenuation.

Final Considerations

In antenna and RF applications, selecting a coaxial cable with appropriate capacitance is essential to ensure effective transmission and reception without significant loss or distortion.

Lower capacitance improves:

• Signal integrity
• Bandwidth capability
• Impedance stability
• Phase accuracy
• Long-distance performance

Electrical properties such as capacitance, attenuation, shielding effectiveness, and impedance must be carefully evaluated when designing communication systems.

Whether deploying LTE infrastructure, 5G networks, WiFi systems, IoT installations, or RF laboratory setups, proper cable selection directly impacts system reliability and performance.