Coaxial Cable Impedance: The Complete Guide

Published by Wassalat Technical Team

Impedance is arguably the most important electrical property of a coaxial cable. Yet, it's also one of the most misunderstood concepts in cabling.

This comprehensive guide explains what impedance is, why it matters, how it's determined, and why matching impedance is critical for signal quality. Whether you're a professional installer or a DIY enthusiast, understanding impedance will help you make better cable choices.

What Is Impedance?

Impedance is the measure of opposition that a cable presents to the flow of alternating current (AC). In simple terms, it's like the "electrical resistance" of the cable, but for AC signals rather than DC.

Unlike resistance (which is constant), impedance depends on frequency. A cable might have an impedance of 50Ω at 100 MHz, but that same cable would have a different impedance at 1 GHz.

Impedance vs Resistance

Why Impedance Matters

Impedance matters because it determines how efficiently signals are transferred from one device to another through the cable.

Three Key Reasons

1. Maximum Power Transfer

Maximum power is transferred when the source impedance equals the load impedance. This is called "impedance matching." If the impedances don't match, some power is reflected back rather than delivered to the load.

2. Signal Integrity

Mismatched impedance causes signal reflections that distort the original signal. This results in:

• Ghosting in video
• Errors in data
• Distortion in audio
• Reduced range in wireless systems

3. Equipment Protection

In high-power applications, reflected signals can damage transmitters. A mismatched antenna feed can cause a transmitter to overheat and fail.

How Impedance Is Determined

A coaxial cable's impedance is determined by three physical factors:

The Formula

Z = (138 × log₁₀(D/d)) ÷ √(εᵣ)

Where:

• Z = Impedance (in Ohms)
• D = Inner diameter of the shield (mm or inches)
• d = Diameter of the center conductor (mm or inches)
• εᵣ = Dielectric constant (relative permittivity)

The Three Factors Explained

Visual Representation

The Standards: 50Ω vs 75Ω

There are two primary impedance standards for coaxial cables: 50Ω and 75Ω.

50Ω – The RF Standard

Why 50Ω?

50Ω represents the optimal balance between:

• Power handling – Maximum power transfer occurs at 50Ω
• Signal loss – Minimum attenuation occurs at 50Ω

Applications:

• Two-way radios
• WiFi and wireless networks
• Amateur radio
• RF test equipment
• LTE and 5G base stations
• Antenna feeds

Common 50Ω Cables:

• RG-58, RG-8, RG-213
• LMR-195, LMR-240, LMR-400, LMR-600
• Belden 9913, 9914

75Ω – The Video Standard

Why 75Ω?

75Ω offers the lowest possible signal loss for a given cable size. This makes it ideal for:

• Long-distance video transmission
• High-bandwidth data
• Broadcast applications

Applications:

• Cable TV (CATV)
• Satellite TV
• CCTV and surveillance
• Broadcast video
• Digital audio

Common 75Ω Cables:

• RG-6, RG-11, RG-59
• Belden 1694A, 1505A
• High-density plenum cables

What Happens When Impedance Mismatches?

When you connect a 50Ω cable to a 75Ω device (or vice versa), you create an impedance mismatch.

The Effects

1. Signal Reflections

At the point where the impedances change, some of the signal is reflected back toward the source. This reflected signal:

• Reduces the power reaching the destination
• Interferes with the forward signal
• Creates standing waves

2. Signal Loss

The reflected signal means less signal reaches the destination. The loss from a direct 50Ω to 75Ω mismatch is about:

• 0.18 dB (about 4% power loss)
• This may not sound like much, but in long runs, it adds up

3. Standing Wave Ratio (SWR)

SWR measures how much power is reflected. A perfect match is 1:1. A 50Ω/75Ω mismatch gives about 1.5:1 SWR.

4. Equipment Damage

In high-power applications (radio transmitters), reflected power can:

• Overheat the transmitter
• Reduce transmitter lifespan
• Cause premature failure

Visual Example

How to Measure Impedance

Measuring cable impedance requires specialized equipment. Here are the methods:

1. Time Domain Reflectometry (TDR)

What it is: Sends a pulse down the cable and measures reflections

Accuracy: Very high

Cost: High ($1,000+)

Common Use: Professional installations, troubleshooting

2. Vector Network Analyzer (VNA)

What it is: Measures S-parameters to calculate impedance

Accuracy: Extremely high

Cost: Very high ($5,000+)

Common Use: Lab testing, manufacturing

3. Impedance Bridge

What it is: Compares cable to known standards

Accuracy: High

Cost: Moderate ($500-$2,000)

Common Use: Professional testing

4. Cable Tester (Simple)

What it is: Continuity and basic impedance check

Accuracy: Low to moderate

Cost: Low ($50-$200)

Common Use: Quick field checks

5. Multimeter (Not Recommended!)

What it is: Measures DC resistance only

Accuracy: Not applicable (doesn't measure impedance)

Cost: Low ($20-$100)

Common Use: NOT for impedance measurement

Connector Impedance

Connectors have their own impedance ratings and are NOT interchangeable between 50Ω and 75Ω systems.

50Ω Connectors

75Ω Connectors

Impedance Adapters and Transformers

Sometimes you must connect 50Ω and 75Ω equipment. In these cases, you need an impedance adapter or transformer.

Types of Adapters

1. Impedance Matching Transformer (Balun)

What it does: Converts impedance from 50Ω to 75Ω or vice versa

Loss: Adds 0.5-1.5 dB of loss

Cost: $10-$50

Use: When you must connect mismatched equipment

2. Tapered Adapter

What it does: Gradually transitions from 50Ω to 75Ω

Loss: Lower than balun (0.3-1.0 dB)

Cost: $20-$100

Use: Professional applications requiring less loss

Should You Use Adapters?

Impedance Calculation Formula

For those who want to go deeper, here's the full formula:

The Full Formula

Z₀ = √(L/C) = (138 / √εᵣ) × log₁₀(D/d)

Where:

• Z₀ = Characteristic impedance (in Ohms)
• L = Inductance per unit length
• C = Capacitance per unit length
• εᵣ = Relative permittivity (dielectric constant)
• D = Inner diameter of the shield
• d = Diameter of the center conductor

Example Calculation: RG-6

Common Impedance Values

While 50Ω and 75Ω are the most common, other impedances exist:

Frequently Asked Questions (FAQ)

1. What is cable impedance?

Impedance is the opposition a cable presents to AC current. It's the ratio of voltage to current, measured in ohms (Ω). Coaxial cables are typically 50Ω or 75Ω.

2. Why are there different impedances?

50Ω offers the best balance of power handling and low loss for RF applications. 75Ω offers minimum signal loss for video and broadband applications.

3. What happens if I use the wrong impedance cable?

You'll get signal reflections, loss, and poor performance. In high-power applications, you can damage equipment.

4. Can I measure impedance with a multimeter?

No. A multimeter measures DC resistance, which is different from AC impedance. You need specialized equipment like a TDR or VNA.

5. What is characteristic impedance?

Characteristic impedance (Z₀) is the impedance that a cable presents to a signal. It's determined by the cable's physical construction and remains constant regardless of cable length.

6. Is RG-6 50Ω or 75Ω?

RG-6 is 75Ω. It's designed for video and CATV applications.

7. Is LMR-400 50Ω or 75Ω?

LMR-400 is 50Ω. It's designed for RF and wireless applications.

8. Can I use a 75Ω cable with a 50Ω antenna?

You can, but you'll lose about 4% of the signal (0.18 dB). It's not recommended for professional installations.

9. What is VSWR?

VSWR (Voltage Standing Wave Ratio) measures the amount of reflected power in a system. A perfect match is 1:1. Higher numbers mean more reflection.

10. What is a balun?

A balun (balanced-unbalanced) is a device that matches impedance between different systems, like 50Ω to 75Ω. It also balances/unbalances signals.

11. Does cable length affect impedance?

No. Characteristic impedance is independent of cable length. A 1-foot cable of RG-6 has the same impedance as a 1000-foot cable of RG-6.

12. How can I tell if my cable is 50Ω or 75Ω?

Check the markings on the cable jacket. If there are no markings, measure the conductor diameter and shield diameter, then calculate or look up the type.

? Related Articles

• Complete Guide to Coaxial Cables
• What Is a Coaxial Cable?
• 50 Ohm vs 75 Ohm Coax
• Coaxial Cable Construction
• RG6 vs RG11
• RG58 vs RG59
• Browse Coaxial Cables →

Last Updated: July 2026

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