Learn about coaxial cable connectors—F-Type, BNC, N-Type, SMA, TNC, and UHF. Compare impedances, frequencies, and...
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.
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.
| Property | Resistance | Impedance |
|---|---|---|
| Definition | Opposition to DC current | Opposition to AC current |
| Frequency Dependent | No | Yes |
| Symbol | R | Z |
| Unit | Ohms (Ω) | Ohms (Ω) |
| Components | Pure resistance | Resistance + Reactance |
Impedance matters because it determines how efficiently signals are transferred from one device to another through the cable.
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.
Mismatched impedance causes signal reflections that distort the original signal. This results in:
In high-power applications, reflected signals can damage transmitters. A mismatched antenna feed can cause a transmitter to overheat and fail.
A coaxial cable's impedance is determined by three physical factors:
Z = (138 × log₁₀(D/d)) ÷ √(εᵣ)
Where:
| Factor | Effect on Impedance | Real-World Impact |
|---|---|---|
| Center Conductor (d) | Smaller conductor = Higher impedance | RG-59 (22 AWG) is 75Ω; RG-58 (20 AWG) is 50Ω |
| Shield Diameter (D) | Larger shield = Lower impedance | RG-6 (6.9mm) vs RG-11 (10.3mm) – both 75Ω |
| Dielectric Constant (εᵣ) | Higher constant = Lower impedance | Air (1.0) vs PE (2.26) – air gives higher impedance |
There are two primary impedance standards for coaxial cables: 50Ω and 75Ω.
Why 50Ω?
50Ω represents the optimal balance between:
Applications:
Common 50Ω Cables:
Why 75Ω?
75Ω offers the lowest possible signal loss for a given cable size. This makes it ideal for:
Applications:
Common 75Ω Cables:
When you connect a 50Ω cable to a 75Ω device (or vice versa), you create an impedance mismatch.
At the point where the impedances change, some of the signal is reflected back toward the source. This reflected signal:
The reflected signal means less signal reaches the destination. The loss from a direct 50Ω to 75Ω mismatch is about:
SWR measures how much power is reflected. A perfect match is 1:1. A 50Ω/75Ω mismatch gives about 1.5:1 SWR.
In high-power applications (radio transmitters), reflected power can:
| Scenario | SWR | Power Loss | Signal Impact |
|---|---|---|---|
| Perfect Match (50Ω→50Ω) | 1:1 | 0% | Optimal |
| Mild Mismatch (50Ω→60Ω) | 1.2:1 | 0.8% | Negligible |
| Standard Mismatch (50Ω→75Ω) | 1.5:1 | 4% | Noticeable |
| Severe Mismatch (50Ω→100Ω) | 2:1 | 11% | Significant |
Measuring cable impedance requires specialized equipment. Here are the methods:
What it is: Sends a pulse down the cable and measures reflections
Accuracy: Very high
Cost: High ($1,000+)
Common Use: Professional installations, troubleshooting
What it is: Measures S-parameters to calculate impedance
Accuracy: Extremely high
Cost: Very high ($5,000+)
Common Use: Lab testing, manufacturing
What it is: Compares cable to known standards
Accuracy: High
Cost: Moderate ($500-$2,000)
Common Use: Professional testing
What it is: Continuity and basic impedance check
Accuracy: Low to moderate
Cost: Low ($50-$200)
Common Use: Quick field checks
What it is: Measures DC resistance only
Accuracy: Not applicable (doesn't measure impedance)
Cost: Low ($20-$100)
Common Use: NOT for impedance measurement
Connectors have their own impedance ratings and are NOT interchangeable between 50Ω and 75Ω systems.
| Connector | Impedance | Common Use |
|---|---|---|
| N-Type | 50Ω | Outdoor RF, WISP, Cellular |
| SMA | 50Ω | WiFi, IoT, Test Equipment |
| BNC (50Ω) | 50Ω | Test equipment, Radios |
| TNC | 50Ω | Military, Aerospace |
| Connector | Impedance | Common Use |
|---|---|---|
| F-Type | 75Ω | CATV, Satellite, Broadband |
| BNC (75Ω) | 75Ω | Video, Broadcast, CCTV |
| RCA | 75Ω | Consumer Video/Audio |
Sometimes you must connect 50Ω and 75Ω equipment. In these cases, you need an impedance adapter or transformer.
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
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
| Scenario | Recommended | Alternative |
|---|---|---|
| Connecting 50Ω cable to 75Ω TV | ❌ No | Use 75Ω cable instead |
| Connecting 75Ω cable to 50Ω radio | ❌ No | Use 50Ω cable instead |
| Testing 50Ω device with 75Ω test equipment | ⚠️ Maybe | Use adapter and account for loss |
| Professional broadcast installation | ✅ Yes (with quality balun) | Re-cable with correct impedance |
For those who want to go deeper, here's the full formula:
Z₀ = √(L/C) = (138 / √εᵣ) × log₁₀(D/d)
Where:
| Parameter | Value |
|---|---|
| D (Shield ID) | 4.57 mm |
| d (Conductor OD) | 1.02 mm (18 AWG) |
| D/d Ratio | 4.48 |
| log₁₀(D/d) | 0.651 |
| εᵣ (Foam PE) | 1.5 |
| √εᵣ | 1.22 |
| 138 / 1.22 | 113.1 |
| Z₀ | 73.6 Ω (≈75Ω) |
While 50Ω and 75Ω are the most common, other impedances exist:
| Impedance | Common Use | Example Cables |
|---|---|---|
| 50Ω | RF, Radio, Wireless | RG-58, RG-8, LMR-400 |
| 75Ω | Video, CATV, Broadcast | RG-6, RG-11, RG-59 |
| 93Ω | Older Digital Systems | RG-62 (ARCnet, IBM) |
| 100Ω | Ethernet (Twisted Pair) | Cat5e, Cat6 (not coax) |
| 300Ω | Old TV Antennas | Twin-lead (not coax) |
| 600Ω | Telephone / Audio | Bell System (historical) |
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.
50Ω offers the best balance of power handling and low loss for RF applications. 75Ω offers minimum signal loss for video and broadband applications.
You'll get signal reflections, loss, and poor performance. In high-power applications, you can damage equipment.
No. A multimeter measures DC resistance, which is different from AC impedance. You need specialized equipment like a TDR or VNA.
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.
RG-6 is 75Ω. It's designed for video and CATV applications.
LMR-400 is 50Ω. It's designed for RF and wireless applications.
You can, but you'll lose about 4% of the signal (0.18 dB). It's not recommended for professional installations.
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.
A balun (balanced-unbalanced) is a device that matches impedance between different systems, like 50Ω to 75Ω. It also balances/unbalances signals.
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.
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.
We stock professional coaxial cables in both 50Ω and 75Ω impedances. Whether you need RG-6 for video or LMR-400 for wireless, we have the right cable for your application.
Last Updated: July 2026
© 2026 Wassalat. All rights reserved.
Complete Guide to Coaxial Cables
50 Ohm vs 75 Ohm Coaxial Cables: What's the Difference?
RG6 vs RG11 Coaxial Cables: Which One Do You Need?
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