In the previous article, we explored the basic structure of RF Coaxial Cables and their two core parameters: Characteristic Impedance and Attenuation. However, when facing a real-world project, you will encounter various cable types on the market, such as RG-58, LMR-400, or more specialized semi-rigid cables.
The focus of this article is to interpret the differentiated selection criteria among these various cable types. For any communication system, choosing the wrong cable is like choosing the wrong pipeline—it can lead to decreased performance at best, and system failure at worst. Our core challenge is to answer: How do you make the optimal trade-off between performance, cost, and physical characteristics?
Deep Dive: Comparison of Three Main RF Cable Types
RF coaxial cables can be broadly categorized into three main types based on their structure, dielectric material, and shielding design. Understanding their characteristics is fundamental to proper selection.
Cable Type
Structural Features
Typical Applications
Key Advantages
Key Disadvantages
RG Series (e.g., RG-58, RG-174)
Standard design, uses solid PE dielectric, braided shield.
Lab testing, general patch cords, short distances, low-requirement antenna feeders.
Lowest cost, highly flexible, easy to bend and install.
Higher loss (attenuation), performance rapidly degrades at high frequencies (>1 GHz).
Outer conductor is a solid copper or aluminum tube, dielectric is PTFE.
Aerospace, military, internal microwave circuit board connections, high-precision test equipment.
Minimal loss, extremely high frequency limit excellent phase stability.
Must be pre-bent into shape using specialized tools, difficult to install.
1. RG Series Cables (Radio Guide)
The RG series is the oldest and most widely used family of RF cables. They are simple in structure, cost-effective, and highly flexible. However, their main drawback is significant signal attenuation at high frequencies (UHF and above) and over long distances. They are generally suitable for general connections where performance requirements are less demanding.
2. LMR Series (Low-Loss)
LMR (or equivalent low-loss) cables are specifically designed for outdoor applications that require long-distance transmission or high frequencies. They use a foamed dielectric to lower the dielectric constant , which reduces signal loss, and employ thicker, more comprehensive shielding structures to improve anti-interference capabilities. If your link budget is tight, or the signal transmission distance exceeds 10 meters, the LMR series is often the optimal choice.
3. Semi-Rigid/Conformable Cables
These cables are the "high-performance sports cars" of RF systems. They offer superb electrical performance, especially in the microwave frequency range. The solid metal outer conductor provides near-perfect shielding, while the PTFE dielectric ensures the lowest possible dielectric loss and excellent phase stability (meaning the electrical length of the cable changes minimally when bent), which is crucial for high-precision phased arrays or test and measurement equipment.
Key Technical Detail Comparison
The performance differences between cable types stem fundamentally from their dielectric materials and shielding structure design.
1. Influence of Dielectric Material:
Solid PE (Polyethylene): Common in the RG series. Low cost, but relatively high loss and poor high-temperature resistance.
Foamed PE (Polyethylene): The core of the LMR series. By introducing air bubbles into the PE, the dielectric constant is effectively lowered. This allows the signal to propagate faster and significantly reduces cable attenuation, making it key to low-loss cables.
PTFE (Polytetrafluoroethylene): Used in semi-rigid and military cables. Features extremely low dielectric loss and can withstand higher temperatures and frequencies.
2. Shielding Structure:
Braiding: Provides good flexibility, but shielding effectiveness can decrease as frequency increases.
Foil + Braid: The standard configuration for LMR lines. The aluminum foil provides 100% coverage, effectively blocking high-frequency interference, while the outer braid provides mechanical strength and additional low-frequency shielding.
Solid Tube: Used in semi-rigid cables. Offers the best shielding performance, minimizing signal leakage, and is the highest performance option.
How to Choose Based on Application Scenario
Correct cable selection is a process of balancing three factors: Performance (Attenuation/Frequency), Cost, and Physical Flexibility.
Scenario
Core Requirement
Recommended Cable Type
Rationale
Outdoor Base Station Antenna Connection
Low loss, weather resistance, long distance.
LMR Series
Ensures that even weak signals reach the base station equipment efficiently; low attenuation is the primary metric.
Internal Test Patch Cord (Within 2 meters)
Flexibility, controllable cost.
RG Series
When frequency is below 1 GHz, the attenuation impact of RG cables is minimal, and installation is convenient.
Internal High-Frequency Connection
High frequency limit, phase stability.
Semi-Rigid/Conformable
Guarantees signal integrity and consistency, suitable for precise connections between RF modules.
Surveillance Video Systems (CATV)
75Ω impedance, moderate cost.
RG-6 / RG-11
Specific 75Ωcables that provide the required high bandwidth and low cost for video transmission.
Summary: Core Principles of Selection
There is no absolute "best" RF cable; only the most suitable one. The core principle of selection is: finding the optimal balance between cost and physical flexibility, while satisfying the performance requirements (frequency, attenuation, VSWR).
If the budget is tight and the distance is short: Consider the RG series.
If the distance is long and high efficiency is required: You must upgrade to LMR or other low-loss series.
If extreme precision and high frequency are required: Consider semi-rigid cables.
Shenzhen Keesun Technology Co.,Ltd was founded in Aug of 2012, a high-tech enterprise specializing in various types of antenna and network cable manufacturing.
