In any Radio Frequency (RF) or microwave system, the coaxial cable is the essential highway that transports signals between antennas, receivers, and transmitters. However, this highway is not perfectly frictionless. As the RF signal travels through the cable, it loses energy. This reduction in signal strength is known as coaxial cable loss or attenuation.
Understanding what causes this loss and how to mitigate it is a fundamental skill for any RF engineer calculating a system’s link budget.
We provide specialized solutions up to 40 GHz.
The Primary Causes of Coaxial Cable Loss
Signal attenuation in a coax cable doesn’t just happen for one reason. It is the cumulative result of three distinct physical phenomena:
1. Conductor Loss (The Skin Effect)
At high frequencies, electrical current does not flow evenly through the entire cross-section of the center copper conductor. Instead, it is forced to the very outer surface (the “skin”) of the wire. This is known as the skin effect. Because the effective area carrying the current is drastically reduced, the resistance increases, turning some of your valuable RF energy into heat.
2. Dielectric Loss
Between the center conductor and the outer shield is an insulating material called the dielectric (often Teflon or foam polyethylene). As the high-frequency alternating current rapidly changes the electromagnetic field, the molecules in the dielectric material vibrate. This continuous molecular friction absorbs RF energy, converting it into heat.
3. Radiated Loss and Leakage
If the outer braided shield or solid jacket of the cable is poorly manufactured or damaged, some of the electromagnetic energy can simply leak out into the surrounding environment, acting like a tiny, inefficient antenna.
How to Overcome High Cable Attenuation
Cable loss increases linearly with the length of the cable and exponentially with the frequency of the signal. So, how do we fix this?
The simplest solution is to use thicker, higher-quality cables (which have lower resistance and better dielectrics) and keep the cable runs as short as possible. However, in real-world applications—like a satellite dish on a tall roof connecting to a server room in the basement—long cable runs are unavoidable.
To overcome the severe signal degradation caused by long cables, engineers typically install low noise amplifiers directly at the antenna feed point. By amplifying the extremely weak received signal before it enters the long, lossy cable, the system preserves the signal-to-noise ratio (SNR) and ensures the receiver gets a clean, usable signal.
FAQ
Q: Does coaxial cable loss increase with frequency?
A: Yes. As the frequency of the signal increases, both the skin effect (conductor loss) and the molecular vibration in the insulator (dielectric loss) become much more severe, leading to higher total attenuation.
Q: What is the difference between insertion loss and return loss in a cable?
A: Insertion loss refers to the energy lost as the signal travels through the cable (due to heat and leakage). Return loss refers to the energy that is reflected back to the source because of impedance mismatches (like a poorly crimped connector).