How To Calculate Coax Cable Loss
By The Calcumatix Team Reviewed by Calcumatix Editorial Review 3 min read
Quick Answer
To calculate coax cable loss, you must multiply your specific cable's attenuation rating by your total cable length. Cable manufacturers publish "Attenuation per 100 feet" ratings for specific frequencies. First, look up the attenuation rating for your exact cable type (like RG-6 or LMR-400) at your operating frequency (like 900 MHz). Next, divide your actual cable length by 100. Finally, multiply that length multiplier by the attenuation rating. For example, if your cable loses 6 dB per 100 feet, and you run 150 feet of cable, you multiply 1.5 by 6. The total loss is exactly 9 decibels (dB).
When you run a coaxial cable from a rooftop antenna down to a basement router, the signal that comes out the bottom is always weaker than the signal that went in the top. This inevitable draining of Radio Frequency (RF) energy is known as cable loss, or attenuation. If you use the wrong type of cable, or if your cable run is simply too long, the signal will degrade into useless static before it ever reaches your equipment. Engineers must carefully calculate exactly how much loss will occur to ensure the system actually works. This guide explains how to calculate coax cable loss based on your specific wire type, length, and frequency.
What Is The Coaxial Cable Attenuation Concept?
Coaxial cables transport high-frequency electrical signals through a copper core, surrounded by a plastic dielectric insulator and a metal shield. As the signal travels through this physical medium, energy is lost as heat due to the electrical resistance of the copper, and some energy leaks through the dielectric shielding.
This loss is measured in decibels (dB). In the RF engineering industry, a loss of 3 dB means you have literally lost half of your original signal power. A loss of 10 dB means you have lost exactly 90% of your power.
The two primary factors controlling loss:
- Cable length: The longer the physical wire, the more time the signal spends fighting electrical resistance. Loss scales completely linearly with length.
- Signal frequency: High-frequency signals (like a 2.4 GHz WiFi signal) bounce off the internal shielding much more frequently than low-frequency signals (like a 50 MHz ham radio signal). High frequencies suffer drastically more attenuation.
Because of these physics, a cheap piece of RG-58 cable might work perfectly for a short, low-frequency CB radio antenna, but it would completely destroy a high-frequency cellular booster signal.
The Step-By-Step Coax Cable Signal Loss Math Rules
To perform this calculation accurately, you must know exactly what frequency your equipment uses (measured in MHz or GHz) and exactly what type of coax you are pulling. You will need to reference a published attenuation chart from the cable manufacturer (like Times Microwave or Belden).
Step by step:
- Identify your operating frequency (e.g., 900 MHz).
- Identify your exact cable type (e.g., LMR-400).
- Find the manufacturer’s published “Loss per 100 feet” rating for that specific frequency and cable combination.
- Measure your total actual physical cable run length in feet.
- Divide your actual length by 100 to find your length multiplier.
- Multiply the length multiplier by the published attenuation rating.
- The result is your total signal loss in decibels (dB).
Worked example: Inputs: You are installing a 900 MHz cellular booster antenna on your roof. You are running 150 feet of RG-6 coax cable down to the indoor amplifier.
Step 1 (frequency): 900 MHz. Step 2 (cable type): RG-6. Step 3 (find rating): According to standard attenuation charts, RG-6 loses approximately 6.0 dB per 100 feet at 900 MHz.
Step 4 (measure length): 150 feet. Step 5 (length multiplier): 150 / 100 = 1.5. Step 6 (multiply): 1.5 × 6.0 = 9.0 dB.
Result: 9.0 dB of total signal loss. This is a massive amount of loss (almost 90% of the raw power). For a 150-foot run at 900 MHz, you would likely need to upgrade to a thicker, lower-loss cable like LMR-400 to maintain a usable signal. Use the coax loss calculator to run this math for your own cable type and length, and see the engineering calculators hub for related RF and electrical tools.
Sources and References
Frequently asked questions
Does the impedance of the cable matter?
Yes. You must use a cable with an impedance (ohms) that matches your radio equipment. Television and cellular antennas typically use 75-ohm cable (like RG-6 or RG-11). Ham radios and Wi-Fi antennas typically use 50-ohm cable (like RG-58 or LMR-400). If you mix a 50-ohm cable with 75-ohm equipment, you create an "impedance mismatch" that reflects signal power backward, causing additional loss.
Do connectors and splitters add to the total loss?
Yes. Every single physical connection or splice adds additional attenuation. A standard barrel connector (connecting two cables together) typically adds about 0.5 dB of loss. A standard two-way signal splitter typically adds an immediate 3.5 dB of loss to both output ports. You must add these hardware losses to your baseline cable loss.
What is the difference between RG-6 and LMR-400?
RG-6 is a thin, flexible, cheap 75-ohm cable designed for low-frequency television signals over short distances. LMR-400 is a thick, rigid, expensive 50-ohm cable designed specifically to preserve high-frequency signals over very long distances. LMR-400 has significantly better shielding and a much thicker copper core, resulting in drastically lower attenuation.
Can I just use a signal amplifier to fix cable loss?
Sometimes. A signal amplifier pushes extra power through the line to overcome the resistance of the cable. However, an amplifier also amplifies the "noise" (static interference) in the line. If the raw signal from the antenna is extremely weak, the amplifier will simply amplify the static, resulting in a perfectly strong, completely unreadable signal.
How does weather affect coaxial cable loss?
Standard coaxial cable is shielded from minor temperature fluctuations. However, if water physically penetrates the outer jacket of the cable and soaks into the dielectric foam, it drastically changes the electrical capacitance of the wire. Water-logged coax will suffer massive, unpredictable signal loss and must be completely replaced.