LMR400 Cable Loss Calculator

Calculator Form

Frequency

MHz

Cable Length

Length Unit

Input Power

Power Unit

Cable Variant

Connector Count

Loss Per Connector

Extra Adapter Loss

Optional System Gain

Target Receiver Level

dBm

Reset

Formula Used

The calculator uses reference attenuation values for LMR400 and estimates the value at your chosen frequency by log interpolation.

Cable loss = attenuation dB per 100 ft × cable length in feet ÷ 100.
Connector loss = connector count × loss per connector.
Total loss = cable loss + connector loss + extra adapter loss.
Input dBm = 10 × log10(input watts × 1000).
Output dBm = input dBm + optional system gain - total loss.
Output watts = 10^(output dBm ÷ 10) ÷ 1000.
Efficiency = 10^(-total loss ÷ 10) × 100.
Voltage ratio = 10^(-total loss ÷ 20).

How To Use This Calculator

Enter the operating frequency in MHz.
Enter the cable length and choose feet or meters.
Enter transmitter power in watts or dBm.
Add connector count and estimated loss per connector.
Add adapter loss when splitters, jumpers, or adapters are present.
Use system gain when you want to include antenna or amplifier gain.
Press the calculate button to view the result above the form.
Download the result as CSV or PDF for records.

Example Data Table

Frequency	Length	Input Power	Connector Setup	Total Loss	Delivered Power
150 MHz	75 ft	25 watt	2 × 0.10 dB	1.325 dB	18.426379 W
900 MHz	50 ft	10 watt	2 × 0.10 dB	2.350 dB	5.821032 W
2400 MHz	30 m	30 dbm	4 × 0.12 dB	7.321 dB	0.185293 W
5800 MHz	100 ft	5 watt	2 × 0.15 dB	13.220 dB	1.503038 W

Understanding LMR400 Cable Loss

LMR400 is a popular low loss coaxial cable for radio, Wi-Fi, cellular, telemetry, and antenna feed lines. Loss still rises as frequency increases. A short run may perform well at VHF, while the same run can waste noticeable power at microwave frequencies. This calculator helps you estimate that change before buying cable, cutting cable, or choosing connectors.

Why Loss Matters

Every decibel of feed line loss reduces power at the antenna. A three decibel loss means about half the transmitter power reaches the load. The same loss also weakens received signals. That matters for weak stations, long outdoor runs, point to point links, and repeater systems. Planning loss early can prevent poor coverage, low signal reports, or unstable data links.

What The Tool Measures

The tool starts with reference attenuation values for LMR400. It interpolates between known frequency points, then scales the result by cable length. It also adds connector loss and any extra adapter loss. If you enter input power, it converts between watts and dBm. The final output shows total loss, delivered power, efficiency, voltage ratio, and optional link margin.

The example table below shows typical planning cases. You can compare them with your own entry. Small changes in frequency or length may look minor, but they can become important when a system already works near its noise limit or power budget during bad weather.

Practical Design Tips

Keep cable runs as short as practical. Use fewer adapters when possible. Select quality connectors and install them carefully. Moisture, sharp bends, damaged braid, and poor crimps can increase real world loss. For outdoor systems, use proper weather sealing and strain relief. If the loss is too high, consider a shorter path, a lower loss cable, or placing equipment closer to the antenna.

Using Results Safely

Treat the result as an estimate, not a field certification. Datasheet values usually assume standard conditions and good terminations. Real installations can vary with temperature, frequency tolerance, connector quality, and cable age. For critical systems, verify the line with a calibrated meter or network analyzer. Still, this calculator gives a strong first check. It makes design choices easier and shows the tradeoff between length, frequency, connectors, and delivered signal power.

FAQs

What does this calculator estimate?

It estimates LMR400 feed line loss, delivered power, efficiency, voltage ratio, and link margin. It also includes connector and adapter losses, so the result is closer to a real installation than a cable-only estimate.

Why does frequency change cable loss?

Coaxial cable loss increases as frequency rises. Higher frequencies create more conductor and dielectric loss. That is why a cable run may be acceptable at VHF but costly at microwave bands.

Can I use meters instead of feet?

Yes. Choose meters in the length unit field. The calculator converts meters to feet internally because the reference loss table is based on dB per 100 feet.

What connector loss value should I enter?

Use the connector datasheet when available. For planning, many users enter 0.05 to 0.20 dB per connector. Poor installation, adapters, corrosion, or moisture can raise this value.

What is delivered power?

Delivered power is the estimated power remaining after cable, connector, and extra losses. If optional system gain is entered, it is added after the loss calculation.

Why include system gain?

System gain helps when you want a wider link budget view. You can enter antenna gain or amplifier gain, then compare the final dBm value with a target receiver level.

Is the result exact?

No. It is a planning estimate. Actual loss can change with temperature, cable age, bend quality, connector workmanship, moisture, and measurement conditions.

When should I test the cable?

Test the cable for critical links, long runs, repeater sites, microwave links, or installations with many adapters. A meter or network analyzer can confirm real installed performance.