coax loss calculator

What this coax loss calculator does

This coax loss calculator helps you estimate how much RF signal is lost along a coaxial cable run. In radio systems, every dB matters. Whether you are setting up a Wi-Fi bridge, cellular modem, SDR receiver, amateur radio station, GNSS feedline, or LoRa gateway, too much feedline attenuation can quietly reduce performance.

The calculator combines:

• Cable attenuation (dB per 100 ft),
• Operating frequency in MHz,
• Total run length, and
• Connector/adaptor insertion loss.

It then outputs total estimated line loss in dB, plus approximate remaining power and voltage percentages.

How coax loss is calculated

1) Cable attenuation scales with length

Manufacturers usually publish coax loss as dB per 100 ft at specific frequencies. If your cable is 50 ft and the datasheet says 6 dB per 100 ft, cable loss is approximately:

6 × (50 / 100) = 3 dB

2) Frequency has a major impact

Coax attenuation increases as frequency rises. A cable that looks fine at 30 MHz can become very lossy at 2.4 GHz. This calculator uses typical attenuation data points and interpolates between them for your chosen frequency.

3) Connector loss is additive

Each connector, adapter, lightning arrestor, or bulkhead transition adds insertion loss. A simple model is:

Total loss = cable loss + (number of connectors × loss per connector)

4) Converting dB to remaining power

Decibels are logarithmic. The remaining power fraction after a loss L is:

Power ratio = 10^-L/10

Example: 3 dB loss means roughly 50% of power remains.

How to use the calculator effectively

• Select a cable type that matches your real feedline.
• Enter the actual operating frequency in MHz, not the advertised band name.
• Include full physical run length with service loops and vertical drops.
• Count every RF transition (connectors, adapters, arrestors, couplers).
• Use realistic connector loss values (0.05 to 0.3 dB each is common depending on quality and frequency).

Choosing the right coax for your project

Short runs at lower frequencies

For short HF or VHF runs, lower-cost cables can work well. The absolute dB penalty might be small enough to ignore.

Long runs or high-frequency systems

For UHF, 900 MHz, 1.8 GHz, 2.4 GHz, and above, cable choice becomes critical. Upgrading from RG-58 to LMR-400 (or better) can recover multiple dB and significantly improve link margin.

System-level tradeoffs

• Sometimes moving the radio closer to the antenna is better than buying premium coax.
• Active devices near the antenna can reduce effective feedline penalty.
• Antenna gain, cable loss, and connector quality should be optimized together.

Real-world factors this estimate does not fully model

This tool is intentionally practical and fast. It does not fully model every RF nuance. In critical designs, also consider:

• VSWR mismatch loss and return loss,
• Temperature drift,
• Water ingress and aging,
• Bend radius violations and crushing,
• Frequency-dependent connector behavior,
• Manufacturing tolerance between cable brands.

Quick interpretation guide

• < 1 dB: excellent feedline performance.
• 1 to 3 dB: generally acceptable for many systems.
• 3 to 6 dB: noticeable performance impact.
• > 6 dB: often worth redesigning (shorter run, better coax, fewer connectors).

Example use case

Suppose you have a 20 m run of LMR-240 at 2.4 GHz with four transitions at 0.15 dB each. The cable loss alone may already be several dB, and connectors add another 0.6 dB. Total feedline loss can easily approach the point where antenna gain is partly canceled out. Running the same link with a larger, lower-loss cable may recover valuable margin.

Bottom line

Coax loss is one of the most overlooked causes of weak RF performance. Use this calculator early in your design process to avoid surprises, and verify with manufacturer datasheets when building high-performance or long-distance systems.