Published on 10/13/2026
A product team wants to know if a 6 m run of RG58 between a 900 MHz radio and its antenna is acceptable. The online calculators give three different answers, because they use different connector loss and different cable data. You can do the same estimate by hand in three lines, and know exactly where the number comes from.
Short answer: total loss in dB equals cable loss per meter times length, plus connector losses, plus a margin. For 6 m of RG58 at 900 MHz that is about 6 x 0.58 = 3.5 dB plus 0.2 dB for two connectors, around 3.7 dB. That leaves about 43 percent of the power at the antenna.
A blog page cannot run a live calculator, so this guide gives the method, a lookup table and worked examples you can follow with a pocket calculator. For a custom estimate, our engineers can run the numbers on request.
On This Page
Step 01
Get dB per meter at your frequency
Use the attenuation chart in this series, or the cable datasheet at your highest frequency.
Step 02
Multiply by length
Cable loss (dB) = dB per meter x length in meters.
Step 03
Add connectors and adapters
Use 0.1 to 0.2 dB per mated pair for SMA, N and BNC below 6 GHz, and 0.2 to 0.3 dB above.
Step 04
Add margin
Add 10 percent for bends, aging and cable spread. Convert to power with 10^(-dB/10).
| Cable at 2.4 GHz | dB/m | 1 m | 3 m | 5 m | 10 m |
|---|---|---|---|---|---|
| RG174 | 1.7 | 1.7 dB | 5.1 dB | 8.5 dB | 17 dB |
| RG316 | 1.4 | 1.4 dB | 4.2 dB | 7.0 dB | 14 dB |
| RG58 | 1.2 | 1.2 dB | 3.6 dB | 6.0 dB | 12 dB |
| RG213 | 0.74 | 0.74 dB | 2.2 dB | 3.7 dB | 7.4 dB |
| LMR-240 class | 0.60 | 0.60 dB | 1.8 dB | 3.0 dB | 6.0 dB |
| LMR-400 class | 0.36 | 0.36 dB | 1.1 dB | 1.8 dB | 3.6 dB |
Values are typical planning figures. Other frequencies are in the attenuation chart. Scale between frequencies with the square root rule: a(f2) = a(f1) x sqrt(f2 / f1).
Key idea: Percent power left is 100 x 10^(-dB/10). 3 dB is 50 percent, 6 dB is 25 percent, 10 dB is 10 percent, 20 dB is 1 percent.
Send frequency, length and connectors
Our engineers will give you a loss estimate and a cable recommendation. Every assembly is VNA-swept for VSWR and insertion loss before shipment.
Talk to an EngineerLoss in coax has two parts: conductor loss, which grows with the square root of frequency, and dielectric loss, which grows linearly with frequency. In small solid-dielectric cables conductor loss dominates up to a few GHz, so the square root rule is a good approximation. At millimeter wave, dielectric loss and surface roughness add more.
The mismatch between a cable and its load adds a small extra term, and standing waves make loss slightly higher than the matched value. At VSWR 1.5:1 the extra loss is under 0.1 dB for a low-loss cable, and bigger for a lossy one.
Avoid
How do I calculate coax cable loss?
Multiply the cable dB per meter at your frequency by the length, add connector loss, and add a small margin.
How much loss does 10 m of RG58 have at 2.4 GHz?
About 12 dB, which leaves about 6 percent of the power.
How do I convert dB loss to percent power?
Power remaining is 100 times 10 to the power of minus loss divided by 10.
How do I scale loss to another frequency?
Multiply by the square root of the frequency ratio.
What is a typical connector loss?
About 0.1 to 0.2 dB per mated pair for SMA, N and BNC below 6 GHz.
Is a calculator exact?
No. It gives an estimate. Measure the finished assembly on a VNA for the true value.
Cable loss is a three-line calculation: dB per meter, times length, plus connectors. Do it for the highest frequency in your band, add margin, and measure the finished assembly.
SigmaRF measures insertion loss on every assembly in India (Vadodara) and the USA (Cleveland area), with sweep data across the band and lead times as short as 2 days.
Order an assembly with measured loss
Standard or custom, VNA-verified.
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