How to read coax loss figures
Coax loss, also called attenuation, is usually quoted in decibels (dB) per 100 feet at a given frequency. Lower is better. Loss increases as frequency goes up and as the cable gets longer. Thicker, better-made cable generally loses less.
The published figure is usually “matched loss,” meaning the antenna presents a perfect match. If SWR is not low, total loss is somewhat higher than the number on the chart.
Approximate loss chart (dB per 100 ft)
These are rounded, approximate values based on typical manufacturer data. Actual cable varies by brand and age, so always check the datasheet for the cable you buy.
| Frequency | RG-58 | RG-8X | RG-213 | LMR-400 |
|---|---|---|---|---|
| 10 MHz (30 m band) | ~1.4 | ~1.0 | ~0.6 | ~0.4 |
| 28 MHz (10 m band) | ~2.3 | ~1.6 | ~1.0 | ~0.6 |
| 50 MHz (6 m band) | ~3.1 | ~2.2 | ~1.3 | ~0.9 |
| 146 MHz (2 m band) | ~5.5 | ~4.0 | ~2.3 | ~1.5 |
| 446 MHz (70 cm band) | ~10 | ~7.5 | ~4.8 | ~2.7 |
Treat the numbers as guidance rather than precise specifications. Foam-dielectric and solid-dielectric versions, and cables from different makers, can differ.
Scaling loss by length
Loss is roughly proportional to length. Divide your length by 100 feet and multiply by the chart figure.
Example: 50 feet of RG-58 at 146 MHz is about half of 5.5 dB, or roughly 2.8 dB. 50 feet of LMR-400 on the same band is about 0.8 dB.
What does dB mean in power?
- 3 dB of loss means about half of the power is lost.
- 1 dB is roughly 20 percent lost.
- 10 dB means only about 10 percent reaches the antenna.
So 100 watts through 3 dB of loss leaves about 50 watts at the antenna. Because loss also applies to received signals, weak signals suffer too.
SWR adds more loss
When SWR is high, power travels back and forth in the cable and each pass loses some. The effect is small on low-loss cable with a good match, but it grows when the cable is lossy and the mismatch is large. That is one more reason to check SWR; see ham radio and antenna basics.
Choosing a cable from the chart
- HF only, short runs: RG-58 or RG-8X is often acceptable; the extra loss is small.
- HF with long runs: RG-213 or a similar low-loss cable is a common choice.
- VHF/UHF: the loss in thin cable adds up quickly. LMR-400 type cable, or larger, is commonly used for runs beyond a few tens of feet.
- Handhelds and jumpers: short pieces of thin cable are usually fine.
You may want the lowest loss when the antenna is up high, as discussed in the height of a VHF radio antenna, since height requires length. Cable types and connector choices are covered in our ham radio coax guide.
Common questions
Is a coax loss calculator more accurate? Calculators typically use the manufacturer’s attenuation data and your length and frequency, so they are handy, but they share the same limitation: they cannot know the condition of your particular cable.
Can I just use a tuner to fix loss? No. A tuner can adjust what the radio sees, but it cannot recover power lost in the cable.