What Is Coax Loss and Why It Matters for Ham Radio
Understanding Coaxial Cable Attenuation
Coaxial cable is not a perfect conductor — resistive losses in the centre conductor and braid, plus dielectric losses in the insulation, convert RF energy into heat rather than radiation. This conversion of usable RF power into waste heat is what we call coaxial cable attenuation or coax loss, and it is measured in decibels (dB). Every length of coax you install between your radio and your antenna will absorb some portion of your transmitted signal, and that loss is permanent — you cannot recover those watts at the antenna end.
There are two primary mechanisms that drive coaxial cable loss. Coax loses signal three ways: conductor resistance dominates at HF, dielectric absorption dominates at VHF and above, and shield radiation stays minimal on quality cable. Understanding which mechanism dominates at your operating frequency helps you make smarter cable choices from the start.
How Signal Loss Affects Your Station Performance
Your transceiver is only as good as the coax connecting it to your antenna. Cheap or incorrectly chosen feedline doesn't just cost you signal — it can waste watts as heat, introduce RF into your shack, and fail completely after a single season outdoors.
The practical impact on your station is direct and measurable. For a 100W transmitter: 1 dB loss means 79W reaches the antenna — barely noticeable; 2 dB loss means 63W reaches the antenna — acceptable for most uses; 3 dB loss means 50W reaches the antenna — like losing half your power; 6 dB loss means 25W reaches the antenna — a significant problem. Critically, the damage is not limited to transmitting. Remember that losses are the same for receiving, so cable loss reduces your ability to hear weak signals as well.
The Relationship Between Frequency and Coax Loss
Cable loss increases with frequency. The same cable that has 1 dB loss at 10 MHz might have 5 dB loss at 450 MHz. This is why VHF/UHF installations require higher quality, lower loss cable than HF installations. This relationship is not linear — it follows a curve that rises progressively faster at higher frequencies, making cable selection increasingly critical as you move from HF to VHF, UHF, and microwave bands. Both conductor loss and dielectric loss increase with frequency — which is why a cable that performs well at HF (below 30 MHz) may be completely unsuitable at 2.4 GHz.
How to Read a Coax Loss Chart
Units of Measurement: Decibels Per 100 Feet
Coaxial cable attenuation data is listed as signal loss in dB per 100 feet for various cable types across a frequency range. This standardized unit makes comparison between cable types straightforward: simply look up your frequency column and read the dB/100 ft value for the cable you're considering. The dB scale is logarithmic, not linear. It's logarithmic — 1 dB = barely noticeable, 3 dB = half your power, 10 dB = 90% gone.
Frequency Bands and Their Impact on Attenuation
In any coax loss chart, you'll notice that the dB values increase as you move across the frequency columns from left (low frequencies like 3.5 MHz) to right (high frequencies like 1296 MHz). This is the fundamental characteristic of all coaxial feedlines — they are inherently lossy at higher frequencies regardless of quality. The difference between cable types is how steeply that loss curve rises. Premium cables like LMR-400 and LMR-600 have shallower loss curves, meaning they maintain their advantage over cheaper cables by an ever-wider margin as frequency increases.
Calculating Total Loss for Your Feedline Length
Loss scales linearly with cable length. If 25 feet of RG-58 at 144 MHz loses 1.5 dB, then 50 feet will lose 3.0 dB, and 100 feet will lose 6.0 dB. To find the total loss for your specific feedline, multiply the dB/100 ft value from the chart by your actual length in hundreds of feet. For example, if you have 150 feet of LMR-400 operating at 146 MHz, and LMR-400 shows 1.5 dB/100 ft at that frequency, your total matched line loss is 1.5 × 1.5 = 2.25 dB. Actual loss increases with cable age, UV exposure, moisture ingress, tight bends, and poor connector workmanship. Measured loss in an installed cable run is typically 10–20% higher than book values.
Coax Loss Chart: Popular Cable Types Compared
The following data tables are compiled from manufacturer datasheets and published attenuation references. Cable type designations like RG-58 or RG-213 describe a general specification, not a single exact product, so the same nominal cable type can vary between manufacturers depending on conductor material, dielectric, and build quality. Use these figures for planning purposes and consult the specific manufacturer datasheet for your exact cable before making a final engineering decision.
RG-8X Coax Loss by Frequency
RG-8X is a flexible mini-8 cable, easier to route than RG-8 but with higher loss. It's a good compromise for portable and short runs. RG-8X (50 ohm) is about 0.24 inch in diameter, suitable for medium power (~350 watts) at HF and Lo-VHF. Based on published attenuation data, RG-8X shows approximately 0.2 dB/100 ft at 1 MHz, rising to 3.0 dB/100 ft at 100 MHz, 4.5 dB/100 ft at 200 MHz, and 8.6 dB/100 ft at 450 MHz.
| Frequency | dB / 100 ft |
|---|---|
| 1 MHz | 0.20 |
| 10 MHz | 0.78 |
| 50 MHz | 2.00 |
| 100 MHz | 3.00 |
| 144 MHz (2m) | 4.70 |
| 200 MHz | 4.50 |
| 450 MHz (70cm) | 8.60 |
| 900 MHz | 12.80 |
RG-213 Coax Loss by Frequency
RG-8 and RG-213 are standard 50-ohm ham radio cable. Good for HF, acceptable for VHF, and heavy and stiff but durable. RG-8 and RG-213 (50 ohm) are about 0.405 inch in diameter, suitable for higher power (~1800 watts) at HF. RG-213 is a workhorse HF cable found in shacks worldwide. Its loss is comparable to RG-8X at lower frequencies but begins to show a steeper rise above 100 MHz.
| Frequency | dB / 100 ft |
|---|---|
| 1 MHz | 0.17 |
| 10 MHz | 0.55 |
| 50 MHz | 1.30 |
| 100 MHz | 1.90 |
| 144 MHz (2m) | 2.80 |
| 200 MHz | 2.50 |
| 450 MHz (70cm) | 5.20 |
| 900 MHz | 8.00 |
LMR-400 Coax Loss by Frequency
The LMR-400 is a 50-ohm, low-loss coaxial cable originally developed as a higher-performance replacement for legacy RG-8 and similar coaxial cables. LMR-400 is widely recognized for its exceptionally low attenuation compared to traditional RG antenna cables. Its loss performance approaches that of semi-rigid and some hardline cables, while retaining flexibility. The following figures come directly from the Times Microwave LMR-400 datasheet:
