Coaxial Cable Loss Calculator: What's Actually Reaching Your Antenna?

You transmit 100 watts. Your antenna is 100 feet away. How much power actually reaches the antenna? The answer depends entirely on your coax cable type, length, and operating frequency. Many hams are unknowingly losing 30-50% of their signal in feedline losses — especially at VHF/UHF.

Understanding coax loss isn't academic. It directly translates to effective power and received signal strength.

The Physics of Coax Loss

Coaxial cable loses signal through three mechanisms:

Conductor loss (resistive): The center conductor and shield have resistance. Current flowing through resistance generates heat. This is the primary loss mechanism at HF frequencies.

Dielectric loss: The insulating material between center conductor and shield absorbs RF energy and converts it to heat. This dominates at higher frequencies (VHF/UHF/microwave).

Radiation loss: A small amount of signal leaks through the shield. Minimal for quality coax with good shielding (95%+ braid coverage or solid shield).

All three losses increase with frequency. A cable that's fine at 7 MHz may be terrible at 432 MHz.

Loss Per 100 Feet for Common Cables

Here are real-world losses for popular amateur radio coax types. These are at 68°F (20°C) for new cable in good condition:

RG-58 (thin, flexible, cheap)

Frequency Loss/100ft 3.5 MHz 1.1 dB 7 MHz 1.5 dB 14 MHz 2.2 dB 28 MHz 3.2 dB 50 MHz 4.1 dB 144 MHz 6.8 dB 432 MHz 12.3 dB

Verdict: Fine for short runs at HF (<25 feet). Terrible for VHF/UHF. Acceptable for test leads and patch cables.

RG-8X (mini-8, popular compromise)

Frequency Loss/100ft 3.5 MHz 0.9 dB 7 MHz 1.3 dB 14 MHz 1.8 dB 28 MHz 2.5 dB 50 MHz 3.3 dB 144 MHz 5.7 dB 432 MHz 10.0 dB

Verdict: Good for moderate HF runs. Popular for portable/POTA because it's lighter than full-size RG-8.

RG-213/RG-8 (full-size, standard ham cable)

Frequency Loss/100ft 3.5 MHz 0.5 dB 7 MHz 0.7 dB 14 MHz 1.0 dB 28 MHz 1.5 dB 50 MHz 2.0 dB 144 MHz 3.6 dB 432 MHz 6.5 dB

Verdict: The standard for HF station feedlines. Reasonable at 6m/2m for short runs. Marginal for 70cm.

LMR-400 (low-loss, semi-rigid)

Frequency Loss/100ft 3.5 MHz 0.3 dB 7 MHz 0.5 dB 14 MHz 0.7 dB 28 MHz 1.0 dB 50 MHz 1.3 dB 144 MHz 2.2 dB 432 MHz 3.9 dB

Verdict: Excellent for VHF/UHF runs. Standard choice for 2m/70cm repeater feedlines. Good for long HF runs (150+ feet).

Hardline (7/8" or 1-5/8" rigid)

Frequency Loss/100ft 144 MHz 0.6-1.0 dB 432 MHz 1.2-1.8 dB

Verdict: Used for tower runs where minimum loss is critical. Stiff, heavy, requires special connectors. Worth it for VHF/UHF contest stations and repeater sites with long runs.

What Loss Numbers Mean in Practice

Decibels are logarithmic. Here's what dB loss means in real power terms:

Loss (dB) Power Reaching Antenna You're Losing 0.5 dB 89% 11% 1.0 dB 79% 21% 2.0 dB 63% 37% 3.0 dB 50% 50% 6.0 dB 25% 75% 10.0 dB 10% 90%

3 dB loss = half your power gone. You might as well cut your output power in half. Or equivalently, you'd need to double your transmitter power to compensate for 3 dB of cable loss.

Real-World Scenarios

Scenario 1: 100W HF Station, 75 feet of RG-213

On 20m (14 MHz):

• Loss = 0.75 × 1.0 dB = 0.75 dB
• Power at antenna = 100 × 10^(-0.75/10) = 84 watts
• Lost: 16 watts (acceptable)

On 10m (28 MHz):

• Loss = 0.75 × 1.5 dB = 1.13 dB
• Power at antenna = 77 watts
• Lost: 23 watts (still acceptable)

Scenario 2: 50W VHF Station, 100 feet of RG-213 to 2m Yagi

On 2m (144 MHz):

• Loss = 1.0 × 3.6 dB = 3.6 dB
• Power at antenna = 50 × 10^(-3.6/10) = 21.9 watts
• Lost: 28 watts! More than half your power is heating the coax!

Fix: Switch to LMR-400 → 2.2 dB loss → 30.1 watts at antenna. You just gained 8 watts (a noticeable improvement) by changing cable.

Scenario 3: 5W QRP, 50 feet of RG-58 on 20m

On 20m (14 MHz):

• Loss = 0.5 × 2.2 dB = 1.1 dB
• Power at antenna = 5 × 10^(-1.1/10) = 3.9 watts
• Lost: 1.1 watts

When you only have 5 watts, losing 1 watt hurts more psychologically than losing 20 watts from 100 watts. But the dB loss is the same relative degradation. At QRP levels, every dB counts.

SWR Makes Loss Worse

The loss figures above assume a perfectly matched load (1:1 SWR). When SWR is high, the actual loss increases because:

1. Reflected power travels back down the cable (absorbing more energy)
2. Total current in the cable is higher than matched conditions
3. Voltage peaks in the cable increase dielectric stress

Additional loss due to SWR (approximate, for already-lossy cables):

SWR Additional loss multiplier 1.5:1 ×1.03 (negligible) 2.0:1 ×1.1 3.0:1 ×1.25 5.0:1 ×1.6 10:1 ×2.5

So if your base cable loss is 2 dB and SWR is 3:1, effective loss is about 2.5 dB. At low SWR (under 2:1), the additional loss is negligible. At high SWR with lossy cable, it becomes significant.

The counterintuitive consequence: Lossy cable can hide SWR problems. If your cable has 6 dB loss, a terrible antenna with 10:1 SWR at the feedpoint appears as roughly 3:1 SWR at the radio end. The cable absorbs both the reflected power and forward power, masking the mismatch. This tricks people into thinking their antenna is fine when it's not.

When to Upgrade Cable

Upgrade to LMR-400 when:

• Operating at VHF/UHF with runs over 50 feet
• Operating at HF with runs over 150 feet
• Running QRP (every dB matters)
• Adding a masthead preamp (you want the most signal possible at the preamp input)

Stick with RG-213/RG-8 when:

• Operating at HF with runs under 100 feet
• Budget is limited and HF is your primary focus
• Flexibility matters (LMR-400 has larger bend radius)

Use RG-58/RG-8X only for:

• Patch cables under 6 feet
• Portable/temporary use where weight matters
• Test equipment connections
• Situations where the convenience outweighs the loss penalty

Hidden Losses: Connectors and Adapters

Each connector adds loss:

• Quality PL-259/SO-239: 0.02-0.05 dB at HF, 0.1-0.3 dB at UHF
• BNC: 0.02-0.04 dB at HF, 0.1 dB at VHF
• N-type: 0.01-0.03 dB through UHF (superior to PL-259 above 100 MHz)
• Barrel adapter: 0.1-0.3 dB (two connector interfaces)
• SMA: 0.05 dB typical

Every adapter you add in the signal path adds loss and a potential failure point. Minimize adapters. If you need a BNC-to-PL259 adapter permanently, consider replacing the cable end with the correct connector instead.

Corroded or poorly installed connectors can add several dB of loss. A green, oxidized PL-259 connector may add 0.5-1 dB or more. Inspect and replace weathered connectors periodically.

Measuring Your Actual Cable Loss

Don't trust published specifications for old or questionable cable. Measure it:

Method 1: Antenna analyzer

1. Short the far end of the cable
2. Measure return loss at your operating frequency with an analyzer at the near end
3. Divide return loss by 2 = one-way cable loss (approximately)

Method 2: Through measurement

1. Connect cable between two ports of a VNA (NanoVNA works)
2. Read S21 (insertion loss) at your frequency
3. This IS your cable loss (direct measurement)

Method 3: Compare with known good cable

1. Measure a signal source through the questionable cable
2. Replace with a short jumper of known-good cable
3. Difference in readings = loss attributable to the long cable

The Receive Side Matters Too

Cable loss affects receive exactly as much as transmit. If you lose 3 dB in your feedline, incoming signals are also 3 dB weaker when they reach your receiver. This is equivalent to reducing your antenna's receive gain by 3 dB.

For weak-signal VHF/UHF work, this is why masthead preamplifiers exist — amplify the signal before it enters the lossy feedline. A 15 dB preamp at the antenna followed by 4 dB cable loss means you net 11 dB of improvement. Without the preamp, those 4 dB are pure loss.