Coax Loss Calculator: Feedline Attenuation by Cable Type & Frequency

Q: How much coax loss is acceptable for ham radio?

Under 1 dB of total feedline loss is considered excellent, 1 to 3 dB is generally acceptable for most amateur use, and anything over 3 dB means more than half your transmitter power is being lost in the cable before it ever reaches the antenna. The same loss applies on receive, reducing the weakest signals you can copy by the same amount.

Q: Does SWR increase coax loss?

Yes, though the effect is smaller than many people assume at the SWR levels most stations actually run. A reasonably well-matched line, under about 2:1 SWR, adds only a small amount of extra loss on top of the cable's matched loss, commonly cited as roughly a few tenths of a dB. The effect grows at higher SWR and compounds with the cable's own base loss, so a lossy cable with a poor match is worse than either problem alone.

Q: Is RG-8X good enough for HF, or do I need LMR-400?

For HF and short to moderate cable runs, RG-8X is generally adequate. Its higher per-foot loss compared to RG-213 or LMR-400 matters far more at VHF and UHF frequencies, and over longer cable runs, than it does on HF below 30 MHz, where even a meaningfully lossier cable still loses only a fraction of a dB over a typical 50 to 100 foot run. LMR-400 earns its higher cost primarily at VHF/UHF and on long runs, not on a short HF feedline.

⚡ Instant Calculator

Feedline Loss by Cable, Length & Frequency

Uses real attenuation formulas (a·√f + b·f) verified against manufacturer datasheets, not a flat table.

Frequency (MHz)

Cable Length (ft)

Cable Type

Total Loss

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Loss per 100 ft

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Power Reaching Antenna

—

Rating

—

Assumes 100W input and a well-matched line (SWR near 1:1). RG-213 and the LMR series use formulas verified against manufacturer datasheets; RG-8X is an estimate scaled from one confirmed manufacturer data point — see the accuracy note below for why.

📋 In This Guide

How coax loss is actually calculated
A note on accuracy & data sources
Cable comparison at common HF bands
Does SWR make it worse?
Which cable should you actually buy?
Common mistakes
Other antenna tools on KE6MGB
Frequently asked questions

📐 How Coax Loss Is Actually Calculated

Unlike antenna length, coax loss doesn't come from one clean formula like 468/f — it comes from two separate physical effects added together: conductor loss (resistive loss in the center conductor and shield, which scales with the square root of frequency) and dielectric loss (energy absorbed by the insulating material, which scales linearly with frequency). Manufacturers publish this as a formula of the form:

a·√f + b·f

dB per 100 ft

a·√f

Conductor loss term

b·f

Dielectric loss term

This calculator uses each cable's specific a and b constants (sourced from manufacturer datasheets where available) rather than reading values off a printed chart, so it can compute loss at any frequency, not just the handful of points a typical chart shows.

🔍 A Note on Accuracy & Data Sources

Cable type names like "RG-58" or "RG-213" describe a general specification, not one exact product — the same nominal cable type genuinely varies between manufacturers depending on conductor material, dielectric, and build quality. We found real disagreement between published sources for the same cable type while researching this calculator, which is normal for this topic, not a sign of bad data.

LMR-195, LMR-240, LMR-400: Formulas taken directly from Times Microwave's published datasheets and verified against their own printed attenuation charts.
RG-213: Formula derived from Belden's published attenuation data, cross-checked against an independent attenuation chart compilation — the two matched closely.
RG-8X: We could only confirm one verified manufacturer data point (Belden 9258, 3.1 dB/100ft at 100 MHz). The figures shown are scaled estimates from that single point, not a fully independent verified curve — treat RG-8X numbers as a reasonable approximation, not a precise spec.

⚠️

For Critical Decisions, Check Your Specific Cable's Datasheet

If you're buying cable for a long or critical run, look up the attenuation chart for the exact product you're purchasing rather than relying on the generic "RG-213" or "RG-8X" name alone — different manufacturers selling cable under the same generic name can have meaningfully different real-world loss.

📊 Cable Comparison at Common HF Bands

At HF frequencies (below 30 MHz), the difference between cable types is much smaller than at VHF/UHF — this is exactly why the conventional wisdom of "LMR-400 isn't worth it for HF" holds up.

Band	RG-213	RG-8X (est.)	LMR-400
80m (3.75 MHz)	0.34 dB/100ft	0.56 dB/100ft	0.24 dB/100ft
40m (7.15 MHz)	0.47 dB/100ft	0.78 dB/100ft	0.33 dB/100ft
20m (14.2 MHz)	0.67 dB/100ft	1.11 dB/100ft	0.47 dB/100ft
15m (21.2 MHz)	0.82 dB/100ft	1.36 dB/100ft	0.57 dB/100ft
10m (28.4 MHz)	0.96 dB/100ft	1.59 dB/100ft	0.66 dB/100ft

Even on 10m with 100 feet of cable, the difference between RG-213 and LMR-400 is under 0.3 dB — genuinely not noticeable on the air. The picture changes dramatically at VHF/UHF, where the same cables show several times more loss per foot.

📈 Does SWR Make It Worse?

Yes, but the effect is smaller than many people assume at the SWR levels most stations actually run. Reflected power travels back and forth through the line, getting attenuated each pass, which adds a small amount of loss on top of the cable's own matched-line loss.

~0.1–0.2 dB

Extra loss at 1.5:1 SWR

~0.3–0.5 dB

Extra loss at 2:1 SWR

~1.0 dB

Extra loss at 5:1 SWR

Below roughly 2:1 SWR, this additional loss is usually small enough not to worry about — focus on choosing a low-loss cable in the first place rather than chasing a perfect match. A lossy cable combined with a poor match compounds both problems, which is the worst-case combination to actually avoid.

🛒 Which Cable Should You Actually Buy?

Situation	Recommendation
HF, run under 100 ft	RG-8X or RG-213 — the loss difference vs. LMR-400 is negligible
VHF/UHF, any length	LMR-400 or better — loss differences become significant
Long runs (100+ ft) on any band	LMR-400 or larger — savings compound with length
Portable/POTA, short run	RG-8X — lighter and flexible, loss is negligible at typical lengths
Permanent station, any band	Worth the upgrade to LMR-400 — install once, use for decades

❌ Common Mistakes

Buying LMR-400 for a short HF run "just to be safe": The loss savings are real but tiny at HF — money better spent elsewhere for short runs.

Ignoring cable choice entirely at VHF/UHF: The same cable type that's fine on HF can lose most of your power at 70cm.

Trusting a generic "RG-213" chart for a specific purchase: Check the actual manufacturer's datasheet for the exact product, especially for long or critical runs.

Chasing a perfect SWR match while ignoring base cable loss: A lossy cable with 1.1:1 SWR still loses more power than a good cable with 2:1 SWR.

Coiling excess coax tightly: Tightly coiled coax can act as an RF choke on some bands — leave gentle loops, not tight coils, for excess cable.

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❓ Frequently Asked Questions

How is coax cable loss calculated?

Coax attenuation comes from two combined effects: conductor loss, which scales with the square root of frequency, and dielectric loss, which scales linearly with frequency. Manufacturers publish this as a formula of the form attenuation equals a constant times the square root of frequency in MHz, plus a second constant times frequency in MHz, giving the result in decibels per 100 feet. Total loss for a given run is that per-100-foot figure multiplied by the actual cable length in hundreds of feet.

How much coax loss is acceptable for ham radio?

Under 1 dB of total feedline loss is excellent, 1–3 dB is generally acceptable for most amateur use, and over 3 dB means more than half your transmitter power is lost in the cable before it reaches the antenna. The same loss applies on receive, reducing the weakest signals you can copy by the same amount.

Does SWR increase coax loss?

Yes, though the effect is smaller than many assume at the SWR levels most stations run. A reasonably well-matched line under about 2:1 SWR adds only a small amount of extra loss on top of the cable's matched loss, commonly cited as a few tenths of a dB. The effect grows at higher SWR and compounds with the cable's own base loss.

Is RG-8X good enough for HF, or do I need LMR-400?

For HF and short to moderate runs, RG-8X is generally adequate. Its higher per-foot loss matters far more at VHF/UHF and over longer runs than on HF below 30 MHz, where even a meaningfully lossier cable still loses only a fraction of a dB over a typical 50–100 foot run. LMR-400 earns its cost primarily at VHF/UHF and on long runs, not on a short HF feedline.

Why do different sources give different loss numbers for the same cable type?

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. The most reliable numbers come directly from a specific manufacturer's datasheet for the exact product being used, rather than a generic chart averaging across many makers.