Feedline Loss Calculator — N1TA

All cables at your length, frequency, power & SWR

Sorted best to worst. Selected cable highlighted. Bar = % of power reaching the antenna.

Selected cable across the ham bands

Same length, power, SWR and connectors, matched-loss model re-evaluated per band. SWR is assumed equal on every band.

Feedline Loss Calculator

Formulas

Matched loss (dB) = (k1·√f + k2·f) × length / 100
 k1 = conductor (skin-effect) term, k2 = dielectric term
ρ  = (SWR − 1) / (SWR + 1)          [at the load]
a  = 10^(matched loss / 10)
Total loss = 10·log10[(a² − ρ²) / (a·(1 − ρ²))] + connectors
SWR at TX = (1 + ρ/a) / (1 − ρ/a)
P out = P in × 10^(−total loss / 10)
EIRP = P out × 10^(gain_dBi / 10)     ERP = EIRP / 1.64

Assumptions

• Each cable is modeled from two nominal published attenuation points (100 and 1000 MHz) and interpolated with the √f + f model. Values are typical, not guaranteed; check the manufacturer's data sheet for critical work.
• Hardline entries are typical foam-dielectric 50 Ω Heliax-style cable. Air-dielectric and corrugated types vary by maker.
• Ladder line is a typical 450 Ω window line. Its loss is low even at high SWR, but the 50 Ω SWR model here is only a rough guide for it, since it is not a 50 Ω line.
• Cable loss rises with temperature (about +0.1%/°F) and as cable ages or takes on water.
• Total loss includes the extra loss caused by SWR on the line, which is why a mismatch hurts more on lossy cable.

Tips & Tricks

• Every 3 dB lost is half your power; 6 dB is about one S-unit on the receive side as well.
• SWR read at the rig looks better than the real antenna SWR because the line absorbs reflected power. Compare the "SWR at transmitter" and "SWR at antenna" lines.
• Going from RG-58 to a larger cable costs less than a bigger amplifier. On VHF and up, go as large as you can afford.
• Connectors, adapters and lightning arrestors each add a little, and they add up on short UHF runs.
• Check cable power handling separately. Small coax can fail from heat at high power and high SWR even when loss looks fine.