The Feedline Loss Calculator

Simple version: pick your band, your run, your cable, and your power. The calculator tells you how much signal reaches the antenna and how much is lost as heat in the cable. A newcomer gets the answer at a glance. An engineer gets a solid reference. Both leave knowing what to run.

It opens in matched mode, the simple case. Set an SWR and pick where your tuner lives, and it switches to the real-world case for a non-resonant Greyline running into a tuner, where the cable you choose starts to matter a great deal. Every dB counts.

How To Read This

Leave SWR on 1:1 and you get the simplest answer: matched-line loss, the best your cable can do. That is the floor. Set a real SWR (your non-resonant Greyline presents a different match on every band) and tell it where your tuner lives, and the calculator shows the real number. Put the tuner at the feedpoint and the whole line runs at 1:1. Put it in the shack and the line carries the SWR, so the cable you chose has to earn its keep. The "SWR your radio sees" line is the honest one: a long lossy run can show a comfortable number at the rig while quietly cooking power in the cable.

The Physics, In Plain English

A decibel fools people because the scale is logarithmic. Three dB is half your power. One dB you will never see move. At HF, with good 50-ohm cable on a matched line, a 100-foot run costs a fraction of a dB -- well below what the station working you can hear. That is why feedline panic is usually oversized.

This is the heart of it, and it is the case the gurus have made for decades. Rauch W8JI on how loosely most S-meters read and where loss really comes from. Devoldere ON4UN, in Low-Band DXing, on why a matched feedline runs through every serious low-band station. Maxwell W2DU, in Reflections, on exactly how reflected power behaves on a real line. And Zavrel W7SX, whose whole gift is making this physics make sense to the rest of us. We point the way. They do the math. Every claim on this page is held to that standard.

For a Greyline specifically: the antenna is non-resonant by design, so the feedline between it and the tuner sees real SWR. Run that line with a big, low-loss cable, or skip the problem entirely by tuning at the feedpoint, and you keep your signal. The calculator above lets you watch both choices play out.

Cable Reference -- Matched Loss, 100 ft

Matched-line loss in dB per 100 feet, lowest-loss cable first. Reference cables (RG-213, RG-8X, RG-58) are shown so you can see the gap; we do not recommend them for a permanent Greyline feed.

*Kit 3 (two-conductor silicone wire in metallic conduit) is modeled from the W7SX design spec; bench confirmation pending. DRF-400 shares LMR-400 electricals -- the numbers are identical. Under SWR every figure rises, and the low-loss lines rise least; the calculator above shows your specific case.