Coax Loss Calculator by Cable Type, Length and Frequency

Feedline loss is the quietest way to throw away a station. It does not announce itself, it does not show up on an SWR meter, and it gets worse every year as water works into the braid. A hundred feet of the wrong cable at 440 MHz can waste more than nine tenths of your transmitter output before it reaches the antenna, and the radio will read a perfect match while it happens.

What the decibels actually cost you

Decibels are logarithmic and most people's intuition for them is wrong in a predictable direction: small numbers matter more than they look. One decibel sounds like nothing and is a fifth of your power. Three decibels is half.

Loss applies to receive as well, which is the half people forget. A feedline that throws away 6 dB on transmit also attenuates every incoming signal by 6 dB, so you are both harder to hear and less able to hear. On VHF and UHF, where the noise floor is low and weak-signal work is normal, that receive penalty is usually the one that hurts.

Which cable for which job

Loss figures are decibels per 100 feet at the stated frequency, from manufacturer datasheets. The pattern to notice is that the ranking barely changes with frequency but the spread widens enormously. At 14 MHz the difference between thin and thick cable is about a decibel over 100 feet, which nobody would hear. At 440 MHz it is six or seven decibels, which is the difference between a working station and a dead one.

The rules that follow from the table

• On HF, buy for power handling and durability, not for loss. A hundred feet of MOOKEERF RG-213 coax, 100 ft with UHF male$115.99 loses under a decibel at 14 MHz. The reason to buy it is that it takes legal-limit power and survives being walked on, not that it saves you a fraction of an S unit.
• On VHF and UHF, loss is the whole decision. A hundred feet of Bolton400 low-loss coax, 100 ft with PL-259$119.95 at 146 MHz loses about 1.5 dB against roughly 5 dB for RG-58. That is the difference between hitting a distant repeater and not.
• Short runs forgive everything. Twenty-five feet of Rydocyee RG-58 coax jumper, 25 ft$17.99 at 14 MHz loses under half a decibel, which is nothing. Use cheap cable for short HF jumpers without guilt.
• Portable inverts the rule. In the field the run is 25 to 50 feet and weight is the constraint, so RG-8X class cable such as JEFA Tech 240-series flex coax, 50 ft with PL-259$49.99 is correct and LMR-400 is a bad trade.
• Buy the run in one piece if you can. Not because connectors are lossy, which they barely are, but because every joint is a place for water to enter, and water in the braid is the failure that turns a good cable into a bad one over a couple of seasons.

What SWR does to feedline loss

A mismatched antenna reflects power back down the line, and that reflected power travels through the same lossy cable a second time. The additional loss caused by SWR is therefore proportional to how lossy the line already is, which produces a result that surprises people: SWR on a low-loss line is almost harmless, and SWR on a high-loss line is expensive.

The practical reading: a 3 to 1 SWR on 40 metres through 100 feet of RG-213 costs you under a decibel in total and is not worth chasing. The same 3 to 1 on 70 centimetres through 100 feet of RG-58 is a disaster, and no tuner at the radio end fixes it, because a tuner in the shack does nothing about the standing wave in the cable beyond it. Full treatment on the SWR calculator and the honest verdict on tuners is on tuner versus resonant antenna.

Velocity factor, and when you actually need it

Signals travel slower in cable than in free space, by a factor set by the dielectric. Solid polyethylene cables run about 0.66, foam dielectric around 0.85, and air-spaced hardline higher still. For an ordinary feedline this is irrelevant: the antenna does not care how long the electrical path is.

It matters exactly when you need a specific electrical length. A quarter-wave matching stub, a half-wave repeater line, a phasing harness between stacked antennas, or a delay line all require the physical length that produces the right electrical length, which is the free-space figure multiplied by the velocity factor. The calculator above gives that number for the selected cable, and the impedance matching calculator works through what those stubs do.

Loss that is not in any datasheet

Every figure on this page assumes new cable in good condition. Three things degrade real installations and none of them appears in a table.

• Water in the braid. The most common cause of a feedline that used to work. Water wicks in through an unsealed connector and travels, and the loss climbs steadily over months. There is no repair; the run is replaced. Seal every outdoor connector with Self-amalgamating rubber splicing tape$9.99 and then Hand-moldable sealant tape$9.69.
• Ultraviolet damage. Non-UV-rated jackets crack in a few seasons of direct sun, which admits water. Black UV-stable jacket, and UV-resistant heavy duty zip ties, 100-pack$6.99 rather than natural-coloured ties, which themselves fail in one summer.
• Bad connectors. A cold-soldered or badly crimped PL-259 can add several decibels, behave intermittently and change with temperature. This is the single most common fault found on an antenna analyser. Practise on scrap using PL-259 solder connectors with reducers, 6-pack$9.99 before doing the run that matters, and read how to install PL-259 connectors.

All three are detectable with a NanoVNA H4 vector network analyzer$89.90, which shows loss and return loss across a sweep and will find a bad connector in a couple of minutes. Under a hundred dollars for a measurement that used to require a laboratory.