Hertzian
Hertzian

Cable Loss

beginner

Every metre of coax quietly eats signal — more at higher frequencies, more on thin cable. How to predict the loss and when it matters.

The idea

A coaxial cable is a pipe for radio energy, and the pipe leaks. Some of the signal turns into heat in the copper, and some is soaked up by the plastic insulation between the centre wire and the shield. Nothing you send down the cable comes out the other end at full strength.

Three things decide how much you lose:

  • Length. Loss grows in direct proportion to length. Twice the cable means twice the dB lost.
  • Frequency. Higher frequencies lose more. At high frequency the current crowds onto the surface of the copper (the skin effect), so less metal carries it, and the insulation absorbs more each cycle.
  • Cable type. Thick, low-loss cable (LMR-400, Heliax) has a fatter centre conductor and better insulation than thin, flexible cable like RG-58. It costs more and bends less, but it leaks far less.

Cable loss is measured in dB, so it slots straight into a link budget: subtract it from your transmit power, and subtract the receive-side cable from your received signal. Because decibels add, a 3 dB cable halves your power no matter how strong the signal was to begin with.

The math

Cable datasheets quote loss per 100 ft (or per 100 m) at a reference frequency. The calculator scales that to your frequency and length:

Lcable=L100 ft(f)×length100 ftL_{cable} = L_{100\,ft}(f) \times \frac{\text{length}}{100\,\text{ft}}

The fraction of power that survives the cable is

Power out=10−Lcable/10×Power in\text{Power out} = 10^{-L_{cable}/10} \times \text{Power in}

Worked example: a 100 ft run at 450 MHz (a UHF radio, at 20 °C). With RG-58 the calculator gives 9.88 dB of loss. That means only 10^(−0.988) ≈ 10.3% of your power reaches the antenna. A 20 dBm (100 mW) transmitter delivers about 10.1 dBm (roughly 10 mW). Swap in LMR-400 for the same run and the loss falls to 2.80 dB, so 52.5% gets through, about 17.2 dBm. That one cable swap buys you 7 dB, which is more than most antenna upgrades.

The same calculator also shows the other two levers. Double the RG-58 run to 200 ft and the loss doubles to 19.76 dB. Drop the frequency to 100 MHz and the 100 ft run loses only 4.5 dB.

Common misconceptions

  • Myth: A cable is just a wire, so it does not change the signal. Reality: At radio frequencies a long, thin cable can throw away 90% of your power before it reaches the antenna.
  • Myth: Cable loss is fixed for a given cable. Reality: It depends on frequency. The same RG-58 that is fine for a short HF run is a poor choice for 100 ft at 2.4 GHz.
  • Myth: You can fix cable loss with a stronger transmitter. Reality: The cable loses the same number of dB at any power, and it costs you on receive too, where you cannot turn up the other station. Shortening the run or using a better cable fixes both directions.
  • Myth: Connectors are free. Reality: Each connector typically adds about 0.1 dB (N-type) to 0.3 dB (PL-259). A run with several adapters can add up to a noticeable loss.

Try it

Open the Cable Loss Calculator, pick RG-58, 100 ft and 450 MHz, and note the loss. Then switch to LMR-400 and use the Compare tab to see several cables side by side. To see the loss in context, drop a cable block into the RF Chain Simulator and watch the link margin shrink as you lengthen it.