Coax Loss and SWR Calculator
How much power actually reaches your antenna, and why feedline loss makes the SWR reading at the radio look better than the truth.
Feedline loss makes your SWR reading look better than it is
The reflected wave travels the length of the cable twice, so it gets attenuated twice. A meter at the shack always reads lower than one at the antenna. A 3:1 antenna behind 3 dB of line reads 1.67:1. Push it further and a sufficiently lossy run reads a beautiful 1:1 with no antenna connected at all, because nothing comes back. So a very flat reading on a long run of cheap cable is a warning sign, not a result.
- The mismatch itself usually costs very little. At 2:1 on decent cable the extra loss is around a tenth of a decibel — completely inaudible at the far end. Chasing 1.5:1 down to 1.2:1 is almost never worth the effort.
- The real reason to care is your transmitter. Most radios fold back their output above about 2:1 to protect the final stage, so a high standing wave ratio costs you power through protection circuitry long before it costs you anything in the feedline.
- Reflected power is not simply lost. Most of it is re-reflected at the transmitter and eventually radiated. What it actually costs you is the extra journey through the cable — which is exactly the mismatch loss above, and why it compounds with a lossy line.
- Loss has two parts. Conductor loss grows as the square root of frequency, because of skin effect; dielectric loss grows linearly. That is why cheap cable is merely poor at HF and hopeless at UHF — the second term catches up.
- Cable is usually the cheapest upgrade you can make. Swapping 30 m of RG-58 for LMR-400 on 2 m more than doubles the power reaching the antenna, which beats anything a tuner will do for you.
How to use
- Pick your cable and enter the frequency, length and transmitter power.
- Enter the SWR at the antenna, or a load impedance to work it out.
- Compare the power delivered against what you put in.
- Note what a meter at the radio would read — it is not what the antenna is doing.
Frequently asked questions
Why does my SWR read better at the radio than at the antenna?
Because the reflected wave travels the length of the cable twice and is attenuated twice on the way. A 3:1 antenna behind 3 decibels of feedline reads 1.67:1 at the shack. Push it further and a sufficiently lossy run reads a beautiful 1:1 with no antenna connected at all, because nothing comes back. A very flat reading on a long run of cheap cable is a warning sign rather than a result.
How much does a bad SWR actually cost me?
On decent cable, remarkably little. At 2:1 the extra loss over a matched line is around a tenth of a decibel, which is completely inaudible at the far end. The mismatch penalty compounds with line loss though, so the same 2:1 on a long run of lossy coax costs several times as much. Chasing 1.5:1 down to 1.2:1 is almost never worth the effort.
Then why does everyone worry about SWR?
Because of the transmitter, not the feedline. Most radios fold back their output above about 2:1 to protect the final stage, so a high standing wave ratio costs you power through protection circuitry long before it costs you anything in the cable. That is the real reason to care, and it is why an antenna tuner at the radio helps even though it does nothing about the standing wave beyond it.
Where does the reflected power go?
Not into heat at the transmitter, which is the common misconception. Most of it is re-reflected at the transmitter end and eventually radiated. What the reflection actually costs you is the extra journey back and forth through the cable, and that is exactly the mismatch loss figure here — which is why it grows so much on a lossy line and barely registers on a good one.
Why is cheap cable so much worse at UHF than at HF?
Because loss has two parts that grow at different rates. Conductor loss goes as the square root of frequency, from skin effect pushing current into the outer surface of the conductor. Dielectric loss grows linearly. At HF the first term dominates and cheap cable is merely poor; by UHF the second has caught up and it is hopeless. RG-58 loses about 1.6 dB per 30 metres at 14 MHz and over 8 dB at 446.
Is better cable worth the money?
It is usually the cheapest real improvement available. Swapping 30 metres of RG-58 for LMR-400 on 2 metres more than doubles the power reaching the antenna, and it helps on receive as well as transmit — which no amount of transmitter power does. Compare that against what a tuner, an amplifier or a bigger antenna would cost for the same result.
Do these loss figures include connectors?
No. Each connector pair costs roughly 0.1 to 0.2 decibels at VHF and more as frequency climbs, and a corroded or badly fitted one can cost far more than the cable it joins. On a run with several adapters that is worth adding by hand, and a connector that has been rained on for a season is worth cutting off and replacing before blaming anything else.
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