Loss is paid twice, and that is the whole point
Feedline loss is the least glamorous number in a station and usually the most important one. Whatever the cable takes out on transmit, it takes out again on receive. Three decibels of run turns 100 watts into 50 at the antenna, and it also turns a signal that would have been comfortably readable into one that is half as strong at the receiver. Nothing at the radio end recovers it, because the loss happens between the radio and the antenna in both directions.
The arithmetic is simple. Decibels from a power ratio are ten times the base-ten logarithm of the ratio, so a total loss L in dB means the fraction getting through is ten to the power of minus L over ten. A tenth of a decibel is 2.3 percent; one decibel is 20.6 percent; three decibels is almost exactly half; ten decibels is nine tenths gone. On the default figures here — 100 feet, 1.5 dB per hundred feet, a 2:1 load, and 0.2 dB of connectors — the total is 1.96 dB and 63.7 watts of 100 arrive at the antenna.
What an SWR actually costs you
Reflected power is not lost by reflecting. It goes back down the line, and most of it comes off the transmitter end and travels out again. What it loses is the extra trip through a lossy cable. That gives a result people find counterintuitive in both directions: on a short run of good cable, a 3:1 match costs almost nothing, and on a long run of lossy cable at a high frequency, the same 3:1 costs a great deal.
Here is what the default run does as the match at the antenna degrades, at 1.5 dB per hundred feet over 100 feet:
| SWR at the antenna | Matched loss | Extra from the mismatch |
|---|---|---|
| 1:1 | 1.50 dB | 0.00 dB |
| 2:1 | 1.50 dB | 0.26 dB |
| 3:1 | 1.50 dB | 0.67 dB |
| 5:1 | 1.50 dB | 1.46 dB |
Even at 5:1 the mismatch is adding slightly less than the cable itself. That is why the honest priority is almost always to fix the cable first and the match second, and why replacing a long run of lossy cable is a bigger improvement than any amount of adjustment at the radio.
Why the meter at the radio lies to you
An SWR meter at the transmitter measures the reflected wave after it has come back down the line, so it has been through the cable twice and arrived attenuated. On the default figures a genuine 2:1 at the antenna reads about 1.62:1 at the radio. On a long run at VHF, where losses are far higher, a badly matched antenna can read close to flat at the operating position.
The rule that follows: an unexpectedly good SWR reading on a long or high-frequency run is a symptom to investigate, not a reassurance. A completely lossy or water-filled cable reads a perfect 1:1 because nothing comes back at all.
Where the loss figure has to come from
There is no cable type selector here on purpose. Loss per hundred feet is a property of a specific cable at a specific frequency, it varies between manufacturers of nominally the same designation, and it rises steeply with frequency — the same run that is almost free at 3.5 MHz can be crippling at 450 MHz. Take the figure from the data for the cable in your hand and enter it against the frequency you are using.
Age and water matter more than most people expect. Coax that has had a cracked outer jacket for a season, or a connector that was never properly weatherproofed, can be several decibels worse than its published figure with no visible symptom other than the SWR reading getting suspiciously good. If the numbers here do not match what the station does, measure rather than assume.
One thing this page will not do is evaluate anything about RF exposure. Exposure limits are published by regulators and evaluating a station against them is a requirement, not a calculation you take from a general-purpose site. That belongs to the regulator and to the person who holds the licence.
Questions people ask
How much power actually reaches my antenna?
The fraction is ten to the power of minus the total loss in decibels divided by ten. On 100 feet of cable rated 1.5 dB per hundred feet, feeding a 2:1 load, with 0.2 dB of connectors, the total is 1.96 dB and 63.7 watts of 100 arrive at the antenna. The remaining 36.3 watts is heat in the cable and its connectors, spread along the run.
Does a high SWR damage my coax or waste all my power?
Neither, usually. Reflected power is not absorbed at the reflection, it travels back down the line and mostly gets sent out again. What it loses is the extra trip through a lossy cable, so the cost depends on how lossy the cable already is. On a 100 foot run losing 1.5 dB matched, a 2:1 load adds 0.26 dB and a 5:1 load adds 1.46 dB. On a very lossy run the same mismatch costs far more.
Why does my SWR read lower at the radio than at the antenna?
Because the reflected wave passes through the cable twice before your meter sees it, so it arrives attenuated. With 1.5 dB of matched line loss, a true 2:1 at the antenna reads about 1.62:1 at the radio. The effect gets larger with longer runs and higher frequencies, which is why a suspiciously flat reading on a long VHF run is a reason to check the cable rather than a sign that everything is fine.
Does an antenna tuner reduce feedline loss?
No. A tuner at the radio end presents the transmitter with a load it is happy to drive, which is genuinely useful, but the wave still travels down the line, reflects at the antenna and comes back. That round trip is where the extra loss occurs and the tuner is not in that path. Reducing feedline loss means shorter cable, better cable, or fixing the match at the antenna end where the reflection happens.
Why does this page not have a cable type dropdown?
Because the number that matters is loss per hundred feet at your frequency, and that is a property of the specific cable rather than of a type name. Nominally identical designations differ between manufacturers, loss rises steeply with frequency, and cable that has aged or taken water can be several decibels worse than its published figure. A dropdown would encourage you to trust a label instead of the data for the cable you actually own.