SWR and Feedline Loss Explained — A Complete Ham Radio Reference
A thorough technical reference covering standing wave ratio, reflection coefficient, feedline loss, and how these interact to determine the efficiency of your antenna system. Covers what SWR actually measures, why high SWR increases coax loss, how to calculate power lost at any SWR value, coax matched-line loss by frequency and cable type, and the practical breakpoints where SWR and feedline loss begin to matter in a real station. Includes a feedline loss calculator and SWR power loss calculator.
Transmission lines and impedance mismatch
A coaxial cable is a transmission line — a structure engineered to carry RF energy from one point to another with minimal radiation or loss. Its characteristic impedance, 50 ohms for standard amateur coax, describes the ratio of voltage to current in a travelling wave on the line under matched conditions. When a transmission line is terminated in a load whose impedance equals the line's characteristic impedance, all the energy travelling down the line is absorbed by the load. No energy is reflected back toward the source.
When the load impedance differs from the line impedance, some of the incident energy is reflected back toward the source. The reflected wave and the incident wave travelling in opposite directions on the same transmission line superimpose to create a standing wave — a pattern of voltage and current maxima and minima that is stationary along the line. The standing wave ratio is the ratio of the maximum voltage amplitude on the line to the minimum voltage amplitude. An SWR of 1:1 means no standing wave exists — perfect match. An SWR of 2:1 means the voltage peaks are twice the voltage troughs. An SWR of infinity means total reflection — an open or short circuit.
Reflection coefficient and the SWR formula
The reflection coefficient Γ (gamma) is the more fundamental quantity, expressing the fraction of the incident wave voltage that is reflected. It is a complex number — it has both magnitude and phase — but for the purposes of SWR calculation only its magnitude matters. The reflection coefficient magnitude ranges from 0 (perfect match, no reflection) to 1 (total reflection, open or short circuit).
SWR and reflection coefficient are directly related by a simple formula. Knowing either one allows immediate calculation of the other. The return loss — the reflected power expressed in dB — is a third way of expressing the same mismatch information, preferred in some professional contexts.
What SWR does not tell you
SWR measured at the transmitter end of a feedline tells you the impedance mismatch at that point — it does not directly tell you the antenna's impedance, the feedline loss, the antenna's radiation efficiency, or whether the antenna is actually working. A perfectly matched 50-ohm dummy load produces SWR 1:1 but radiates nothing. A lossy feedline reduces reflected power back to the transmitter, which can cause a high-SWR antenna system to show low SWR at the radio — the coax has absorbed the reflected wave, masking the true mismatch at the antenna feedpoint.
This masking effect means that long runs of lossy coax can show low SWR at the transmitter even when the antenna is severely mismatched. A 50-metre run of RG-58 at 28 MHz with a 10:1 SWR at the antenna end may show only 3:1 SWR at the transceiver — the line has attenuated the reflected wave sufficiently to mask the problem. The antenna is still operating at 10:1 SWR and losing significant power to mismatch and cable heating, but the radio's SWR meter gives a false sense of acceptable performance.
Where SWR actually matters operationally
The primary practical consequences of high SWR at the transmitter are three. First, most solid-state transceivers automatically reduce transmit power when SWR exceeds approximately 2:1 to 3:1, using fold-back protection circuits. At SWR 3:1 many rigs have already reduced output power by 30 to 50 percent. At SWR 5:1 some rigs have reduced to 25 percent output or less. Second, high SWR increases the RF voltage on the feedline, which can exceed the breakdown rating of cheap coax connectors or cause arcing in damaged cable sections. Third, high SWR on a lossy feedline multiplies the cable heating effect — a problem at high power levels on long cable runs.
For a station running a well-matched resonant antenna with short coax at modest power, SWR below 2:1 is perfectly adequate and the difference between 1.2:1 and 1.8:1 is operationally meaningless. The situation changes when the feedline is long, the frequency is high, or the power is high — these are the conditions where SWR and feedline loss interact destructively and where careful attention to match and cable quality pays dividends.
Coax Feedline Loss Calculator
Calculates matched-line loss, mismatch additional loss, and total system loss for common coax types.
SWR Mismatch Power Loss Calculator
Shows reflected power, mismatch loss, and transmitter fold-back at the antenna feedpoint (no feedline).
Matched-line loss — the baseline
Matched-line loss is the signal attenuation a coaxial cable produces when it is terminated in its exact characteristic impedance — a perfect 50-ohm load with SWR 1:1. Under these conditions no standing wave exists, current and voltage are constant along the line, and all loss is due to resistive heating in the conductor and dielectric loss in the insulator. Matched-line loss increases with frequency because skin effect concentrates current in a progressively thinner layer of conductor surface as frequency rises, increasing the effective resistance per unit length.
Matched-line loss is typically specified by cable manufacturers in dB per 100 feet at specific spot frequencies. A 30-metre run of RG-58 has a matched-line loss of approximately 0.9 dB at 7 MHz, rising to 1.8 dB at 28 MHz, and 4.5 dB at 144 MHz. This escalating loss with frequency is why the choice of coax matters far more at VHF and UHF than at HF — the same RG-58 that is perfectly adequate for 40m HF becomes a significant liability at 2m.
Additional loss from SWR on the line
When SWR is greater than 1:1, the standing wave pattern on the feedline creates voltage and current maxima that are higher than the matched-line values. These elevated voltages and currents produce higher heating in the cable's resistance, increasing loss above the matched-line baseline. The additional loss from SWR is proportional to both the SWR and the matched-line loss of the cable — a very low-loss cable like LMR-400 experiences less SWR-induced additional loss than a high-loss cable like RG-174 at the same SWR.
The interaction of SWR and cable loss produces a result that surprises many operators: a low-loss cable at high SWR often performs better than a high-loss cable at low SWR. Running LMR-400 at 3:1 SWR may deliver more power to the antenna than RG-58 at 1.5:1 SWR over the same length run at the same frequency. This is because the LMR-400's low matched-line loss leaves little margin for SWR to amplify, while the RG-58 is already losing significant power to matched-line loss before any SWR penalty is added.
The ATU does not fix feedline loss
One of the most persistent misconceptions in amateur radio is that an antenna tuner eliminates the problem of high SWR on a long coax run. An ATU at the transmitter transforms the impedance seen by the transmitter to 50 ohms, which satisfies the radio's SWR protection circuitry and restores full output power. But the ATU does not change the SWR on the coaxial cable between the ATU and the antenna. If that cable has 5:1 SWR on it, the additional loss from that mismatch is still present regardless of what the ATU is doing at the transmitter end.
The correct solution for a mismatched antenna fed by a long cable run is to match the antenna at the feedpoint — either by using a resonant antenna, adding a remote matching unit at the antenna base, or using an antenna design with a natural 50-ohm feedpoint. An ATU at the transmitter is useful for trimming a near-resonant antenna or for multiband operation with acceptable cable loss, but it cannot compensate for a severely mismatched antenna on a lossy feedline. Moving the tuner to the antenna feedpoint — where the SWR issue exists — is always more effective than tuning at the transmitter end.
Ladder line and open-wire feedline
Parallel open-wire transmission line — commonly called ladder line or window line — operates on a fundamentally different loss model from coaxial cable. The matched-line loss of 450-ohm or 600-ohm open-wire line is very low — typically 0.02 to 0.1 dB per 30 metres at HF frequencies, compared to 0.5 to 2 dB for RG-58 over the same distance. More importantly, the additional loss from SWR on open-wire line is minimal even at very high SWR values, because the low matched-line loss means there is little energy in the conductor for the standing wave current peaks to heat.
This is why the all-band doublet antenna — a centre-fed dipole of convenient length fed with open-wire line to an ATU at the shack — can operate efficiently at high SWR on many bands. The SWR on the open-wire line may be 10:1 or 20:1 on some bands, but the low matched-line loss of the feedline means the additional SWR-induced loss is small in absolute terms. The same antenna fed with 20 metres of RG-58 would lose much more power to the high-SWR penalty on the coax. Open-wire line cannot be used with coax components or standard coax fittings and requires an ATU with balanced output, but its low loss at high SWR makes it the ideal feedline for multiband wire antenna systems.
| Coax Type | 3.5 MHz | 7 MHz | 14 MHz | 28 MHz | 50 MHz | 144 MHz | 432 MHz | Impedance | VF |
|---|---|---|---|---|---|---|---|---|---|
| RG-174 | 1.9 | 2.7 | 3.9 | 5.6 | 7.5 | 13.8 | 26.0 | 50Ω | 0.66 |
| RG-58 | 0.8 | 1.1 | 1.6 | 2.4 | 3.2 | 6.1 | 12.0 | 50Ω | 0.66 |
| RG-8X (mini-8) | 0.5 | 0.7 | 1.1 | 1.6 | 2.2 | 4.3 | 8.5 | 50Ω | 0.78 |
| RG-213 / RG-8 | 0.3 | 0.5 | 0.7 | 1.0 | 1.4 | 2.8 | 5.6 | 50Ω | 0.66 |
| LMR-240 | 0.2 | 0.3 | 0.5 | 0.7 | 1.0 | 2.0 | 4.1 | 50Ω | 0.84 |
| LMR-400 | 0.1 | 0.2 | 0.3 | 0.4 | 0.6 | 1.2 | 2.4 | 50Ω | 0.85 |
| LMR-600 | 0.07 | 0.10 | 0.14 | 0.21 | 0.28 | 0.57 | 1.2 | 50Ω | 0.87 |
| Heliax 1/2" (LDF4-50) | 0.04 | 0.05 | 0.08 | 0.11 | 0.15 | 0.30 | 0.6 | 50Ω | 0.88 |
| RG-6 (75Ω CATV) | 0.3 | 0.4 | 0.6 | 0.9 | 1.2 | 2.4 | 5.0 | 75Ω | 0.82 |
| 450Ω window line | 0.02 | 0.03 | 0.04 | 0.06 | 0.08 | 0.16 | 0.4 | 450Ω | 0.91 |
Loss values in dB per 30 metres (approx 100 feet). Matched-line loss at SWR 1:1. Actual loss varies by manufacturer and connector quality.
| SWR | Reflection coeff |Γ| | Return loss (dB) | Power reflected (%) | Mismatch loss (dB) | Typical TX fold-back | Operational verdict |
|---|---|---|---|---|---|---|
| 1.0:1 | 0.000 | ∞ | 0.0% | 0.00 | None | Perfect match |
| 1.2:1 | 0.091 | 20.8 | 0.8% | 0.04 | None | Excellent — negligible loss |
| 1.5:1 | 0.200 | 14.0 | 4.0% | 0.18 | None to minimal | Very good — acceptable for all use |
| 2.0:1 | 0.333 | 9.5 | 11.1% | 0.51 | Minimal on most rigs | Good — mismatch loss small; ATU useful |
| 2.5:1 | 0.429 | 7.3 | 18.4% | 0.88 | Mild on some rigs | Acceptable — use ATU if available |
| 3.0:1 | 0.500 | 6.0 | 25.0% | 1.25 | Moderate — 50–75% power | Marginal — improve match or use ATU |
| 4.0:1 | 0.600 | 4.4 | 36.0% | 1.94 | Significant — ~40% power | Poor — feedline loss penalty significant |
| 5.0:1 | 0.667 | 3.5 | 44.4% | 2.55 | Severe — ~25% power | Bad — most power reflected; fix the antenna |
| 10.0:1 | 0.818 | 1.7 | 66.9% | 4.81 | Severe fold-back | Very bad — two-thirds of power reflected |
When SWR genuinely does not matter
For HF operation at 40m and below, with a resonant or near-resonant antenna, and a short coax run of 10 to 15 metres, SWR between 1:1 and 2:1 is of virtually no practical consequence. The matched-line loss of common coax at these frequencies is already very low — a 15-metre run of RG-58 loses about 0.35 dB at 7 MHz under matched conditions. Adding a 2:1 SWR penalty increases this to about 0.5 dB. The difference of 0.15 dB is completely inaudible and unmeasurable on an S-meter. Energy spent worrying about trimming a 40m dipole from 1.8:1 to 1.2:1 SWR is better directed toward raising the antenna 2 metres, which would produce a far more noticeable improvement.
When SWR and feedline loss matter significantly
SWR and feedline loss become genuinely important when the frequency is high, the cable is long, or the cable is lossy. A 30-metre run of RG-58 at 144 MHz with 3:1 SWR loses approximately 9 dB total — less than one-eighth of the transmit power reaches the antenna. The same 30-metre run at 7 MHz with the same SWR loses only about 1.5 dB total. At VHF and UHF, choosing the right coax type and keeping runs short is far more important than at HF. At HF, antenna height and resonance matter more than cable quality within the range of common coax types.
Connector quality and its effect on SWR
Poorly made coax connectors are a common source of SWR problems that are frequently misattributed to the antenna. A PL-259 connector with an inadequate solder joint, a cold solder joint in a BNC plug, or a corroded barrel adapter can introduce SWR of 1.5:1 to 3:1 at HF and significantly more at VHF. These connector-induced SWR readings are indistinguishable on the meter from a mismatched antenna. When troubleshooting high SWR, always verify connector integrity before adjusting the antenna. Wiggling the cable near each connector while watching the SWR meter is a quick field diagnostic — a SWR change when the cable is moved indicates a connector problem.
Water ingress and coax degradation
Water entering coaxial cable through a connector, a nick in the jacket, or a cable end left unterminated causes the dielectric loss to increase dramatically as the water migrates through the braid and saturates the foam or solid dielectric. A cable that measures 1 dB loss when new may measure 3 to 5 dB loss after several months of water ingress. The SWR also rises as the characteristic impedance of the wet section changes. Outdoor coax installations must be weatherproofed at every connector with self-amalgamating tape or coax seal compound. Cable ends left outdoors without connectors must be capped. Any coax run that shows progressively worsening SWR over months without any antenna change should be suspected of water ingress and inspected or replaced.
Does a tuner fix high SWR on a long coax run?
An ATU at the transmitter end of the coax satisfies the transceiver's SWR protection and restores full output power, but does not reduce the SWR on the coax itself. The cable between the ATU and the antenna still has high SWR and the associated additional loss. For long lossy cable runs, the correct approach is to match at the antenna feedpoint — using a remote ATU, a matching transformer, or a resonant antenna. An ATU at the transmitter is a workable compromise for short cable runs and near-resonant antennas.
Is SWR 1.5:1 really good enough?
Yes, in virtually every amateur radio context. SWR 1.5:1 means only 4 percent of transmit power is reflected, producing a mismatch loss of 0.18 dB. This is completely inaudible and produces no perceptible change in signal reports. Chasing SWR below 1.5:1 produces diminishing returns so small as to be unmeasurable in practice. The effort is far better invested in antenna height, orientation, or power. SWR below 2:1 is the practical target for any resonant amateur antenna system.
What is return loss and how does it relate to SWR?
Return loss is the reflected power expressed in dB — how many decibels below the incident power the reflected power sits. A return loss of 20 dB means the reflected power is 100 times lower than the incident power, corresponding to SWR 1.22:1. A return loss of 6 dB means the reflected power is only 4 times lower, corresponding to SWR 3:1. Return loss is preferred in some VHF/UHF and professional contexts because it increases with improving match — higher numbers are better — which is more intuitive for some purposes than SWR, where lower numbers are better.
Can I use 75-ohm CATV coax for amateur radio?
Yes, with caveats. RG-6 and other 75-ohm CATV coax has lower loss than RG-58 at HF and is usable for receive and low-power transmit. The 75-ohm impedance mismatch with a 50-ohm antenna produces SWR of 1.5:1 in a well-matched system — acceptable for most HF use. F-connector to PL-259 adapters are needed, and the maximum power handling of RG-6 is lower than RG-213. For receive-only SDR installations on HF, RG-6 is an economical and perfectly adequate feedline choice.
Why does my SWR change when I key the transmitter?
SWR that differs between the analyser reading and the transmit reading is usually caused by one of three things: the analyser uses a different frequency or sweep rate than the transmitter; the antenna is temperature-sensitive and changes impedance with the heat of transmitter power; or there is a connection problem in the RF path that behaves differently under the higher voltages of transmit power versus the low-level analyser signal. A connector with a marginal solder joint, a coax centre conductor barely touching the connector pin, or a cracked connector body may make intermittent contact at analyser power but arc or open under transmit voltage.
How much power is lost at SWR 3:1?
At the antenna feedpoint, SWR 3:1 reflects 25 percent of the incident power back toward the transmitter — a mismatch loss of 1.25 dB. In a lossless system this reflected power is re-reflected from the transmitter and eventually radiated, so the actual power lost to mismatch alone is small. The larger issue at SWR 3:1 is the additional loss it imposes on the feedline, which is significant on long lossy coax runs, and the transmitter fold-back it triggers in most solid-state transceivers, which may reduce output to 50 to 75 percent of rated power.
Is ladder line really better than coax for multiband antennas?
For multiband wire antennas fed through an ATU, yes — ladder line's very low matched-line loss and minimal SWR penalty make it dramatically better than coax when the antenna is operated well off resonance on multiple bands. An all-band doublet on 450-ohm window line loses perhaps 0.2 dB on the worst band. The same antenna on RG-58 might lose 4 to 6 dB on bands where the SWR is very high. The trade-off is that ladder line cannot be buried, cannot pass through standard connectors, and requires an ATU with a balanced output — constraints that suit fixed stations but not portable or stealth installations.
What SWR will damage my transceiver?
Modern solid-state transceivers incorporate automatic fold-back protection that reduces output power as SWR rises, preventing damage under normal circumstances. Most rigs reduce power significantly above 2:1 to 3:1 SWR and reach minimum output before damage can occur. Transmitting into an open circuit or short circuit — infinite SWR — with fold-back protection active is generally safe for the transceiver. Damage risk arises from sustained operation into high SWR with fold-back disabled, or from high-voltage arcing in connectors at elevated RF voltages caused by high SWR on long cable runs at high power.