Coax at QRO: What High SWR Really Changes

The familiar √SWR rule is real—but conditional. It describes voltage and current maxima when the same net power is transported through an ideal lossless line. It is not a universal multiplier for a transmitter’s forward-power reading, and it is not a complete thermal derating rule for real coax.

First Decide Which Power You Mean

“1.5 kW at 20:1 SWR” is not a complete engineering condition. At minimum, the statement must identify whether 1.5 kW means:

• forward travelling-wave power at a specified point;
• net power crossing that point, equal to forward minus reflected power;
• power delivered to the load after feedline loss;
• transmitter or tuner input power; or
• PEP or continuous-average power for the stated modulation and duty cycle.

Those quantities can be very different in a mismatched, lossy, tuned system. A directional coupler separates travelling waves at its own location. It does not directly report the largest voltage, largest current or hottest section anywhere else on the line.

The Two Correct Stress Formulae—For Two Different Conditions

For an ideal lossless line of real characteristic impedance Z0, let S be VSWR and let the reflection-coefficient magnitude be:

Case A: Forward Power Is Held Constant

If P+ is the forward travelling-wave power, the forward-wave RMS values are:

• V+ = √(P+ × Z0)
• I+ = √(P+ / Z0)
• Vmax = V+ (1 + |Γ|)
• Imax = I+ (1 + |Γ|)

Relative to a matched line at the same forward power, the maxima grow by 1 + |Γ| = 2S/(S + 1). That factor approaches 2—not infinity—as SWR rises. Meanwhile, the net power crossing the plane falls to P+ (1 − |Γ|²).

Case B: Net Transported Power Is Held Constant

If Pnet is held constant, the incident wave must grow as mismatch increases. In the same ideal lossless model:

• Vmax = √(Pnet × Z0 × S)
• Imax = √(Pnet × S / Z0)

This is the valid √SWR rule. It is appropriate only after stating that net transported power is held constant. In a tuned system, large forward and reverse travelling-wave components can coexist while their difference remains the net power flow. That does not mean the transmitter independently generates the sum of both directional powers.

One 1.5 kW, 20:1 Example—Two Very Different Answers

Take an ideal 50 Ω line with S = 20, so |Γ| = 19/21 ≈ 0.9048.

For the constant-net-power case, the directional powers at that ideal plane are about 8.27 kW forward and 6.77 kW reflected. Their difference is 1.5 kW. These are wave-decomposition quantities inside the standing-wave system, not a claim that an ordinary 1.5 kW amplifier suddenly manufactures 8.27 kW of net energy.

A real line is lossy, so power and reflection coefficient vary with distance. The tuner, cable length, source boundary and load all matter. At high mismatch the exact lossy-line calculation—not one multiplier—must establish conditions along the entire run.

Heating Is Not Governed by √SWR Alone

Conductor heating is strongest near current maxima; dielectric heating is strongest near voltage maxima. Because those positions differ, mismatch can create a periodic temperature pattern. Cable length, attenuation, thermal conduction, ventilation, bundling and nearby materials then shape the actual temperature.

A useful first-order indicator comes from comparing the sum of forward and reflected travelling powers with their net difference. At one plane:

A 20:1 example may produce a voltage that remains below one cable’s stated insulation figure while still producing unacceptable conductor loss, connector heating or tuner current. Passing a voltage check does not establish thermal or current margin.

“RG-213 Class” Is Not a Power Specification

Power capability must come from an identified product and its stated conditions. Revision 0.599 of the official Belden 8267 RG-213 data, dated 20 February 2026, lists maximum power values that fall from 9,295 W at 1 MHz to 2,761 W at 10 MHz, 1,122 W at 50 MHz and 748 W at 100 MHz. The same page identifies it as a commercial, non-QPL product and lists different voltage values for different regulatory contexts.

Those numbers apply to Belden 8267 under the manufacturer’s applicable rating basis. They do not rate every cable sold as RG-213, and they do not authorise operation at the listed matched power under arbitrary SWR, ambient temperature, connector, duty cycle or installation. They do prove why “decent RG-213 class coax” is too vague for a QRO safety conclusion.

The Times Microwave cable calculator likewise starts by asking for frequency, length and an exact cable selection. Loss is not a property of diameter or folklore; it belongs to a specified product and operating point.

What Can Limit a QRO Feedline

The weak point may be:

• the cable dielectric or conductors;
• a PL-259, N connector, adapter or bulkhead feed-through;
• a soldered or crimped termination;
• a tuner capacitor, inductor, relay or PCB trace;
• a balun, transformer or common-mode choke;
• water, salt, dirt, corrosion or a damaged jacket; or
• an installation that traps heat or violates bend-radius limits.

The published matched-cable limit and the connector limit are therefore only two rows in the system assessment.

A Defensible QRO Derating Workflow

1. Identify the exact path. Record cable manufacturer and part number, length, connectors, adapters, tuner, choke, switching and enclosure.
2. Define power correctly. State forward, reflected, net and load power at specified planes, plus PEP, average power and duty cycle.
3. Define every operating state. Include each band, normal mismatch, tuner search, band changes, rain or ice, antenna faults and credible operator error.
4. Use a lossy-line model. Calculate voltage, current and dissipation versus position with the manufacturer’s attenuation and velocity data at the relevant frequency and temperature.
5. Check separate limits. Compare peak voltage, peak current and average heating with the applicable cable, connector and component ratings. Do not convert one rating into another.
6. Apply manufacturer derating. Account for ambient temperature, altitude, bundling, installation and mismatch using stated methods. If the manufacturer gives no usable conditions, the claimed margin is uncertain.
7. Validate conservatively. Begin below the intended power, use controlled key-down time and monitor remote instruments. De-energise before close inspection. Odour, noise, unstable readings, discoloration or unexpected temperature rise are stop conditions.

The Practical Verdict

High SWR can raise voltage, current and feedline heating—but not by one universal multiplier. √SWR correctly gives the ideal-line maxima when net transported power stays constant. At constant forward power, the maxima follow 1 + |Γ| and delivered power falls. Real coax adds position-dependent attenuation and heat.

No general QRO conclusion follows from an SWR, a wattage and a generic cable class alone. Calculate the exact cable system under a fully defined power condition, then verify voltage, current and thermal margin separately.

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