The Real Reason Your Coax Is Heating Up: Understanding Reactive Power and SWR Loss
While high SWR is often blamed for “ruining” a radio, the physical reality of why your coax gets warm involves a fascinating tug-of-war between forward and reflected waves. When an antenna isn’t perfectly matched, the coax becomes more than just a pipe—it becomes a storage device for reactive power.
Here is the breakdown of why that heat happens and how SWR changes the math of your transmission line.
1. The Interaction of Waves
When your transmitter sends a signal (Forward Power) toward a mismatched load, not all of that energy is radiated. The portion that is “rejected” travels back down the cable (Reflected Power).
- Constructive Interference: At certain points along the line, the forward and reflected waves add together, creating voltage peaks.
- Destructive Interference: At other points, they partially cancel out, creating voltage nulls.
This creates a Standing Wave. The “Heating” occurs because these peaks increase the stress on the physical components of the cable.
2. Where the Heat Comes From
Coax isn’t perfect; it has inherent “matched line loss” (attenuation) based on its construction. However, high SWR multiplies these losses through two primary mechanisms:
Dielectric Loss
Coax uses an insulator (dielectric) between the center conductor and the shield.
- High SWR creates Voltage Maxima much higher than what the cable sees at a 1:1 match.
- These high-voltage points “stress” the dielectric, causing molecular friction that generates heat. If the SWR is high enough, the dielectric can melt or arc.
Ohmic (Resistive) Loss
Every wire has some resistance.
- High SWR creates Current Maxima.
- Since heat is calculated by P = I2R, doubling the current at a specific point in the cable doesn’t just double the heat—it quadruples it.
3. The “Ping-Pong” Effect
This is the most critical concept for understanding total loss. When power is reflected back from the antenna, it doesn’t just disappear.
- The reflected power travels back toward the transmitter.
- As it travels back, it is subject to the cable’s attenuation a second time.
- If you have an antenna tuner (ATU) at the rig, it reflects that energy back toward the antenna again.
- The power bounces back and forth (“ping-pongs”) until it is either radiated by the antenna or dissipated as heat in the coax.
Key Takeaway: The longer and “lossier” your cable is, the more power is absorbed during these multiple bounces. This is why a high SWR is much more damaging on a long run of thin RG-58 than on a short run of high-quality LMR-400.
4. Reactive Power vs. Real Power
In a perfectly matched system, the voltage and current are “in phase,” and the power is Real (Resistive)—it’s doing work (radiating).
In a mismatched system, the energy becomes Reactive. This energy is stored temporarily in the magnetic and electric fields of the coax. While reactive power doesn’t “do work” at the antenna, it still requires Real Current to move it back and forth. That extra current is what toasts your connectors and softens your jacket.
Summary of SWR Impact
| Factor | Effect of High SWR | Result |
| Voltage | Increases at peaks | Risks dielectric breakdown/arcing |
| Current | Increases at peaks | Increases I2R heat loss |
| Line Loss | Power bounces multiple times | Total system efficiency drops |
| Component Stress | Heat transfers to connectors | Can desolder or oxidize joints |