Reflected Power SWR What Really Happens!

"Reflected power" is an excellent model or mechanism for understanding and calculating operating impedance changes along a mismatched transmission line. However, it confuses people into thinking it is a real backward power flow to the PA device. Even worse some claim or think it is power bouncing back and forth in a transmission line until it is either dissipated or radiated. Reflected power is a valid tool that is easily and often misused. Key takeaways from this video: 1.) Power does not flow two directions in the transmission line at the same time. 2.) "Reflected power" or a load mismatch simply sets up a standing wave in the transmission line. 3.) The standing wave describes a fixed or stationary operating impedance change along the transmission line's length. 4.) Mismatch loss, % forward power, and % reflected power shown in common SWR tables only apply to certain special sources. They are not applicable to mismatched coaxial cable losses in our systems. The cable loss change from load mismatch is due primarily to current and to a lesser extent voltage change along coaxial transmission lines at HF and low VHF. 5.) Transmitters should have their designed impedance range presented to them. Incorrect impedance termination at the output port can cause various changes or problems. 6.) A particular termination error affects different final amplifiers in different ways. A mismatch can increase or decrease PA heat, increase or decrease PA component voltages, or might change operation at all. One has to know how a particular PA behaves when termination impedance changes. The same holds true for transmission lines. Tables that correlate a specific system loss to a specific (V)SWR and reflected power are misleading, as we will see. This video focuses on a 10:1 VSWR and power loss through 25 feet of LMR-400 on 20 Meters when compared to loss with a 1:1 perfect match through the same cable and equipment. If you disagree with or have trouble following this video the book "Reflections" by W2DU will help. Walt wrote that book on a simple level to help Hams move away from the false idea that reflected power is backward power flow that makes it back into the PA where it is dissipated. Power does not flow backward into the PA. Reflected power is just another way of expressing or explaining the change in operating impedance along a transmission line. The load mismatch and "reflected power" sets up a standing wave, a stationary wave representing the operating impedance along the transmission line. Even with a 10:1 or 20:1 load-end SWR, near 100% power transfer is possible. All we need do is follow the maximum power transfer theorem (also known as the "conjugate match theorem") and have a reasonably good transmission line. This doesn't mean we want every radio or amplifier to see the wrong load impedance, it simply means reflected power is not real backward flowing power that dissipates in the power amplifier. I'm not great at videos. I don't have music, CW, or dancing bears. My audio isn't studio, I speak like I speak to everyone from my dogs and best friends to strangers on the street. My main focus is getting an idea across. I hope you enjoy this effort. Most of all I hope it helps us have a better picture of how a mismatch, even a gross mismatch, affects our systems.