Originally Posted by NN5I
Answer, with discussion:
Briefly, an antenna tuner intercepts power reflected back down the feedline by a mismatched antenna, and reflects it back up the feedline, returning it to the antenna for another try.
Consider a 100-watt 50Ω transmitter, connected with a 50Ω feedline to a 50Ω antenna. Everything is perfectly matched. One hundred watts come out of the transmitter, go into the feedline, and are radiated by the antenna. This system will have an SWR of 1.0 everywhere. Let’s assume the feedline is perfect and has no losses.
Replace that antenna by a folded dipole whose impedance is about 300Ω. The SWR is now about 6. The antenna radiates just half the power, or 50 watts, and reflects the other 50 watts back down the feedline. These 50 watts will re-enter the transmitter.
So what? Well, let’s assume that that transmitter is 80% efficient. It takes 125 watts from its power supply, puts out 100 watts of RF signal, and dissipates the other 25 watts as heat. Its designer will have provided enough cooling (heat sinks and fans) to handle those 25 wasted watts. Now, add 50 watts of reflected power that the amplifier must also get rid of, and the cooling load on the amplifier is trebled. The final stages may burn up, a common failure in transmitters. That’s why many modern rigs are designed to reduce power, or even shut down, with high SWR.
What to do? We could improve the antenna, but it’s easier to use an antenna tuner. The tuner will intercept and re-reflect that reflected power, sending it back up toward the antenna. Now there’s 150 watts going up the feedline even though the transmitter puts out only 100 watts. The antenna is radiating half the original power, and half the re-reflected power.
Of the re-reflected 50 watts, the antenna will radiate 25 and reflect the other 25 back down the feedline. The tuner will send it back up. Now there’s 175 watts going up the feedline.
After enough round trips, there will be 200 watts going up the feedline; and the mismatched antenna will be radiating 100 watts, which is what the transmitter puts out. Put a wattmeter in the line (between the tuner and the feedline) and – sure enough – it will show 200 watts going up and 100 watts coming back. This may have been surprising in the past, but now you understand it. The “extra” 100 watts has been making multiple round trips up and down the feedline.
Notice that some of the power is delayed by its extra travels. If the feedline is really long, this may actually be noticeable in the transmitted signal and is an example of ringing.
This also explains why, with a lossy feedline that loses some power as heat, the losses increase disastrously with high SWR. Much of the power goes up and down multiple times, with losses each way. That’s why an antenna tuner, though it saves the transmitter, isn’t as good as an antenna that’s properly matched to the feedline. It also explains why a feedline that’s perfectly adequate with a well-matched antenna may fail with a mismatched one. Imagine: If the mismatch were so bad (SWR=100) that the antenna radiated only 4% of the power that reached it, there would be 2500 watts going up and 2400 watts coming back. Common amateur feedlines can’t handle 4900 watts. Fortunately, amateur tuners can’t handle SWR=100 either.
After you have adjusted the tuner so that no reflected power re-enters the transmitter from the feedline, the transmitter will “see” a load that matches the impedance for which it is designed (probably 50+j0 Ω). Thus, if you answered that the tuner “matched” or “transformed” the feedline (or antenna system, really) impedance to the transmitter, your answer is strictly correct but unenlightening.
There are other ways to keep reflected power out of the transmitter. One of these is by the use of a terminated circulator (also called an isolator). That’s a device that has a third port on which you place a dummy load, so that direct power goes up the feedline, while returned power goes out the third port and into the dummy load. This would waste the reflected power; better to use a tuner to send it back up to the antenna.
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