How Antenna Tuners Work
How do antenna tuners work?
The majority of the material in this section comes from the following three sources:
The question of why you may need an antenna tuner has already been answered in the "Do you need an antenna tuner section". My observation is that literally
every ham that I have ever met has one. Look at ham shack pictures on Pinterest or Facebook and you'll see that everybody has one. Why? Because most likely every
ham has some form of matching issue that they would like to correct.
Personally, I just think that antenna tuners are cool, and I think Doc Brown would agree.
How does an antenna tuner work
Review the article listed above from Karinya. There are lots of great articles on the site, but I find this one to be very interesting. In short the purpose of the
antenna tuner is to match impedance of your radio's output with that of the the feed line and load. How it does this is very interesting. Basically an antenna tuner does 2 things:
1) Provide a correct conjugate match for the impedance as seen by the tuner looking toward the transmission line and antenna.
2) Provide the proper resistive impedance match for your radio.
Typical amateur radios have an output section designed for a 50 ohm resistive impedance and this is what a tuner needs to provide to the transceiver in order for it to produce its full rated output.
What is a Conjugate Match?
Quoting from Maxwell:
"The term conjugate match identifies a condition where the impedances on opposite sides of a junction have identical resistive components, and reactive components, if any, that are equal in magnitude but opposite in sign. For example, a conjugate match exists when a source impedance of 50 + j10 ohms feeds a load impedance of 50 – j10 ohms. When a conjugate match is accomplished at any of the junctions in a system, all reactances in the system are canceled, including any reactance in the load. This reactance cancellation establishes resonance in the entire system, and the generator delivers its maximum available power to the load. This means that a non-resonant antenna as the load is tuned to resonance by the conjugate match."
The statement from Maxwell, is broad and a lot of things can be read into it. This is where the experiments and writeup from Karinya are a big help. Take
a moment to read the work published by Karinya and pay particular attention to how the antenna tuner CAN change the impedance of the system
from the perspective of the load, AND from the perspective of the source. Basically what Karinya says is that the antenna tuner cannot change the impedance of the load, but what it can do is change the impedance of the system from the perspective of the load looking back toward the source. Additionally, the tuner
cannot change the 50 ohm characteristic load of the source (transmitter), but it can change the impedance of the system from the perspective of the source
looking into the tuner. i.e, the tuner can provide a conjugate match for the feed line/load impedance, which will eliminate SWR at the tuner, and it can provide
at pure 50+0j ohm load to the transmitter. Everybody is happy!
Confused? Refer back to the AT&T video presented by John Shive and recall what happened to the reflected waves as they returned to the source and were re-reflected back to the load. This is what an antenna tuner does for you. By presenting a perfect conjugate match to the reflected waves, all of the reflected wave's energy is returned back to the load. In the AT&T video this is accomplished by fixing (or holding in this case) the first rod on the machine steady. When the reflected wave, traveling back from the load meets this the immovable first rod (the conjugate match), 100% of the wave's energy is re-reflected back toward the load. Note also that at this point the re-reflected wave is in phase with new forward wave (power coming from the transmitter) and that these waves superpose on one another creating a new voltage/current peak that is greater than that of the original forward wave. In other words, if we were to measure the power going towards the antenna AFTER the tuner, we would measure more power coming OUT of the tuner than we measured going INTO the tuner. Mr. Shive illustrates this in the video as well.
At the load end, a perfect conjugate match does not exist. When the forward wave from the transmitter meets the junction between the feed line and antenna, some of the power goes into the antenna and some of the power is reflected back to the source. This can be seen in the AT&T video when John Shive attaches a second machine to the output of the first machine. The impedance of the second machine is different. The second machine has smaller cross arms and it is clear what happens when the forward wave hits this impedance mismatch. In the example, some of the power moves into the second machine and some of the power is reflected back toward the source. Again, by providing a perfect conjugate match at the source, that represents the opposite impedance of the antenna and feed line at the source, 100% of the reflected wave's energy is re-reflected back to the load. When the re-reflected wave hits the feed line / load junction again, some of the power in the re-reflected wave moves into the load and some of the re-reflected power is reflected back to the source, again.
Reflected wave energy bounces back and forth between the load and conjugate match presented by the tuner until all of the energy has either been lost due to feed line attenuation, or the power is radiated out through the antenna. There are losses in the feed line and the antenna tuner. What we can do to help overcome those losses in the tuner is to use a tuner that is more efficient. How do we do that???? Read on....