How Antenna Tuners Work
How do antenna tuners work?
The majority of the material in this section comes from the following three sources:
Anderson: Antenna Tuners: Transient and Steady State Reflections
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 and Jeff Anderson are a big help. Take
a moment to read the work published by Karinya and Jeff 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 return 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 (Read on though, this is a bit more complicated than you might first think). 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, most 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 a minor amount of losses in the antenna tuner.
How does an antenna tuner re-reflect reflected energy but yet allow received signals to pass through?
Good question, and there are very few easy to understand references to this on the web. Fortunately, we have K6JCA to help us out with this. Here's the link the Jeff's work in this area. Antenna Tuners: Transient and Steady State Reflections
The following is a short summary of Jeff's work in this area and I have added a few things just to help clarify what is going on. The short answer to the question posited in this section is that antenna tuners don't re-reflect 100% of reflected power. Just like any impedance discontinuity, they reflect some energy and allow the rest of that energy to pass through. But let's not get ahead of ourselves, let's dive into the details of this and highlight what is actually going on.
There are two states that need to be considered in order to describe what the antenna tuner does and what the radio sees. On initial start of the RF signal, the initial waves of energy (think of the energy as individual packets or pulses) move towards the tuner. These initial pulses of energy are subject to reflections coming from the input port of the antenna tuner. On reaching steady state, these reflections disappear and the reflection coefficient of the tuner's input port becomes 0.
The following illustration depicts a simple test setup that was used by K6JCA to identify what happens on signal start and what happens when the signal (and reflections) reach a steady state. The first diagram illustrates the physical model with a 50ohm generator, 200 ohm load (antenna), 50ohm transmission line and a simple matching transformer. The transmission line on the load side is 1/2 wavelength long at the operating frequency, which essentially makes the transmission line transparent to the model.
In this second illustration, the formulas for calculating the coefficient of reflection and the transmission coefficient for each boundary have been added. The results of the equations are visible in the second half of the illustration.
And finally, the last illustration with all of the coefficient values added to the model.
It's obvious from the math that the transformer does not have a reflection coefficient of 1 on the output side, which means that some of the energy coming from the load (either a received signal or a reflection from a transmitted signal) is going to pass through the tuner and onto the radio. The same applies on the generator (radio) side of things. The tuner does not provide (initially) a reflection coefficient of 0 and thus some of the energy coming from the radio is going to be reflected back towards the radio.
The key word in the above paragraph is "initially", that is during the transient state between signal startup and steady state operation. Keep in mind that the transmission line on the load side is 1/2 wavelength long and thus there is a time factor involved for signals traveling from the radio to the tuner and on to the load.
Consider the following screen shot consisting of a time based recording illustrating reflections from the tuner back to the radio on the left side and the pulses of energy coming from the tuner towards the load on the right side. This experiment was done with a 12 cycle burst of RF power and the results of the transmissions and reflections recorded.
Note that on the left hand side, the initial reflections from the antenna tuner start out strong and as the cycle continues they begin to dissipate. On the right hand side we can see the signal coming from the antenna tuner moving toward the load. Note that the signal starts out a bit weak and builds to a stronger signal as the cycle progresses.
So, what's happening here. Let's add one more screen grab to illustrate what is going on.
Here's our circuit model with everything filled in, including transmitted power (voltage in this case) and all of the reflections that are happening. Looking at the left hand side between the radio and tuner first you can see that there is a reflection from the tuner going back towards the radio. The power from this reflection is reduced over time as the power (reflections) coming from the load make their way through the tuner. The diagram above depicts the steady state of things after a period of time. The important things to note are that the tuner does not present a reflection coefficient of 1 to reflected signals coming from the load and that the tuner does reflect power back to the radio initially. In this case the reflection coefficient of the output port (looking from the antenna into the tuner) is actually 0.6 so some power from the antenna will make it through the tuner back towards the radio.
The key to everything that's happening here is located in the left hand frame. Note that the reflected power from the incident wave is equal in magnitude to the reflected power from the load (-0.6 and 0.6). These signals are out of phase and thus cancel each other out once the steady state has been reached. This would seem to be a contradiction of the law of energy conservation, but let's look at the other calculated values for some reassurance. Note the power calculation in red below the left hand side of the illustration of 20mw. This is what the transmitter is producing. Now look at the power calculation below the right hand side of the illustration, which also has a value of 20mw. no power, other than initial startup and shutdown power, is lost during steady state operation. The reason for this is that the power coming out of the tuner is a composite of a brand new wave along with the addition of reflected power from a previous wave. Note that on the left hand side reflected power and power coming from the load (reflected) are destructive and that power coming from the source (radio) and the power coming from the tuner (re-reflected power) are constructive due to their phasing. Power on the left is out of phase (180 degrees) and power on the right is in phase and thus adds together. This is a very simple model and as such complex impedances have not been considered, i.e., there are no inductive and capacitive values at the load to deal with. The math remains the same, but will be a bit more complicated.
So, how does an antenna tuner re-reflect 100% of reflected signals from the load but also allow received signals to pass? It doesn't. There's a lot more going on, as you can see. Imagine the above illustration in receive mode. All of the blue lines (power from the transmitter) go away and and direction of the green line is reversed. That is, the antenna is now acting as the generator and the receiver is the load. Reflections of power still happen between the tuner and the antenna, but some of the power makes it through the tuner each time. Additionally, just like in the previous paragraph, re-reflections from the antenna are added to the incoming power from the antenna and thus all of the power coming from the antenna reaches the radio during steady state reception. There are no waves that cancel out here, as we don't have reflections on both sides of the tuner.
For more information on this, and a LOT more detail, please see Jeff's work at the link provided above.