The information in this document is not of my development, but from years of listening to others who are much more versed in antennas, RF circuits, transmission lines, etc. I have also Pillaged, Robbed, Copy and Pasted many useful bits of information off the Interweb, by Copy and Paste, and then editing the lot to hopefully make a coherent document.
In this Thesis I will refer to SWR as a simple means of stating a mismatch between two components operating at RF. I will also refer to load and antenna meaning the same, as the antenna is the load we work with. I will also refer to feed lines and transmission lines which are the same. The load could be something different, but we are interested in working with transmission lines and antennas and how they interact and work together.
The Antenna Tuner - ATU or AMU
An antenna tuner is a device that transforms one impedance into another. Impedance is the resistive and inductive or capacitive value in Ohms of a load such as an antenna. When using coax or feed line to feed power from a transmitter to an antenna this impedance is typically desirable to be 50 Ohms. Many co-ax cables used today are 50 Ohms, which means if the load (antenna) impedance at the end of the feed line is 50 Ohms then one will get a good match and all the power fed into the feed line will travel up the feed line and into the load. However, we seldom find this perfect match in the real world. More often, especially when an antenna is being used on multiple bands, the antenna will have a different impedance than the feed line. Meaning that of the power fed into the feed line some of it will not go into the load or antenna and is reflected back to the transmitter. This is where an antenna tuner can help.
Some misconcepts about antenna tuners: 1. They tune the antenna (false). 2. They do nothing more than make the radio happy and do little to help the antenna system perform better (false and false again).
When using an antenna tuner, the antenna itself is more important the antenna system (radiator and feedline and the surroundings), and it can be adjusted so as to get more power into the radiator or antenna element. As an example.... If one has an antenna with say 4:1 SWR, this means 36% of the power going to the antenna is reflected back down the coax toward the source or transmitter. So, if one puts 100 watts into the coax/feed line this 100 watts when it arrives at the antenna, only 64 watts goes into the antenna to be radiated/transmitted and 36 watts is reflected back to the transmitter. This is not good, for one has lost power not being radiated/transmitted and also it can go back into the transmitter possibly causing damage to the final power amplifier.
This is where the tuner comes in. The antenna tuner is placed in the feed line between the transmitter and antenna. It can be placed at the transmitter end, the antenna end or anywhere in between. Where it is placed has to do with what type antenna one might have or for convenience might be at the transmitter end of the feeder where the tuner can be adjusted. Some tuners are "automatic" type meaning they automatically adjust themselves when power is applied and often these tuners are placed at the antenna. In a mobile this might be a better performing system.
A tuner has coils and capacitors inside to make its adjustments. The amount of inductance from the coils or capacitance from the capacitors depends on the load to be "compensated" for, and adjusted for. That is as we have an antenna with a high SWR or load much different from our coax impedance the more or less capacitance or inductance is needed.
The tuner is a circuit that transforms our load into the desired impedance of the transmitter. This is usually the 50 Ohms required by modern transmitters. To adjust the tuner, one applies or transmits power from the transmitter and using a SWR meter between the transmitter and to adjust the tuner settings for lowest SWR. Once this is done then the transmitter will be happy seeing a good load (50 Ohms) and the SWR working into a good load. However, the antenna has not changed. It still has a 4:1 SWR and for every 100W going up to it the antenna reflects 36W back toward the transmitter. So how does a tuner help ?.
Well here is the secret !! .....
When the power going up to the antenna gets there, some goes into the antenna and is radiated and some is reflected back down the coax as in our 100W forward / 36W reflected example. This 36W reflected travels back down the feed line to the antenna tuner. When this power gets to the tuner there is an infinite SWR or it acts this way and all of the 36W is reflected from the tuner back up the coax. (I am not gong into how this is done, but it mainly has to do with the phase of the voltage and currents in the reflected 36W). When this 36W gets back to the antenna with 4:1 SWR 64% goes into the antenna and 36% of it is reflected (64% of 36W=23W into the antenna and 36% of 36W=13W reflected). At this point the total power that has gone into the antenna is 64W + 23W = 87W. Now the 13W reflected repeats this same scenario with it going down to the antenna tuner and all being reflected back up to the antenna with 64% (8.3W) going into antenna and 36% (4.7W) being reflected. We now have 64W + 23W + 8.3W = 95.3W going into the antenna. This process continues until all the power goes into the antenna.
We need to take a breath and note some other things that are happening.
1. The example is assuming there are no coax losses, that is if we put 100W into the coax then all of that power will get to the antenna. We know in the real world this does not happen for we know some of the power will be lost and converted to heat. This is for all the power including the 100W, the 36W reflected, 23 W, 9W, the 8.3W, etc. But for our demo we ignore this loss just to make the point. In real life one wants to use the lowest practical loss coax feed line.
2. Another issue is in real life is that the tuner is not perfect, so some power is lost due to the tuner not being able to tune to a perfect 1:1 SWR at the transmitter. Many automatic tuners tune until they get a 1.5:1 SWR and quit. This is often good enough for most transmitters. A 1.5:1 SWR means if 100W forward we have 4W reflected and is hardly a power level we need to be concerned with.
3. The antenna itself has not been changed thus the tuner did not tune the antenna. It did tune the "antenna system" and provided an impedance the transmitter can handle. The tuner did tune the antenna system keeping in mind the antenna system is the radiator, feed line, connectors and the surroundings such as buildings, gutters, fence post, garage doors, cans, and of course the Earth ground. In the case of a vehicle antenna it can also include the people walking around and the vehicle itself and where it is mounted. The antenna characteristic can be changed in a moment affecting what the tuner must do.
4. The antenna, whatever is used, has a radiation pattern and characteristics. Thus, however we get the power into the antenna and the radiated characteristics will remain the same. The tuner is only helping to get more power into the antenna. What happens to this power depends on the antenna. Some power may actually go to ground and some in a direction we do not need. That is radiate it up or away from the desired receiving station.
Antenna tuners do work. They can make the antenna system more efficient. But again they do not improve the antenna or radiator performance. Just makes it look that way because more power gets into the antenna.
An issue important here is the antenna pattern. Some use a long antenna such as 80 Metre dipole to operate on all the HF and maybe 6 Metres and higher. As an antenna gets longer it tends to radiate more off its ends. So on 80 m the radiation pattern, the way the power leaves the antenna, might be more of a wide figure 8. But at say 10 Metres it might be a very flat figure 8 with 90% of the radiation off the ends in the direction of the dipole wire. So if the antenna wire is mounted to go (say) east/west on 80 Metres it will radiate well north and south, but on 10 Metres will radiate mostly to the east and west. Of course if you want it to do this then you get good results. The point is just because you have a good SWR and the tuner is getting more power into the antenna it does not mean you are getting the performance you need or want.
How the RF on the Feedline Acts...... and why it gets reflected from the tuner back up the feedline.
There are two ways to look at the reflected power being reflected from the tuner back up the feed line to again try to go into the antenna and be radiated.
One way is we know we tune the tuner so that an SWR power meter placed between the transmitter and tuner shows no reflected power if the tuner is tuned properly. If we place a SWR power meter between the tuner and antenna we do see power being reflected if the feed line does not match the antenna. Where does this reflected power go. It could go back into the tuner, but since the tuner does not get hot (it may get warm just because it does have some loss) the power cannot be going into the tuner and is not going from the tuner to the transmitter.
So if the SWR power meter between the transmitter and tuner shows no reflected power and the power is not being absorbed by the tuner and we cannot destroy or create power (law of Physics) then it would seem reasonable to assume the reflected power is going back up the feed line to the antenna.
To try to explain why this is happening.........
First what happens to the power as it gets to the load/antenna.
To start with, we take a feed line that is open at the load end; no load or antenna connected. As the RF power goes up the feedline it is made up of a RF voltage and current which are in phase. When this power gets to the open end the current goes to zero since the end is open, but the voltage remains. Since there is no where for the power to go it gets reflected back down the feedline toward the source/transmitter/tuner. The reflected voltage will be in phase with the forward voltage coming up the feedline, but the reflected current will be 180 degrees out of phase with the forward current.
Next we take the situation where the feedline is shorted at the load end. As the forward power reaches this shorted end the voltage goes to zero (a short causes this zero voltage). Again since the power has no where to go it gets reflected back down the feedline, but now the reflected voltage is 180 degrees out of phase with the forward voltage and the reflected current is in phase with the forward current. In both of these cases all of the power is reflected down the feed line.
We can also take a situation where there is a load/antenna not being a short or open, but not a perfect match to the feedline; the load is at a different impedance than the feedline and we have an SWR higher than 1:1. Here some of the power will go into the antenna. How much depends on the SWR or mismatch between the feedline and antenna. Of this reflected voltage and current one will be in phase and one will be out of phase. If the load is lower than the feed line impedance the current will be out of phase with the forward current and the reflected voltage will be in phase with the forward voltage. If the load impedance is higher than the feed line impedance the reflected voltage will be out of phase from the forward voltage and the reflected current will be in phase with the forward current.
The tuner is tuned so as to transform the impedance of the feed line at the tuner output to the transmitter impedance typically 50 Ohms resistive. One way this is done is tuning out the reactance (XL or XC impedance) at the tuner output. The tuner basically tunes to have the opposite L or C from the feed line C or L. So if the feed line has say a XC impedance of 40 Ohms the tuner will provide a XL of 40 Ohms, but these two XL and XC impedances will be 180 degrees difference in phase. This phase relationship will cause the forward current or voltage to be 180 deg out of phase from each other and will cancel. This effect will make the tuner look like an infinite SWR to the reflected power thus it will be reflected from the tuner back up the feed line to again to try to go into the antenna and be radiated. The reflected voltage or current being out of phase with the forward voltage or current is the reason the reflected power gets reflected by the tuner and goes back up the feedline to the antenna. As when the reflected voltage or current were made to be 180 deg out of phase with the forward voltage or current, the tuner will now also produce a 180 deg phase shift of the voltage or current. Since the reflected voltage or current was 180 deg out of phase with the forward and it again gets changed 180 deg the voltage or current reflected by the tuner is now in phase with the forward voltage and current.
Restating the reflected voltage or current became out of phase by 180 deg at the antenna. Then when this reflected voltage or current returned to the tuner it was reflected, but again the reflected voltage or current became out of phase by 180 deg a second time making it now in phase with the forward voltage and current. This power now adds and goes up the feed line. This process continues until all the power ends up in the antenna. We are ignoring losses here just to keep it simple.
One note is we now see power going up and down the feed line a number of times. How much power makes more than one trip up and down the feed line depends on the antenna mismatch. Since the feed line is now handling more power at any given time than coming out of the transmitter this can affect the power handling of the feed line. We are not creating power, but it is that the same power travels over the feed line a number of times so when selecting a feed line one must make sure it can handle higher power than the specifications for a particular feed line. This is also why a small coax like RG58 can handle say 1,000 watts of power at 80m if one has a low SWR, but cannot if the SWR is high.
SWR, What is it ?
When power is transmitted up a feed line and it gets to the antenna and the load impedance is the same as the impedance of the feed line all the power goes into the antenna. Thus there is no reflected power, voltage and current coming down the feed line...all the power goes into the antenna. If, however, the load impedance is different from the feed line then some of the forward power is reflected. Either the voltage or current will be 180 deg out of phase with the forward voltage or current. As this reflected out of phase voltage or current travels back down the feed line it adds or subtracts with the forward voltage or current. This causes the value of the voltage or current to be different at different points along the line. The SWR is the ratio of the differences in this max and min voltage or current along the line.
VSWR (voltage SWR) is the ratio of the max voltage between different points on the line. Also, SWR = (load impedance) divided by (feed line impedance) or SWR = (feed line impedance) divided by (load impedance)
SWR is always given as 1 or greater and a ratio of 1 or higher to 1 (ex. 2:1).
A measure of forward power to reflected power ratio the following SWR for forward to reflected power: SWR = 1.5:1 forward power of 100W reflected power of 4W (4%) SWR = 2:1 forward power of 100W reflected power of 14W (14%) SWR = 3:1 forward power of 100W reflected power of 26W (26%) SWR = 4:1 forward power of 100W reflected power of 36W (36%) SWR = 10:1 forward power of 100W reflected power of 70W (70%)
One can see an SWR of 3:1 or more becomes significant and to be concerned about. So by knowing the SWR one can determine how good or bad the match is between the feed line and the antenna.
SWR is usually measured with a SWR meter or bridge. The meter works by having a short piece of feed line inside and sampling at 2 different points along this line. For the forward power the sample is taken closest to the transmitter. For the reflected the sample is taken closer to the antenna. These samples are most often only a few inches apart.
The RF voltage is sampled, rectified to DC and fed to 2 meters which will allow one to read the ratio. Some meters have one meter with a switch to connect to the 2 samples. Also often the forward sample has a variable resistor to adjust the meter for the max reading and then the reflected sample is measured with a calibrated scale to indicate the SWR reading. Other methods are used with some just giving forward and reflected power or voltage, some have 2 meters in one housing with scales for where the 2 meter movements meet to indicate VSWR. These are called cross needle SWR meters. In all cases 2 voltage samples are taken and compared.
Nearly all SWR meters are really VSWR meters in that they measure the max to min voltages on the feed line. Also most power meters are really volt meters measuring the voltage on the feed line with a meter scale calibrated in power assuming a fixed impedance such as 50 Ohms. It is really difficult to measure real power for there are many variables in this measurement.
Since power = V^2 / R (voltage squared divided by R) by measuring voltage and knowing R we can determine the power. Again most RF power meters do this using a calibrated meter scale and assuming an R of 50 Ohms. If another impedance such as 300 Ohms is used then a different scale and calculation is required. Also the sampling device is through capacitive coupling to the coax feeding the antenna. The distant between the pick up point on the coax and the sampling pick up determines the amount of voltage that will be sampled. As the frequency of the RF increases or decreases this sampling value will change affecting the power reading. Usually power meters have a frequency range for the power scale they are used for. Some meters have different sampling elements to adapt the meter for different frequencies. The point is one must make sure the meter used for measuring power be made for the frequency in use.
Power can also be measured by measuring current. Power = I^2 x R (current squared times R)
Testing Antennas ...a few thoughts
Making antennas is one of the most popular aspects of our hobby - and for good reason. Even the most "practically-challenged" radio amateur can make an antenna. And the great thing about antennas is that it's almost impossible to make an antenna that does not "work" to some extent. By "work" I mean radiate radio waves. To that extent, antenna projects are always a success! But how do we test our antennas? The most common measure of a "good" antenna is its Standing Wave Ratio. Good antennas have a low SWR, right? Unfortunately, although SWR is easy to measure and a low SWR at the output of our transmitter is a good thing, SWR tells us virtually nothing about how "good" the antenna is. In fact a good (low) SWR can be an indication of serious antenna problems!
Power Transfer and SWR
For a transmitter to deliver maximum power into the antenna, the antenna impedance must be the same as the output impedance of the transmitter - this is a low SWR. Moritz von Jacobi published the maximum power(transfer) theorem around 1840 and this theorem indicates that maximum power transfer occurs when the load (antenna) resistance is the same as the source (transmitter) impedance. However, the easiest way to get a great match with an antenna is to make it lossy (RF power gets turned into heat, not radiated). A example of this is a dummy load where the objective is to turn all RF power into heat and radiate none at all. A good dummy load is a handy piece of test equipment for any shack, it will have a low SWR over a wide band - but will be a terrible antenna as it will radiate virtually no energy.
To a lesser extent all antenna systems (antenna plus feeder) turn some RF energy into heat and in doing so their SWR is usually improved by losses. Many wideband antennas actually incorporate resistors. Where small antennas, such as magnetic loops, are being used, a good SWR over a wide bandwidth without re-tuning is almost always a bad sign. Curiously at least one loop supplier tries to make low SWR over a whole band without the need to re-tune, into a feature! This is a clear example of low SWR indicating a design fault.
An example of an antenna that has a high SWR is a doublet fed with open wire line. The SWR at the feedpoint of a doublet can be very high (10 or more) but because the feeder losses of the open wire line are so low, the overall antenna system is still a good performer. Interestingly a doublet is an example of antenna that does not even need to be resonant to work well. Low SWR and resonance are two very over-rated measures of antenna performance.
As an aside, another measure of antenna performance is the current flowing in the antenna. Antenna current in the radiating element is what you want to maximise. If using home-made transmitters and "tricky to match" antennas,such as short loaded verticals, antenna current is often the most reliable measure of antenna performance. Despite these thoughts, SWR cannot be dismissed as irrelevant as most modern transmitters have SWR protection circuits which lower the transmitter power if a high SWR is detected at the antenna socket so to keep your transmitter "happy" a low SWR at the radio is a good thing - provided you don't think it is telling you much about your antenna. Radiated Power
If we accept that a dummy load or an antenna could have a low SWR for a variety of reasons (including losses - excessive or otherwise) how could we decide which was the better "antenna"? The difference is clearly how much power is radiated (and in what direction). In the case of a dummy load it probably gets hot showing that the power is being turned into heat. Measuring radiated power for an antenna is difficult to do in any meaningful way. All parts of the antenna system will have losses but as they are small, measuring heating effects may require specialised equipment and is not generally practical. Even if we can measure those losses, there will be there loss factors due to the environment that the antenna is in, which will mean that identical antennas in different locations will have different overall losses - with subsequent differences in performance. This moves into the realm of measuring overall system performance. Done accurately, this is a fascinating way to see just how good your antenna really is!
Practical Measurement of Radiated Power The real test of any antenna is how well it performs in real life. SWR, antenna current and even radiated power only show part of the story. In the past, the performance of antennas has been evaluated in a variety of ways including plying field strength meters around in helicopters! Radio amateurs have occasionally driven round with field strength meters, although measuring the performance of HF antennas by measuring field strength near the ground is not a very good technique and is fraught with problems.
One possible way to test performance is using the Reverse Beacon Network. In essence this is a network of receivers with special software that can detect stations calling CQ on CW and pass their callsigns to an internet database. this works well but to use it effectively you need to be a CW operator - and call CQ a lot! It is especially useful for SOTA/NPOTA activators to check propagation. In many ways, a better system is the Weak Signal Propagation Reporter(WSPR) network (by Joe Taylor, K1JT). Many remote receivers detects signals well below the noise and report back to a central website. The power levels used are low (often less than a Watt) but because of the way the system works, the performance of these low power transmitters is the equivalent of tens, hundreds, even thousands of Watts! Getting meaningful results from this system requires the user to do a lot of additional work though.
These pages are continually under construction and change - Please call back frequently to check them out again!