Antenna Tuners - Setting the Record Straight
I am quite sure that few if any, in the Amateur community, will argue with the premises that the antenna is one of the most important components of their station. In fact I will be so bold as to say “the most important component”. We all know that the best rig money can buy will NOT give a good account of itself if it is coupled to a poor antenna. We also know that a rig that is many years old, but in good repair, WILL give a good account of itself if it is properly coupled to a good antenna.
Antenna System Defined
Properly coupled to the antenna? Let’s examine that concise statement and see what it means. In order to do this we need to examine our thoughts about what does an antenna consist of. It consists of a radiator, a transmission line, and a ground. These three components are all a part of the antenna and must be dealt with as a “SYSTEM”. Even with a resonant half-wave dipole, the ground is part of the system because of the distributed capacity from the radiator to ground. Therefore, from here on we will be thinking about an “ANTENNA SYSTEM” and not its individual components.
Enter the antenna tuner, this piece of station equipment has had many words written about it. Unfortunately, these words are often, either misunderstood, or in some cases they are just plain incorrect. Some of the bad raps attributed to the antenna tuner are:
- An antenna tuner only fools the transmitter into thinking it is seeing a matched load.
- An antenna tuner, which is located at the transmitter end of the transmission line, cannot do anything to the radiator itself.
- An antenna tuner wastes much of the transmitter output power in dissipated heat. All of these ideas are FALSE. Let me hasten to say that a poorly designed or poorly adjusted antenna tuner can make all of the above bad things, and much more, come true.
Resonance
Before we get into the antenna tuner details, let’s look into some generalities about antennas. Remember, a resonant antenna, one in which the radiator is cut to a specific length, is resonant at one frequency only. As we move either up or down in frequency from resonance the radiator is no longer resonant and therefore, its feed point impedance becomes a combination of resistance and reactance rather than a pure resistance. As we go higher in frequency, the radiator will show inductive reactance as a part of its feed point impedance. It follows that as we go lower in frequency, the radiator will show capacitive reactance as a part of its feed point impedance. In conventional antenna theory, this condition is true regardless of the antenna type or its configuration. This does not mean the antenna stops radiating off-resonance, but it does mean the feed point impedance is no longer a pure resistance and its magnitude is now different than it was at the resonant frequency. It also means that if the transmission line was exactly matched to the radiator at its resonant frequency, no reflected RF energy. This is no longer true. This however is another subject, which will be discussed in some detail later in this article in connection with transmission line losses due to standing wave ratio (SWR).
We will now explore in practical terms what an antenna tuner can do to an antenna system and also what it can’t do, and what effect the “can dos” and “can’t dos” will have on the system. When the antenna tuner is properly adjusted, it tunes the antenna SYSTEM to resonance. Resonance is reached in any L/C circuit when the inductive reactance and the capacitive reactance are made equal. A properly designed and adjusted antenna tuner performs what is known as “conjugate matching”. This is just another way of saying, the system is tuned to resonance since it means, in a broad sense, the inductive and capacitive reactance in the circuit are equal. Conjugate match theory states: “In a multi-port system, if the reactance (inductive or capacitive) is tuned out at one port in the system, it is tuned out at all ports in the system,” and an antenna system is certainly a multi-port system.

For purposes of discussion, let us consider a center-fed dipole. Its feed point impedance is influenced by the radiator length, its height above real ground, the distance to and size of near-by conducting objects. As in most cases but not always, our antenna tuner is located at the transmitter end of the transmission line. The electrical length and natural impedance of the transmission line will also influence the magnitude of the impedance presented to the antenna tuner. Remembering conjugate match theory, the tuner must cancel out the reactance presented by the transmission line to the tuner, and it must also transform the remaining resistive component to fifty (50) ohms, which is what all modem Amateur transmitters want to see. It does this as follows: if the reactance presented to the tuner by the transmission line is “inductive”, then the tuner must present to the transmission line an equal amount of capacitive reactance.
Transformation
The following electrical configuration of the tuner is what transforms the remaining resistive component to fifty (50) ohms. The majority of present day antenna tuners are configured as “Hipass T” tuners, although there are numerous other configurations. At the radiator end of the transmission line, the reactance (if any) will also be canceled out. This means that the radiator is now tuned to the operating frequency, regardless of its length, height above ground, or the influence of surrounding conducting objects. It does not mean the natural impedance of the transmission line has been matched to the feed point impedance of the radiator. Despite this, with a given RF power from the transmitter and since the radiator is now at resonance at the operating frequency, the same RF current will flow in the radiator that would flow in a radiator that is resonant (cut to an exact length) at that operating frequency. Neither does the conjugate match change the SWR on the transmission line, but it does present an exact impedance match between the transmitter output and the transmission line input.
Without a doubt these three facts are what give rise to the idea that the antenna tuner only fools the transmitter into thinking it is matched to the antenna and nothing else. The point here, that more often than not, the fact that maximum RF current is now mel-2.gif (4421 bytes)flowing in the radiator is overlooked. And this is the name of the game, since it is the RF current in the radiator that couples the energy to space (radiates). In a properly designed and adjusted antenna tuner, this conjugate match process will be carried out with better that ninety five percent efficiency. Another likely point of confusion is the well-established concept that in order to obtain maximum power transfer in a system, all interfacing impedance must be matched.

But, in the case of the antenna system the objective is to obtain maximum RF current flow in the radiator, in order to couple maximum RF energy to space. The conjugate match does this without all interfacing impedance in the system being matched.
Since there are numerous antenna systems and antenna tuner types, it would take a rather large textbook to cover all of them. And remember, the purpose of this article is to give rise to the thought that the antenna tuner is not the “bad guy” it is too often portrayed to be. Therefore, it will be left to the serious reader to follow up on this rather complex subject, if they are interested. A very good start on this road is the ARRL Antenna Handbook, for those who wish to learn more about the subject.
Can’t Do This
This is not quite the end though, we will now take a look at some of the significant effects of the “can’t dos” of an antenna tuner. One of the first to consider is the fact that the SWR on the transmission line has not been altered by the conjugate match. If our center-fed dipole is cut to the center of the 3500 to 4000 MHz Amateur band, and it is operated over the entire band with maximum RF current flowing in the radiator (which is possible with a tuner), RF voltages can develop on the transmission line that exceeds several thousand volts. This of course raises the possibility of “flash over” which can destroy the transmission line and other components in the system, particularly at the higher transmitter power output levels allowed an amateur transmitter. The antenna tuner components must not only be high “Q” but they must withstand the high voltage which can be produced by high SWR. If there is a broadband transformer type balun involved, flash over can also occur there. The balun flash over problem does not exist with the “choke” or “current balun”.
Another “can’t do” effect is that transmission line loss increases with high SWR on the line, and very significantly in some cases. Coaxial transmission lines in general suffer from this malady to a much greater degree than 450-ohm open wire line does. Again this subject cannot be covered in a single brief article and the serious student of the subject is encouraged to turn to any one of the good textbooks on the subject. However, before we sign off a few more points are in order.
Achieve Broadband
I do not propose that an antenna system which incorporates an antenna tuner will satisfy everyone’s antenna system needs, but if your antenna system must serve multi-bands, and you want it to be broadband, it is a piece of equipment to which you should mel-3.gif (4532 bytes)give serious consideration. There are limitations in such an antenna system. For example: When the radiator is center-fed and at or near a multiple of half waves either side of the feed point, its feed point impedance can become so high that the tuner will not compensate, in some cases, for this condition. In addition under these conditions the SWR, even with a 450-ohm open wire line, can be unmanageable.

When an open wire, or balanced transmission line is used it is essential that a balun be incorporated between the transmitter end of the line and the antenna tuner. This is so since it is the purpose of a balun to make transition from a balanced load to an unbalanced load, and all modern day transmitters have an unbalanced output, hence the need for the balun.
About Baluns
Another misconception, held by some amateurs, is that the purpose of a balun is to match impedance. This is an incorrect idea, although some baluns have an impedance transformation ratio, such as 4:1 or 9:1, but this is a coincidence of the balun not its purpose. I strongly recommend the use of the “choke balun” or “current balun”, which always has a 1:1 impedance transformation ratio. The balun that has an impedance transformation ratio greater than one is always a special case of a broadband transformer, and is subject to bandwidth limitations and also flash over under high SWR conditions. The “current balun” on the other hand does not suffer from either of these conditions. It is not bandwidth limited nor will it flash over under high SWR conditions. A properly constructed current balun is not bandwidth limited if it is assembled by placing close fitting ferrite beads over a length of coax (usually about twelve inches long [30cm]) after the outer plastic jacket has been removed, and then the length of coax is resealed in some manner. If the correct bead material has been selected, the balun will do its job from 160 to 10 meters with ease.
Conclusion
In conclusion we have looked at the fact that an antenna tuner does much more that fool the transmitter into thinking that it is matched to its antenna system. In fact it is matched to the system by virtue of the “conjugate match theory”. We have also given thought to the fact that an antenna tuner can directly effect the radiator of an antenna system, even when it is located at the transmitter end of the transmission line. Finally the overall efficiency of an antenna system will suffer less that five percent when a well designed and adjusted antenna tuner is incorporated into the system. And by the way, I repeat: The purpose of a balun is not to match impedance, but rather to make transition from a balanced to a unbalanced condition.
Editor’s Note
I fully agree with what is explained about antenna tuners in this article. There is great deal misunderstood about the function of an antenna tuner. I have owned many tuners in the past years and the only thing I can add to this is to note that the efficiency of each of the different types of tuning design can vary quite a bit. Sure they will all get the VSWR down to 1:1, but how much does the received signal change between different tuner designs? That is the big question as far as I am concerned.
As an example of what I am saying, I conducted a test between two of my tuners several years ago on 40 meters. I was talking to a ham approximately 150 miles (240 Km) from my home and, just on a whim, decided to compare the two tuners one at a time to see what the difference, if any could be.
What I found was a tremendous difference between the two. The first tuner was a widely sold T-network made by a popular ham equipment manufacturing company, and the second was a very old 275-watt E.F. Johnson Matchbox. The antenna used is a conventional 40-meter inverted V, fed with 300-ohm twin lead.
The first tuner got a report of S-9. Then, the Matchbox was connected and the report went up to 20 over 9. This switch was made several times. This test was made when the band was at its most stable time of the day, which was at noon in the middle of summer here. The resultant difference in signal reports stayed the same I believe that the superior design of the Matchbox was the deciding factor in the large difference in signal reports. The other design uses a minimum of parts, and is simple to construct. Not so the Matchbox. It is a lot more complicated in design and uses things like differential capacitors and other rather expensive components. But that is what made the difference in signal strength at the other end of the circuit. I had the kilowatt version of the Matchbox and now wish I had not sold it. It is a superior design to all other designs that I have seen. It is unfortunate no one else has seen fit to make and sell a comparable antenna tuner today that is designed and constructed identical to the old Johnson Matchbox design.
If there were one like the old Matchbox, on the market, I am sure that it would sell very well. But I am afraid that the cost would be prohibitive and is why we don’t see them anymore.
Originally posted on the AntennaX Online Magazine by Mel Murphy, W6VEJ
Last Updated : 30th April 2024