The decision-making aid: antenna tuners - selection made easy
Whether or not I need an antenna tuner/coupler depends mainly on the antenna I have chosen and the type of feed, as well as on the environmental conditions on site.
A look into the antenna jungle:
The different configurations
End-fed horizontal or vertical wire antennas operate asymmetrically to earth and can be high, medium or low impedance, depending on length and frequency. Mostly they are set up in the form of an L-antenna. The low-impedance current feed always requires a good high-frequency grounding by a counterweight.
Earth or counterweight
The high-impedance, so-called voltage feed, on the other hand, works largely independent of the earth. Extremely high-impedance (> 1 kΩ) or low-impedance (< 50 Ω) feed points, however, can cause matching problems if they lie outside the matching range of a coupler. Wire antennas with medium impedance at the feed point can always be matched without problems.
Earth or counterweight
Earth or counterweight
Classic vertical antennas are operated in 1/4 lambda resonance and are designed to be feeded with coaxial cable.Here it is really only a matter of reducing the standing wave ratio at the band ends from S = 3.0 or 2.0 for the transmitter output to s = 1.5 or less.
The same applies to all other resonant, coaxial cable-fed wire antennas such as mono or multi-band dipoles, windom or loop and delta-loop antennas. A simple, unbalanced antenna tuner with a coaxial cable output is suitable for this purpose, and its impedance matching range should not be too demanding. What is important here is that the coupler can also handle impedances below 50 Ω.
The so-called Levy or double-Zepp antenna only works with a tuned two-wire line, but loop and windom antennas, as well as the classic "one-legged" Zeppelin antenna, are also fed with a two-wire line. Together with a balanced antenna tuner with a correspondingly wide matching range, these aforementioned antennas enable true multi-band operation. Another advantage of the balanced feed is that the antenna is largely independent of the earth.
Which circuit suits you best? Circuit concepts at a glance
The simple L element with its four different wiring options as a low-pass or high-pass filter in upward and downward transformation only has a limited matching range with the usual inductance and capacitance values. High maximum inductance and capacitance values are required for a wide matching range, meaning that the values and components quickly become unwieldy.
For this reason, this circuit is only used in simple unbalanced antenna tuners for coaxial cable-fed antennas, where it is only a matter of bringing the SWV at the band ends to a value that is compatible with the transceiver. The L element does have one major advantage: with its two components and optimised dimensioning, the L element is the lower-loss matching circuit. It is therefore often used for matching antennas at a fixed frequency.
The classic Pi filter is a further development of the L element and consists of the combination of a downward and upward low-pass filter and combines these two properties in one circuit. As a low-pass filter, it offers good harmonic suppression, but requires components with higher inductance and capacitance values for a wide matching range and especially for the lower shortwave bands. The Pi filter does not manage with the 220 to 330 pF variable capacitors commonly used in other coupler circuits. Consequently, the otherwise very versatile Pi filter is rarely found in current antenna tuners.
The T-high-pass circuits and their modified forms are able to handle extremely different impedances over a continuous frequency range from 1.5 to 30 MHz. With their relatively low, common capacitance values of a maximum of 220 to 330 pF for the two variable capacitors and a maximum inductance of 25 to 30 µH, the T-highpass has become established in most unbalanced antenna tuners available on the market.
Based on the basic circuits, other, more complex circuit concepts of current antenna tuners follow. With the use of a differential rotary capacitor, the T-high pass has two control elements, namely a rolling or switching coil and a rotary knob, which makes the operation of this circuit particularly convenient and simple. One axis simultaneously increases one partial capacitance and decreases the other by the same amount or vice versa.
If one replaces the rotary capacitor on the transmitter side in the conventional T-high-pass circuit with a differential rotary capacitor that is parallel to the coil, this forms a capacitive voltage divider on the input side, whose total capacitance together with the coil also works as a resonant LC circuit. This circuit is known as a transmatch and offers, in addition to a very large pick-up, additional preselection on the receiving side.
For high-quality, fully symmetrical antenna tuners, the symmetrical, double L element is the ultimate goal. For a downward or upward transformation, you can install a variable capacitor on each side and set the one you do not need to its minimum capacitance (initial capacitance). On the upper bands, however, the initial capacitance is noticeable, which is why in practice a variable capacitor is usually switched between the two sides via RF-compatible relays. With a considerable expenditure of components and mechanics, the already excellent symmetry characteristics can be perfected even further.
Roller Inductor or Switchable Inductor? Inductors and their Advantages and Disadvantages
The coil is the component of an antenna tuner that always introduces the greatest amount of loss! To make the inductance variable within wide limits, there are at least three possibilities: A so-called air core or bridge coil or toroidal coil with switchable taps is the simplest and most cost-effective solution. To ensure that there are no gaps in the frequency and matching range, there must be as many taps as possible. Devices with only one tap per band offer only a limited working range.
The roller inductor has the advantage that its inductance is continuously adjustable, which ensures a continuous tuning range of the antenna tuner. But roller inductors also have disadvantages! Depending on the quality of the mechanical design, the pick-up contact - the moving roller - can introduce losses into the circuit due to excessive contact resistance if the contact is poor. In extreme cases, the contact resistance also changes continuously when the reel is spun, so that the SWR display often changes abruptly when setting. One can only be warned against such components! Unfortunately, high-quality roller coils are expensive components.
There is one common disadvantage that is shared by both roller coils and air core or toroidal coils: the unused coil sections must either be short-circuited or disconnected. In the case of rolling coils anyway, but also in the case of cylindrical and toroidal coils, the first method mentioned is used almost exclusively: common practice is to short-circuit the unused coil section. The short-circuited windings, however, form an autotransformer with the windings of the "active" section of the coil, whose unused coil section is short-circuited. A not inconsiderable amount of power can be lost in the short-circuited section. If the unused turns are left open and "electrically hanging in the air", so to speak, unwanted resonances can form together with the circuit capacitances.
Stray field and losses in antenna tuners:
The advantages of toroidal coils
Another problem arises from the stray field of any cylindrical or roller coils. In particular, housings made of sheet steel that are too small cause unwanted current losses. In the range of commercially manufactured antenna tuners, you can, unfortunately, find some examples in which a cylindrical or roller inductor has been squeezed between two sheet-steel housing shells with a gap of only 1 to 2 cm. A few minutes of continuous operation with 100 W transmitting power, and you can feel the heated spot in the housing cover with your hand from the outside where the coil is mounted in the unit. The most compact design possible and, on the other hand, low losses are therefore two requirements that exclude each other.
With regards to this problem, toroidal coils are clearly at an advantage, as they cause as little stray field as possible. This is why they are mainly used in compact units for portable operation. However, other criteria have to be considered when using toroidal coils. The toroidal material must be suitable for the frequency range and the toroidal size must be dimensioned for the maximum power so that the toroid does not go into saturation. Therefore, only large iron powder toroidal cores are suitable.
Rotary capacitors and switches -
What should be considered?
Furthermore, you should pay attention to the mechanical design and the quality of the variable capacitors. In simpler, inexpensive antenna tuners, often only one-sided rotary capacitors are installed, which rather deserve the designation trimmer capacitors. In these cases, the contact to the rotor is usually primitive. Better versions of real variable capacitors have a double-sided rotor axle with ball bearings and ceramic cover plates. A critical point, as already mentioned, is always the contact pick-up at the rotor. Here, the material, the contact pressure and the size of the contact area are decisive. The plate spacing determines the dielectric strength and thus the maximum power load capacity of the tuner.
With a split-stator variable capacitor, contact pick-up from the rotor can be avoided completely. If this variable capacitor is mounted isolated on the unit chassis, the rotor forms a variable series circuit with the two separate stator packs, whereby the dielectric strength is doubled, but the resulting total capacitance is unfortunately halved. For use in the lower shortwave range, especially 160 m, it can be difficult to apply sufficient capacitance, as there are hardly any such variable capacitors with correspondingly large capacitance values. Even with a specimen with 2 × 500 pF, only 250 pF total capacity remains.
For band selector switches for coil taps and other switching functions in the HF power range, ceramic designs are always the first choice. Switching under load should always be avoided, as even with the best switch it will always affect the contacts to the point of complete failure.
Balanced antenna outputs
and their challenges
in antenna tuners
A tricky issue is the subsequent balancing at the antenna-side output with a 1:4 balun or a "1:1 balun transformer for undefined resonances". In many unbalanced antenna tuners this method is used to provide the additional feature of a "balanced output" for the operation of two-wire fed antennas with minimal effort. Another, but even more inconvenient option is to operate such a balancing transformer outside the station via a coaxial cable of greater length. If the two-wire line happens to be largely low -or medium- resistance at its lower end and has little or no reactive components, this can work quite reliably. However, voltage feed and high reactive components cannot be handled with this. And the fact that high reactive components occur at the transformer is probably predominantly the case. However, a balun transformer is not designed for this operating condition.
Incidentally, a transmission ratio of 1:4 is ultimately used for cost reasons, because production is then possible with a simple bifilar winding. A transmission ratio of 1:1 would serve the same purpose, as it is only a matter of balancing. The transmission ratio is actually irrelevant, since continuously changing impedances occur anyway over the entire frequency range in question, at different antennas and two-wire cable lengths. It makes more sense to pass the whole thing through 1:1 with a so-called "balun for indefinite impedances".
Challenges of forced balancing with common mode currents in antenna tuners
The forced balancing of an unbalanced tuner by means of a common mode choke inserted at the transmitter-side input must also be viewed critically. Although this measure leads to a symmetrical feed of the feed line and the antenna, the power balance looks sobering at the latest at shortened dipoles.
Raised unbalanced antenna coupler with common mode choke (CMC) at the transmitter-side input,
TX, CMC, AK, Ant
unbalanced antenna coupler with CMC at the transmitter input and 1:4 balun at the antenna output,
TX, CMC, AK, Ant
Balancing at the transmitter side input of a balanced antenna coupler,
TX, CMC, AK, Ant
The common mode choke eliminates the symptom of "sheath waves" by noticeably converting the misdirected energy in the ferrite material into heat. This part of the power is still not available for radiation. Only the annoying consequences such as "hot ground", irradiation and interference with your own electronics, etc. can be easily solved. In addition, a manually operated antenna tuner is disturbed by the "hot" HF leading housing and the hand sensitivity during operation. Actually, this method is only suitable for isolated, automatic antenna tuners. It should be noted that the operating voltage and control lines must also be included in the common mode choke.
The best option for balanced antennas is always a true balanced antenna tuner, where the balancing at the transmitter input of the tuner is done by a true balun transformer or a sheath wave trap. In any case, this makes the situation clear. Especially with shortened antennas and on the low bands of 80 m and 160 m, this choke must have the highest possible blocking attenuation!
Optimum operation: Tips for the correct setting of your antenna tuner
Finally, a few considerations on the correct manual operation and adjustment of antenna tuners. Again and again we see that the operating elements of an antenna tuner are turned around blindly and haphazardly, and after long attempts at tuning the result is "Don't go". It is absolutely not helpful to hastily and unsystematically turn the control elements of the roller coil and variable capacitors. Finding the correct tuning of an antenna tuner should not be left to chance. Ideally, one should understand what is going on in the unit and what the consequences are if the capacitance values and inductance measurements are increased or decreased.
A rough pre-setting is already possible in terms of reception by setting both variable capacitors to a medium value and using the roll or switching coil to set to a maximum received signal, or more precisely, to a maximum receiver noise. This basic setting provides a good starting point for approaching the optimum setting while observing the SWR trend.
If, for example, we have increased the capacitance of the transmitter-side variable capacitor, the capacitance of the antenna-side variable capacitor must be reduced by the same amount or vice versa. Or put it in other words and more simply: If we turn one rotary knob to the left, we must turn the other to the right. The operation of the two variable capacitors is therefore always alternately in the opposite direction in each case. If one knob is turned to the left, the other must be turned to the right and vice versa.
It is the same with the inductance. If we increase the inductance, the capacitance of the variable capacitors must be reduced - or vice versa.
You will quickly realise that a good SWR can be achieved with quite different settings. The optimal setting with the lowest losses in the coupler is always the one with the smallest necessary inductance - i.e. the one with as few coil turns as possible.
Tip: When tuning, always turn upwards from the initial inductance. The initial setting with which an SWR of 1.0 can be achieved is always the one with the lowest required inductance and thus the lowest losses - i.e. the optimum one.
Automatic antenna tuners:
the future of tuning
Automatic couplers mainly use unbalanced Pi filters, or more precisely, balanced Pi filters in low-pass configuration. In these filters, coils and capacitors are divided into many fixed, binary staggered individual values and connected in series or parallel. For this purpose, all individual coils are connected in series and the capacitors in parallel. Each coil has a relay contact in parallel and each capacitor has a relay contact in series. By opening and closing the relay contacts, each individual value can be added or removed. In this way, any values between minimum and maximum inductance and capacitance can be displayed in small increments within the framework of the binary staggered values. The relays are controlled by a microcontroller that evaluates the SWR on the input side.
Automatic antenna tuners are popular and widely used because the tuning process runs automatically in a very short time, and the tuning values found once are stored and can be reproduced from memory at any time. As unbalanced "coaxial couplers" they are used at the station end and, like the automatic tuners integrated directly into the transceivers, are intended to extend the operating range of a resonant coaxial cable-fed antenna up to the SWR corner points of about s = 3.0.
Automatic antenna tuners in weatherproof housings with unbalanced connection are designed for end-fed wire or vertical antennas of almost any length. These automatic tuners must be placed outside the station, directly at the feed point of the antennas. This can solve many electrical and spatial antenna problems.
The HF reaches the feed point of the antenna via the coaxial cable without radiation. Only one thing can become a problem here, too: Without a really good HF ground, even an automatic coupler can only "do its job" poorly or not at all. Unfortunately, a good HF earth is not always present where the coupler finds its place. The actual earthing begins at the earth's surface and every earth wire from the coupler to this point acts like a part of the antenna. It radiates and causes losses.
Furthermore, one should keep the following in mind when using automatic antenna tuners: What is really going on at the feed point often remains hidden. If there is no indication in a display about the setting values, you will miss a change at the feed point, e.g. if the contact resistance at the connection has increased due to corrosion or water has penetrated the balun or coaxial cable. The coupler adjusts to the changed conditions and readjusts with changed settings and, in the worst case, does not go on strike until the antenna has fallen down.
Unbalanced automatic tuners in the variants for coax-fed antennas and end-fed wire antennas are now available from numerous manufacturers "like sand by the sea". Among the few fully balanced automatic couplers for connecting two-wire lines, the choice is not so large. Without exception, these couplers are housed in weatherproof enclosures and are intended for outdoor installation. There are fully automatic couplers that start and carry out the tuning process automatically - and those with an external control unit on which the setting values can be selected manually and stored once the optimum setting has been reached. Radio amateurs have also developed their own remote-controlled symmetrical antenna tuners, which are available as circuit board kits.
Mechanically and financially more complex is the use of motor-driven tuning elements, i.e. roller coils and variable capacitors. This method is used in a few high-end units.
Additional preselection and harmonic suppression
Finally, two more positive aspects of an antenna tuner should not go unmentioned. A Pi filter and an LC low-pass L element provide additional harmonic suppression. However, this does not really matters anymore with current, commercially manufactured amateur radio transceivers, since the required values of harmonic suppression are already exceeded by the devices. The additional harmonic suppression may only be helpful for DIY devices.
On the receiving side, an antenna tuner provides additional preselection by attenuating the frequencies above or below the receiving frequency, depending on the type of circuit.
Actually, you should not pay so much attention to the standing wave ratio (SWR): Even at a SWR of s = 3.0, only 1.25 dB of power is lost, at S = 2.0 it is only 0.5 dB. No one hears either at the other end of the transmission path! However, most modern transceivers already start to reduce the power at about s = 1.5, but at the latest at s = 2.0. If, for example, you transmit with only 20 W instead of 100 W, you will notice a minus of about one S-step. That is why one still strives for a perfectionist SWR of s ≤ 1.5 and less.
Why SWR 1.5 or better?
Spoiled for choice:
antenna tuners from top manufacturers
The antenna tuner category offers plenty of choice. Among the manufacturers Acom; Ameritron; CG Antenna; Flexradio; Icom; LDG; mAT; MFJ; Palstar; SGC and Yaesu, you will find mainly stand alone antenna tuners, but also transceiver specific antenna tuners. Starting from simple, manually operated unbalanced antenna tuners, over a variety of unbalanced automatic couplers up to fully balanced top of the range units as automatic couplers or for manual operation. The power classes range from small and light QRP antenna tuners to the kilowatt class, both in versions as table-top units or as remote-controlled versions for outdoor mounting.