Matching device: the purpose and principle of construction

In the Amateur practice not so often you can find antenna in which the input resistance is equal to the characteristic impedance of the feeder and the output resistance of the transmitter. In the majority of cases to detect such compliance is not possible, so you need to use a specialized matching device. Antenna, feeder, as well as the output of a transmitter included in a single system in which energy is transferred without any losses.

How to do it?

To realize this rather complicated task, you need to use a matching device in two key areas – the connection point between the antenna and the feeder, and the point where the feeder connects to the output of the transmitter. The most widespread today received specialized transforming the device from oscillating resonant circuits and ending with coaxial transformers, made in the form of individual lengths of coaxial cable of the desired length. All of these matching devices are used for impedance matching, which allows you to ultimately minimize the total losses in transmission lines and, more importantly, to reduce out-of-band radiation.

Resistance and its features

In the majority of cases, the output impedance is standard in modern broadband transmitters within 500 m. it should be noted that a coaxial cable is used as feeder, also have the standard value of the characteristic impedance at 50 or 750 m. If we consider the antenna, which can be used by matching device, depending on the design and type in input resistance has a fairly wide range of values, ranging from a few Ohms and ending with the hundreds and even a large number.

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It is Known that in a single-element antennas input impedance at the resonant frequency is almost active, the greater the transmitter frequency will differ from the resonance in the other hand, the greater will be the reactance inductive or capacitive character in the input resistance of the device. At the same time, multi-element antennas have an input impedance at the resonant frequency, has a complex character due to the fact that in the process of formation of the reactive component contribute to the various passive elements.

If the input resistance refers to active, it can be confirmed by the resistance using a specialized matching device for antenna. It should be noted that losses are practically insignificant. However, immediately after in input resistance will begin to form the reactive component, the reconciliation process will be more complex, and will need to use more and more complex matching device for the antenna, which will provide compensation for undesirable reactivity, and it should be located directly at the point of supply. If reactivity is not compensated for, this would adversely affect the SWR in the feeder but will also significantly increase the total losses.

What to do?

An Attempt to fully compensate the reactivity in the lower end of the feeder is unsuccessful, because it is limited to the performance of the device. Any adjustment frequency of the transmitter within the boundaries of the narrow sections of the Amateur bands in the end will not result in a significant reactive component, with the consequence that often there is no need to compensate. It is also worth noting that the right design of multi-element antenna also does not provide a large reactive component existing in the input resistance that does not require compensation.

In the air you can quite often find the various disputes about the role and the appointment has a matching device for antenna ("long wire" or other type) in the process of negotiating with her transmitter. Some lay on him quite high expectations, while others just consider him just another toy. That is why you need to properly understand what really might in fact help antenna tuner and where its use would be superfluous.

What is it?

First and foremost, you need to understand that tuner is a high frequency transformer resistances, with which if necessary it will be possible to compensate the reactance inductive or capacitive nature. It is possible to consider a fairly simple example:

Split a vibrator that at the resonant frequency is the active input impedance at the level of 700 m, and it uses coaxial cable with a transmitter that has an input resistance of about 500 m. the Tuners are set at the transmitter output, and in this situation will represent to any antenna (including "long cable") of the matching device between the transmitter and the feeder, without any difficulties coping with its main task.

If in the future to reconstruct the transmitter to a frequency thatdifferent from the resonant frequency of the antenna, in this case, the input resistance of the device may receive the reactivity, which then almost immediately starts to manifest itself at the lower end of the feeder. Thus a matching device “R” of any series will also be able to compensate, and the transmitter will again have consistency with the feeder.

What is the output where the feeder connects to the antenna?

If you are using a tuner exclusively on the output of the transmitter, in this case, fail to provide full compensation, and the device will begin to occur, as losses will not be present until the end of the alignment. In this situation, you will need to use another that connects between the antenna and the feeder, which will fully remedy the situation and compensate the reactivity. In this example, the feeder acts as a coherent transfer line having an arbitrary length.

Another example

The loop antenna, in which the active input impedance has a value of about 1100 m, should be coordinated with the transmission line to 50 Ohms. The output of the transmitter in this case is set to 500.

Here you will need to use a matching device to the transceiver or antenna that will be installed at the point where the feeder connects to the antenna. In the majority of cases, many fans prefer to use RF transformers of various types, equipped with ferrite cores, but is actually more convenient to manufacture quarter-wavelength coaxial transformer, which can be made from a standard 75-a lot of cable.

How to implement it?

The Length used of the segment of cable must be calculated according to the formula A/4*0.66, where A is a wavelength, and 0.66 is the velocity factor used for the majority of modern coaxial cables. The matching device of the HF antenna in this case will be connected between 50-Ohm feeder and antenna input, and if they roll into the Bay with a diameter of 15 to 20 cm, in this case, it will also act as a balun device. The feeder will be automatically aligned to the transmitter, as well as the equality of their resistances, and in such a situation it will be possible to completely refuse services to a standard antenna tuner.

Another option

For this example, we can consider another optimal coordination method – using multiple half-wave or half-wave coaxial cable in principle to any wave resistance. It is included between the tuner located near the transmitter and the antenna. In this case, the input impedance of the antenna, having a value at 110 Ohms is transferred to the lower end of the cable, and then using the antenna matching device, transformirovalsya in the resistance of 500 m. In this case, provides for the complete harmonization of the transmitter with the antenna and feeder is used as a repeater.

In more severe situations, when the input impedance of the antenna is irrelevant to the characteristic impedance of the feeder, which, in turn, does not match the output impedance of the transmitter requires the matching device of the HF antennas in the amount of two pieces. In this case one is used at the top to be aligned with the antenna feeder, while the other provides the coordination of the feeder with the transmitter at the bottom. However, there is no possibility to make some kind of matching device with your hands, which can be used one for coordination of the entire chain.

The Occurrence of reactivity will make the situation even more complicated. In this case, the matching device HF band will significantly improve the harmonization of the transmitter with the feeder, thus providing significant relief work amp, but more of them should be expected. Due to the fact that the feeder is misaligned with the antenna, there will be losses, so the efficiency of the device is low. Activated SWR meter installed between the tuner and the transmitter, ensure fixing of SWR=1, and between the feeder and the tuner such effect will be achieved as there is a mismatch.

Conclusion

The Use of a tuner is that it allows you to maintain the optimal mode of the transmitter in the process of working on an inconsistent load. But it cannot be achieved by improving the efficiency of any antenna (including "long wire”) – the matching device is powerless, if it is misaligned with the feeder.

P-contour, which is used in the output stage of the transmitter, can also be used as an antenna tuner, but only in the case if you will attend the operational change of the inductance and capacity of each. In the majority of cases, both manual and auto-tuners represent a contour tunable resonant devices, regardless of the intention of they factory, or did someone decided to make a matching device for antenna with their hands. In the manual there are two or three of the regulatory element, and they are not operational in the work, while the automatic are expensive, and to work in a serious capacitythe cost can be extremely high.

Broadband matching device

This tuner meets, most of the variations in which it is necessary to provide coordination between the antenna and the transmitter. Such equipment is very effective in the process of working with antennas used at the harmonics, if the feeder is a half-wave repeater. In this situation the input impedance of the antenna is different on different bands, but the tuner allows an easy alignment with the transmitter. The proposed device can operate at power up to 1.5 kW in the frequency range from 1.5 MHz to 30 MHz. Such a device can be done even with your hands.

The Main elements of the tuner is the RF autotransformer on a ferrite ring from a deflection system of the CNT TV-35, and the switch designed for 17 positions. Provides for the use of the conical rings from the models of UNT-47/59 or any other. In the coil has 12 windings, which are wound in two wires, with the beginning of one is combined with the end of the second. The diagram and table numbering through turns, while the wire is stranded and enclosed in Teflon insulation. Insulation wire diameter is 2.5 mm, providing taps from each coil, starting from the eighth, if we start counting from the grounded end.

The Autotransformer is installed extremely close to the switch, with connecting wires between them must have a minimum length. Provides for the use of the switch 11 of the regulations, will be saved if the design of the transformer with a large number of taps, for example, from 10 to 20 round, but in this situation there will be a reduction and interval of the transformation of the resistances.

Knowing the exact value of the input resistance of the antenna, you can use this transformer in order to align the antenna with the feeder 50 or 750 m, using only the necessary bends. In such a situation it is placed in a special waterproof box, and then fill with paraffin and put in directly in the feed point of the antenna. In itself is a matching device can be run as a standalone design or may be included in special antenna-switching block some of the radio station.

For clarity, the label mounted on the handle of the switch, shows a resistance value that corresponds to a given position. In order to provide full compensation of reactive inductive component, provides the possibility of connecting a variable capacitor.

The following table clearly indicates how the resistance depends on the number of your turns. In this case, the product of the calculations were carried out based on the resistance ratio, which is in quadratic dependence on the total number of turns made.