Coordinating devices: purpose and principle of construction

2024 Author: Henry Conors | [email protected]. Last modified: 2024-02-12 02:41

In amateur practice, it is not often possible to find antennas in which the input impedance is equal to the wave impedance of the feeder, as well as the output impedance of the transmitter. In the overwhelming majority of cases, it is not possible to detect such a correspondence, therefore, specialized matching devices must be used. Antenna, feeder, and also the output of the transmitter are included in a single system in which energy is transmitted without any losses.

How to do it?

To accomplish this rather complicated task, you need to use matching devices in two main places - this is the point where the antenna connects to the feeder, and also the point where the feeder connects to the output of the transmitter. The most widespread today are specialized transforming devices, ranging from oscillatory resonant circuits to coaxial transformers, made in the form of separate pieces of a coaxial cable of the required length. All of these matchers are used to match impedances, ultimately minimizing overall transmission line loss and, more importantly, reducing out-of-band emissions.

Resistance and its features

In most cases, the standard output impedance in modern broadband transmitters is 500 m. It is worth noting that many coaxial cables used as a feeder also differ in the standard value of wave impedance at the level of 50 or 750 m. If, however, consider antennas for which matching devices can be used, then, depending on the design and type, the input impedance in them has a fairly wide range of values, ranging from a few ohms to hundreds and even more.

It is known that in single-element antennas, the input impedance at the resonant frequency is practically active, while the more the transmitter frequency differs from the resonant one in one direction or another, the more the reactive component of an inductive or capacitive nature will appear in the input impedance itself devices. At the same time, multi-element antennas have an input impedance at the resonant frequency, which is complex due to the fact that various passive elements contribute to the formation of the reactive component.

If the input impedance is active, it can be matched to the impedance using a specialized antenna matching device. It should be noted that the losses here are practically negligible. However, immediately after a reactive component begins to form in the input resistance, the matching procedure will become more and morecomplex, and more and more complex antenna matching will need to be used, with the ability to compensate for unwanted reactivity, and should be located directly at the feedpoint. If reactivity is not compensated, this will negatively affect the SWR in the feeder, as well as significantly increase the overall losses.

Should I do this?

An attempt to fully compensate for reactivity at the lower end of the feeder is unsuccessful, since it is limited by the characteristics of the device itself. Any changes in the frequency of the transmitter within the narrow sections of the amateur bands will ultimately not lead to the appearance of a significant reactive component, as a result of which there is often no need to compensate for it. It is also worth noting that the correct design of multi-element antennas also does not provide for a large reactive component of the available input impedance, which does not require its compensation.

On the air, you can often find various disputes about the role and purpose of a matching device for an antenna (“long wire” or another type) in the process of matching a transmitter with it. Some have rather high hopes for it, while others simply consider it an ordinary toy. That is why you need to correctly understand how an antenna tuner can really help in practice, and where its use will be superfluous.

What is this?

First of all, you need to correctly understand that the tuner is a high-frequency resistance transformer, with which, if necessary, it will be possible to compensate for inductive or capacitive reactivity. Consider an extremely simple example:

Split vibrator, which at the resonant frequency has an active input impedance of 700 m, and at the same time it uses a coaxial cable with a transmitter having an input impedance of about 500 m. Tuners are installed at the output of the transmitter, and in this situation will be for any antenna (including a "long cable") matching devices between the transmitter and the feeder, coping with its main task without any difficulties.

If further the transmitter is tuned to a frequency that differs from the resonant frequency of the antenna, then in this case, reactivity may appear in the input resistance of the device, which subsequently almost immediately begins to appear at the lower end of the feeder. In this case, the matching device "P" of any series will also be able to compensate for it, and the transmitter will again receive consistency with the feeder.

What will be the output where the feeder connects to the antenna?

If you use the tuner exclusively at the output of the transmitter, then in this case it will not be possible to provide full compensation, and various losses will begin to occur in the device, since there will be incomplete matching. In such a situation, you will need to useone connected between the antenna and the feeder, which will completely correct the situation and provide reactivity compensation. In this example, the feeder acts as a matched transmission line of arbitrary length.

Another example

Loop antenna, which has an active input resistance of about 1100 m, must be matched with a 50 ohm transmission line. The transmitter output in this case is 500 m.

Here you will need to use a matching device for the transceiver or antenna, which will be installed at the point where the feeder connects to the antenna. In the vast majority of cases, many hobbyists prefer to use various types of RF transformers equipped with ferrite cores, but in fact, a quarter-wave coaxial transformer, which can be made from standard 75 ohm cable, is a more convenient solution.

How to implement it?

The length of the cable section used should be calculated using the formula A/40.66, where A is the wavelength and 0.66 is the velocity factor used for the vast majority of modern coaxial cables. The HF antenna matching devices in this case will be connected between the 50-ohm feeder and the antenna input, and if they are rolled into a bay with a diameter of 15 to 20 cm, then in this case it will also act as a balancing device. The feeder will be fully automatically matched to the transmitter, as well asequality of their resistances, and in such a situation it will be possible to completely refuse the services of a standard antenna tuner.

Another option

For such an example, you can consider another optimal way of matching - using a multiple of half a wave or a half-wave coaxial cable, in principle, with any wave impedance. It is included between the tuner located near the transmitter and the antenna. In this case, the input impedance of the antenna, which has a value of 110 ohms, is transferred to the lower end of the cable, after which, using an antenna matching device, it is transformed into a resistance of 500 m. In this case, full matching of the transmitter with the antenna is provided, and the feeder is used as a repeater.

In more severe situations, when the input impedance of the antenna is inappropriate for the characteristic impedance of the feeder, which, in turn, does not correspond to the output impedance of the transmitter, two HF antenna matching devices are required. In this case, one is used at the top to match the feeder to the antenna, while the other is used to match the feeder to the transmitter at the bottom. At the same time, there is no way to make some matching device with your own hands, which can be used alone to match the entire circuit.

The emergence of reactivity will make the situation even more complicated. In this case, HF matching devices will significantly improvematching the transmitter with the feeder, thus providing a significant simplification of the work of the final stage, but you should not expect more from them. Due to the fact that the feeder will be mismatched with the antenna, losses will appear, so the efficiency of the device itself will be underestimated. An activated SWR meter installed between the tuner and the transmitter will ensure that the SWR=1 is fixed, and this effect cannot be achieved between the feeder and the tuner, since there is a mismatch.

Conclusion

The benefit of the tuner is that it allows you to maintain the optimal mode of the transmitter in the process of working on an inconsistent load. But at the same time, an improvement in the efficiency of any antenna (including the “long wire”) cannot be ensured - the matching devices are powerless if it is mismatched with the feeder.

P-circuit, which is used in the output stage of the transmitter, can also be used as an antenna tuner, but only if there is an operational change in the inductance and each capacitance. In the vast majority of cases, both manual and automatic tuners are resonant contour tunable devices, regardless of whether they are assembled at the factory or someone decided to make a matching device for the antenna with their own hands. There are two or three regulating elements in manual ones, and they themselves are not operational in operation, while automatic ones are expensive, and for work at serious capacities, their cost can be extremely high.

Broadband matching device

This tuner satisfies the vast majority of variations in which it is necessary to ensure the matching of the antenna with the transmitter. Such equipment is quite effective in the process of working with antennas used on harmonics, if the feeder is a half-wave repeater. In this situation, the input impedance of the antenna differs on different bands, but the tuner allows for easy matching with the transmitter. The proposed device can easily operate at transmitter powers up to 1.5 kW in the frequency band from 1.5 to 30 MHz. You can even make such a device with your own hands.

The main elements of the tuner are an RF autotransformer on a ferrite ring from the deflecting system TV UNT-35, as well as a switch designed for 17 positions. It is possible to use cone rings from models UNT-47/59 or any other. There are 12 turns in the winding, which are wound into two wires, while the beginning of one is combined with the end of the second. In the diagram and in the table, the numbering of the turns is through, while the wire itself is stranded and enclosed in fluoroplastic insulation. For insulation, the wire diameter is 2.5 mm, providing taps from each turn, starting from the eighth, if counting from the grounded end.

The autotransformer is installed as close as possible to the switch, while the connecting conductors between them must have a minimumlength. It is possible to use a switch with 11 positions, if the design of the transformer with a not so large number of taps is saved, for example, from 10 to 20 turns, but in such a situation, the resistance transformation interval will also decrease.

Knowing the exact value of the input impedance of the antenna, you can use such a transformer to match the antenna with a 50 or 750 m feeder, using only the most necessary taps. In such a situation, it is placed in a special moisture-proof box, after which it is filled with paraffin and placed directly at the feed point of the antenna. The matching device itself can be performed as an independent design or included in a special antenna-switching unit of some radio station.

For clarity, the label mounted on the switch handle shows the resistance value that corresponds to this position. To ensure full compensation of the reactive inductive component, it is possible to subsequently connect a variable capacitor.

The table below clearly shows how the resistance depends on the number of turns you have made. In this case, the calculation was carried out based on the ratio of resistances, which is in quadratic dependence on the total number of turns made.