In amateur practice, it is not so 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, such a correspondence cannot be detected, therefore, specialized matching devices must be used. The antenna, feeder, and transmitter output are part of a single system in which energy is transmitted without any loss.
How to do it?
To realize this rather complicated task, you need to use matching devices in two main places - this is the connection point of the antenna with the feeder, as well as the point where the feeder is connected to the output of the transmitter. The most widespread today are specialized transforming devices, ranging from vibrational resonant circuits to coaxial transformers made in the form of separate segments of a coaxial cable of the desired length. All of these matching devices are used to match the resistances, which ultimately minimizes the overall loss in the transmission line and, more importantly, reduces out-of-band emissions.
Resistance and its features
In the predominant majority of cases, the output impedance is standard 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 the impedance at the level of 50 or 750 m. If we consider antennas for which matching devices can be used, then, depending on the design and type in them, the input resistance has a fairly wide range of values, starting from several ohms and ending Vai hundreds or even more.
It is known that in single-element antennas, the input impedance at the resonant frequency is practically active, and the more the transmitter frequency differs from the resonant in one direction or another, the more the reactive component of an inductive or capacitive nature will appear in the input impedance of the device itself. At the same time, multi-element antennas have an input impedance at a resonant frequency, which is complex in nature 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 almost negligible. However, immediately after the reactive component begins to form in the input impedance, the matching procedure will be more and more complicated, and it will be necessary to use an increasingly complex antenna matching device, the capabilities of which will allow for compensation of undesirable reactivity, and it should be located directly at the point nutrition. If the reactivity is not compensated, this will negatively affect the SWR in the feeder, as well as significantly increase the overall loss.
Is there any need to 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 tunings 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 for its compensation. 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 resistance, which does not require compensation.
On the air, you can quite often find various disputes about what role and purpose the matching device for the antenna (“long wire” or another type) has in the process of matching the transmitter with it. Some have high hopes for it, while others simply consider it an ordinary toy. That is why you need to correctly understand what the antenna tuner really can help in practice, and where its use will be superfluous.
What it is?
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 provide compensation for the reactivity of an inductive or capacitive nature. You can consider an extremely simple example:
A split vibrator that has an active input impedance of 700 m at the resonant frequency, and 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 they will represent or antennas (including the “long cable") matching devices between the transmitter and the feeder, without any difficulties coping with its main task.
If, in the future, 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 impedance 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 not be an exact coordination. In such a situation, it will be necessary to use another one, connecting between the antenna and the feeder, which will completely correct the situation and provide compensation for reactivity. In this example, the feeder acts as an agreed transmission line having an arbitrary length.
One more example
A loop antenna with an active input impedance of about 1100 m must be matched with a 50 ohm transmission line. The transmitter output in this case has a value of 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 is connected to the antenna. In the predominant majority of cases, many amateurs prefer to use RF transformers of various types equipped with ferrite cores, but in fact a more convenient solution would be to manufacture a quarter-wave coaxial transformer, which can be made from a standard 75-ohm cable.
How to implement this?
The length of the cable section used should be calculated using the formula A / 4 * 0.66, where A represents the wavelength and 0.66 is the shortening factor used for the vast majority of modern coaxial cables. In this case, the HF antenna matching devices 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 it will also act as a balancing device. The feeder will be completely automatically matched with the transmitter, as well as if their resistances are equal, and in this situation it will be possible to completely refuse the services of a standard antenna tuner.
Another variant
For this example, one more optimal matching method can be considered - using a multiple half wave or a half-wave coaxial cable, in principle, with any wave resistance. It is turned on between the tuner located near the transmitter and the antenna. In this case, the input impedance of the antenna, having a value of 110 Ohms, is transferred to the lower end of the cable, after which, using the antenna matching device, it is transformed into a resistance of 500 m. In this case, the transmitter and the antenna are fully coordinated, and the feeder is used as a repeater .
In more severe situations, when the input impedance of the antenna is inconsistent with the wave impedance of the feeder, which, in turn, does not correspond to the output impedance of the transmitter, matching devices of HF antennas in the amount of two pieces are required. In this case, one is used at the top to achieve matching of the feeder with the antenna, while the other provides matching of the feeder to the transmitter at the bottom. In this case, there is no way to make some kind of matching device with your own hands, which can be used alone to coordinate the entire circuit.
The occurrence of reactivity will make the situation even more complicated. In this case, the HF band matching devices will significantly improve the coordination of the transmitter with the feeder, thus providing significant simplification of the operation of the terminal stage, but you should not expect more from them. Due to the fact that the feeder will be inconsistent with the antenna, there will be losses, 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 is fixed = 1, but this effect cannot be achieved between the feeder and the tuner, since there is a mismatch.
Conclusion
The tuner is useful in 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, the efficiency of any antenna (including the "long wire") cannot be improved - the matching devices are powerless if it is inconsistent with the feeder.
The 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 predominant 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. In manual there are two or three regulatory elements, and they themselves are not efficient in operation, while automatic ones are expensive, and for work at serious capacities their cost can be extremely high.
Broadband Interconnect
Such a tuner satisfies the predominant 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 work with antennas used at harmonics, if the feeder is a half-wave repeater. In this situation, the input impedance of the antenna differs in different ranges, but at the same time, the tuner allows for easy coordination with the transmitter. The proposed device can easily operate with transmitter powers up to 1.5 kW in the frequency band from 1.5 to 30 MHz. Such a device can even be made with your own hands.
The main elements of the tuner are a high-frequency autotransformer on a
ferrite ring from the deflecting system of the CNT-35 television, as well as a switch designed for 17 positions. The possibility of using cone rings from the models UNT-47/59 or any other is provided. 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 table, the numbering of the turns is end-to-end, while the wire itself is stranded and is enclosed in fluoroplastic insulation. For insulation, the diameter of the wire is 2.5 mm, providing bends from each turn, starting from the eighth, if you count from the grounded end.
The autotransformer is installed extremely close to the switch, while the connecting conductors between them must have a minimum length. It is possible to use a switch to 11 positions, if the design of the transformer with not so many taps, for example, from 10 to 20 turns, is saved, but in this situation, the resistance transformation interval also decreases.
Knowing the exact value of the input impedance of the antenna, you can use such a transformer to match the antenna with a feeder of 50 or 750 m, using only the most necessary taps. In this situation, it is placed in a special moisture-proof box, after which it is poured with paraffin and placed in directly at the antenna's power point. The matching device itself can be implemented as an independent structure or be included in the special antenna-switching unit of a radio station.
For clarity, the label mounted on the handle of the switch shows the resistance value that corresponds to this position. To ensure full compensation of the reactive inductive component, it is possible to subsequently connect an alternating capacitor.
The table below clearly indicates how the resistance depends on the number of turns you made. In this case, the calculation was carried out based on the ratio of resistances, which is in a quadratic dependence on the total number of turns made.