In amateur practice, it is not so often possible to meet antennas in which the input resistance is equal to the feeder impedance , as well as the output impedance of the transmitter. In the majority of cases, it is not possible to detect such a correspondence, therefore it is necessary to use specialized matching devices. The antenna, the feeder, and the output of the transmitter are included in a single system in which the energy is transmitted without any loss.
How to do it?
To realize this rather complicated task, it is necessary to use matching devices in two main places - the point of connection of the antenna with the feeder, and also the point where the feeder is connected to the transmitter output. The most widespread today are specialized transforming devices, ranging from vibrational resonant circuits and ending with coaxial transformers made in the form of separate lengths of a coaxial cable of the required length. All these matching devices are used to match the resistances, which ultimately minimizes total losses in the transmission line and, more importantly, reduces out-of-band emissions.
Resistance and its features
In the majority of cases, the output impedance is standard in modern broadband transmitters of 500 m. It should be noted that many coaxial cables used as a feeder also differ in the standard value of wave resistance at the level of 50 or 750 m. If we consider antennas for which Can be used matching devices, then depending on the design and type in them, the input resistance has a fairly wide range of values, ranging from a few ohms and finishing Vai hundreds or even more.
It is known that in single-element antennas the input impedance at the resonance frequency is practically active, and the more the frequency of the transmitter will differ from the resonant one to the other, the more reactive component of the inductive or capacitive character will appear in the input impedance of the device itself. At the same time, multiple-element antennas have an input impedance at a resonant frequency, which has a complex character due to the fact that various passive elements contribute to the formation of the reactive component.
If the input resistance refers to the active resistance, it can be matched to the resistance using a dedicated matching device for the antenna. It should be noted that the losses here are almost insignificant. However, just after the reactive component begins to form in the input resistance, the matching procedure will be more and more complicated, and it will be necessary to use an increasingly complex matching device for the antenna, whose capabilities will allow compensation of undesirable reactivity, and should be located directly at the point Power supply. If reactivity is not compensated, this will negatively affect the SWR in the feeder, and will also significantly increase the overall losses.
Do I 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 changes in the frequency of the transmitter within the boundaries of narrow sections of the amateur bands will ultimately not lead to the appearance of a significant reactive component, so that there is often no need for compensation. It should also be noted that the correct design of multi-element antennas also does not provide for a large reactive component of the existing input resistance, which does not require its compensation.
On the air you can often meet various disputes about the role and purpose of the matching device for the antenna ("long wire" or other type) in the process of coordinating with it the transmitter. Some place great hopes on him, while others simply consider him an ordinary toy. That's why you need to understand correctly what the antenna tuner really can help in practice, and where its use will be superfluous.
What it is?
First of all, it is necessary to understand correctly that the tuner is a high-frequency transformer of resistances, with the help of which, if necessary, it will be possible to provide compensation of reactivity of inductive or capacitive nature. We can consider an extremely simple example:
The cut-off vibrator, which at the resonant frequency has an active input resistance of 700 m, and at the same time uses a coaxial cable with a transmitter that has an input resistance of about 500 m. The tuners are installed at the output of the transmitter, and in this situation they will be for some other reason. (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 re-tuned to a frequency that is different from the resonant frequency of the antenna, then in this case, reactivity may appear in the input resistance of the device, which will then begin to appear almost instantly at the lower end of the feeder. In this case, the matching device "R" of any series can also compensate it, and the transmitter will again get consistency with the feeder.
What will be on the output, where does the feeder connect 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 the device will begin to experience various losses, since there will be incomplete exact reconciliation. In this situation, you will need to use one more, 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 a coherent transmission line having an arbitrary length.
One more example
The loop antenna, in which the active input resistance is about 1100 m, must be coordinated with the transmission line by 50 ohms. The output of the transmitter in this case is 500 m.
Here it will be necessary 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 majority of cases, many fans prefer to use different types of RF transformers 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 used cable length should be calculated by the formula A / 4 * 0.66, where A is the wavelength, and 0.66 is the coefficient of shortening used for the majority of modern coaxial cables. The matching devices of HF antennas 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 it will also act as a balancing device. The feeder will be fully automatically coordinated with the transmitter, as well as if their resistances are equal, and in such a situation it will be possible to completely abandon the services of a standard antenna tuner.
For such an example, we can consider another optimal method of matching - with the help of a multiple half wave or half-wave coaxial cable in principle with any wave impedance. It is inserted 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, and then, using an antenna matching device, is transformed into a resistance of 500 m. In this case, a complete 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 inadequate to the feeder impedance, which in turn does not correspond to the output impedance of the transmitter, two-piece matching HF antennas are required. In this case, one is used at the top to achieve the alignment of the feeder with the antenna, while the other ensures the matching of the feeder to the transmitter at the bottom. In this case, there is no way to make any 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 difficult. In this case, the matching devices of the HF bands will significantly improve the coordination of the transmitter with the feeder, thus providing a significant simplification of the end-cascade operation, but more of them should not be expected. Due to the fact that the feeder will be mismatched with the antenna, there will be losses, so the efficiency of the device itself will be understated. The activated SWR meter installed between the tuner and the transmitter will provide fixation of the SWR = 1, and between the feeder and the tuner this effect will not be achieved, since there is a mismatch.
The advantage of the tuner is that it allows to maintain the optimum mode of the transmitter in the course of work on an uncoordinated load. But at the same time, there can be no improvement in the efficiency of any antenna (including "long wire") - the matching device is powerless if it is mismatched 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 operative change in inductance and each capacitance. In the majority of cases, both manual and automatic tuners are resonant contoured tunable devices, regardless of whether they are assembled factory or someone decided to make a matching device for the antenna with their own hands. There are two or three control elements in manual ones, and they are not operative themselves, while automatic ones are expensive, and for operation at serious capacities their cost can be extremely high.
Broadband matching device
Such a tuner satisfies most of the variations, in which it is necessary to ensure the matching of the antenna with the transmitter. Such equipment is quite effective in working with antennas used on harmonics, if the feeder is a half-wave repeater. In such a situation, the input impedance of the antenna differs on different bands, but the tuner makes it easy to match the transmitter. The proposed device can easily function at transmitter powers up to 1.5 kW in the frequency band from 1.5 to 30 MHz. Such a device can be made even with their own hands.
The autotransformer is installed extremely close to the switch, and the connecting conductors between them must have a minimum length. It is possible to use the switch for 11 positions if the transformer design is retained with not a lot of taps, 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 antenna input impedance, you can use such a transformer to match the antenna with a feeder of 50 or 750 m, using only the most necessary outlets. In such a situation, it is placed in a special moisture-proof box, after which it is filled with paraffin and put in directly at the power point of the antenna. By itself, the matching device can be implemented as an independent design or be included in a special antenna-switching unit of some radio station.
For clarity, the mark 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 connect the variable capacitor afterwards.
The table below clearly indicates how the resistance depends on the number of turns you have made. In this case, the product of calculations was carried out based on the ratio of the resistances, which is in a quadratic dependence on the total number of turns made.