Pro antennas for the smallest

Pro antennas for the smallest

Let's try to figure out how antennas work and why electromagnetic energy from a comfortable conductor is radiated into an alien dielectric, and we can do without matan, which will, of course, require very serious simplifications and even vulgarization , but still allows you to get an initial presentation and, I do not exclude, the desire to read materials for the more advanced.

If you are a radio engineer, an experienced amateur radio amateur, or just know physics well, then you are strictly not recommended to read the following in order to avoid negative consequences for your mental health. You were warned.

Let's start with the boring basics. In the good old days, when there was neither the Internet, nor your Fido, the known phenomena of electricity and magnetism were not considered as one thing, having a common nature, until two hundred years ago, the Danish Oersted discovered that the flow of electric current through a conductor causes a deviation the arrows of the compass, i.e. It creates a magnetic field accessible to observation and measurement with the simplest instruments.

Soon, the Frenchman Ampere derived the law of the name of himself, describing the dependence of the electric current and the magnetic field arising from it, and a little later the Englishman Faraday turned on and discovered and mathematically expounded the phenomenon of electromagnetic induction. After quite a bit of time, the Scotsman Maxwell creates the electromagnetic field theory on which we would have to rely on in the following story, but we agreed to do without matan as much as possible so that even the most inveterate humanities could feel the taste for technology instead of being frightened by complicated formulas. All these works led to the fact that in 1887 the German Hertz experimentally proved the existence of radio waves by building a radio transmitter and radio receiver, which, quite unexpectedly, turned out to be workers. However, Hertz himself did not appreciate the prospects of his radio program (the first in the world!), And therefore the invention of radio was more often associated with Italian Marconi, who, in addition to an undeniable engineering genius, was also successful in terms of commercialization. Yes, if anyone is interested, the first broadcast of the voice belongs to Canadian Fessenden, who managed to do this in 1900.

The current in the conductor creates a magnetic field. Why do we need to take his hand on the bare wire? Then, in order to easily remember the direction of the magnetic field vector depending on the direction of the current in the conductor - the “right-hand rule”.

So, now we know that the flow of electric current in a conductor leads to the fact that a magnetic field occurs near the conductor. Here it is, if it is very, very simplistic, and there is electromagnetism. Therefore, the first thing we can learn: the radiation of the antennas is associated with the flow of electric current in them.

Radio communication uses alternating current of various frequencies (or wavelengths — speaking of antennas, it is often more convenient to talk about wavelength, and about radio engineering as a whole, about frequency).
Different frequencies allow you to simultaneously carry out many independent transmissions and separate their reception, choosing the necessary frequencies and discarding unnecessary ones. There are quite a few ways to do this, but they are the subject of separate articles. Alternating current has one unpleasant feature: although it is completely subject to Ohm’s law (interdependence of voltage, resistance of the circuit and current in it), voltage and current may not coincide in time. Yes, “phase shift” is not necessarily in the head, it is more than an electrical and radio engineering term. That's what happens. If we applied alternating voltage to some ideal resistor, then the common-mode alternating current in this circuit would be equal to the voltage in volts divided by the resistance in ohms, as well as a decent direct current.But if instead of a resistor we have an inductance coil, then the matter becomes more complicated. When we apply a voltage to the coil, it seems to resist the current through it, so the current lags behind the voltage in phase. By the way, if you turn off the power supply from the coil, it will also resist and try to maintain the current flowing through itself (to the extent that the coil can store energy) - the voltage is gone, and the current is still flowing. This is the resistance, it is called reactive, the higher, the higher the frequency. That is, with increasing frequency with equal inductance or with increasing inductance with equal frequency, resistance to alternating current increases. With capacitors everything is the same, but only vice versa. When a voltage is applied to a capacitor, the current first falls into it, like in an empty hole, ahead of the voltage, and then drops as it is charged. The ease with which an alternating current enters the capacitor means that with increasing frequency at equal capacitance, the resistance to alternating current decreases, and at equal frequency, with increasing capacitance, the resistance to alternating current also drops. Therefore, we take note: the reactance, that is, the inductive or capacitive resistance to alternating current, depends on the frequency.

On the left there is a traditional sinusoidal oscillogram, on the right there is a phase shift using the example of “lagging” the current from the voltage in the presence of an inductive resistance circuit.

The total resistance consisting of an active component (a conditional resistor that consumes power “purely” without affecting the phase) and a reactive component (phase-shifting inductance and/or capacitance) is called complex impedance or impedance.

So, the antenna is a conductor to which electrical energy is supplied and which radiates it into the surrounding space. Emit electric current in a conductor that creates a magnetic field around the conductor.

Why does electromagnetic energy come out of a conductor comfortable for it into a vacuum that is uncomfortable for it? And she does not come out! Energy creates field oscillations, but does not move by itself. Let's compare with sound waves. When the speaker (antenna) creates oscillations, the air (ether) does not move, the wind does not arise, but the oscillations propagate in the air (ether). The same happens with electromagnetic waves, except for electromagnetic energy that is distributed not in the air, but in the air. Later, however, they will find out that the proposed ether does not exist, and that the earth is also not flat, but the electromagnetic field feels fine and in vacuum but we know that there is ether, and the earth is, of course, not flat, but slightly convex . That is, once again, energy is not transferred along with the medium (more precisely with the field), but is transferred due to the propagation of waves in a generally stationary medium (in the field).

Antenna as an oscillating circuit. Before we talk about the specific designs of simple antennas, on the basis of the device which we can understand and in the device complex, let's talk about electrical resonance. To do this, go back to the reactive resistance. An antenna can be represented as a distributed capacitance and a distributed inductance - as a coil unwound to a direct wire and as degenerated to the same wire of a capacitor plate. The presence of reactance in the circuit, as we recall, separates the current and voltage phases. However, if we pick up a certain combination of inductance and capacitance (which only works on one particular frequency, because we remember that reactance changes with frequency), it turns out that capacitance and inductance cancel each other out and we see purely active resistance in load. Such mutual compensation and the result in the form of a purely active resistance as a result of compensation is called electrical resonance.By itself, for the operation of the antenna, it is unimportant, because, as we have already found out, the antenna emits a current in a conductor. However, there are a number of reasons for striving to achieve resonance in the antenna. The fact is that, unlike DC, for AC it is important that the characteristic impedance (I recall Ohm’s law, namely that the circuit’s resistance is numerically equal to the applied voltage divided by the current) of the generator, transmission line and load, i.e. The antennas themselves were equal. If there is no equality, a part of the electromagnetic energy will be reflected back onto the generator, which will lead to a whole spectrum of undesirable phenomena. Significant reactance leads to a strong mismatch and a significant reflection of energy. However, this also applies to the active component of the impedance, which is easier to reconcile with a small, easily compensated reactive component. Therefore, technically try to create such antennas, in which the reactive component is missing or easily compensated, and the active component is equal to the impedance of the generator or is easily transformed. In the case of the simplest antennas, the creation of a specific antenna capacitance or a specific inductance simply means a sizing. Therefore, the antenna dimensions are usually measured not in linear units, but in fractions of the wavelength.

The simplest full-sized antenna. Half-wave dipole, quarter-wave groundplayn and similar designs.

As you can see, the distribution of currents and voltages are the same. Only if in the quarter-wave groundplayne one half of the dipole is a pin, and the second half is ground, then in the half-wave dipole the second half is its second half.:)

To get acquainted with the principles that are the same for any more complex antennas, I propose to understand the device and operation of the basic antennas - a symmetrical half-wave dipole or an asymmetrical quarter-wave groundplane. To a certain extent, they are identical and a half-wave dipole can be considered as an extreme case of a quarter-wave groundplane, the radial (counterweight) angle of which reached 180 ° to the radiating probe, therefore most of the considered features are equally applicable to both antennas.

As you can see, such an antenna has an electric resonance, because an integer number of half-waves of current and an integer number of half-waves of voltage are placed in its conductor. They are out of phase relative to each other, but their reactivity is mutually compensated.

If the antenna were a little shorter than half-wave, then it would have a capacitive impedance component and would have to be compensated by inductance (doesn’t resemble coils at the base of sibish auto-antennas?), And if you lengthen it, you will receive an inductive component that needs to be compensated by capacitance .

Radiation resistance. There is nothing special about radiation resistance. Or rather not. Radiation resistance in the physical sense does not exist, it is an analytical value that is used to determine the efficiency of the antenna. It is easiest to imagine the radiation resistance as the active component of the impedance of the entire antenna, which is spent on radiation. Actually, there is the term "radiation loss" and this is useful "loss" if we are talking about an antenna, but this is not equal to the radiation resistance, so do not confuse. There is no imaginary resistance of the medium to imaginary radiation into it or anything else - there are different properties like dielectric constant that we will not consider yet.

Even in the antenna there is a loss resistance in the form of conductor resistance, which is spent on its heating, various losses in structural elements and matching links.Knowledge of the radiation resistance is necessary to understand the efficiency of the antenna: for some antennas, the radiation resistance can be several Ohm units and the loss resistance is several times greater, which means that the efficiency of such an antenna is extremely low despite the fact that its design is adequate for the rest. In simple antennas like the dipole or groundplane under consideration, the radiation resistance is close to the impedance of the antenna itself, because the conductor losses are relatively small, but in any case these are not identical concepts.

Let's return to the dipole. As long as we supply energy at its geometric center, where the current is maximum and the voltage is minimal, the radiation resistance is small. Theoretically, it is approximately 73 Ohms, and almost a little less, depending on the relative thickness of the material. As one of the halves of the dipole splits into separate radials, the resistance will decrease slightly and fall to approximately 36 ohm and an angle of 90 ° to the pin. This obviously affects the efficiency of the antenna. But, for clarity, we will consider exactly the dipole. As the power point shifts from the center to the edge, we will see that the current drops, and the voltage rises, that is, the radiation resistance increases, which reaches its maximum when powered from the end. This circumstance does not affect all other antenna characteristics, it still radiates with the same radiation pattern, which means it has the same radiation efficiency (but not the efficiency of the entire antenna assembly, because the efficiency depends on relative losses).

The impedance of the antenna is equal to the voltage at the feed point divided by the current delivered. And it consists of, as we have already found out, the resistance of the radiation, on which we usefully lose energy to the radiation we need, and the resistance of loss, on which we lose energy, is useless. In many ways we can influence the impedance of the antenna. Without changing the geometry, we can shift the power point. We can use various transforming elements (including literally transformers with windings at those frequencies where their use is rational). All these manipulations do not affect the radiation efficiency of the antenna and are needed only to match the antenna with the generator (transmitter). For example, a center-wave half-wave dipole with a resistance of approximately 73 Ohms, through a simple 1: 4 transformer, can be matched with a generator designed for an antenna of 18 Ohms or 300 Ohms - depending on how you connect the leads. This will not affect the operation of the antenna, except for the effect of losses in the transformer on the efficiency of the entire assembly.

If it seems to you that the antenna has only a monopole - some kind of pin, a piece of wire, or just a track on a printed circuit board, then in fact this is a variant of the groundplayin, which has no dedicated radials, but the radials are the earth, the operator’s body (portable radio station, for example ) or ground polygons on the board. The losses in such radials are obviously greater than in those specially created as part of the antenna, so the efficiency of such structures is always lower, as well as the degree of impedance matching due to the unpredictability of situational instead of calculated radials.

As the antenna length increases beyond the half-wave dipole, the radiation resistance first increases, reaching a maximum with an even number of half-waves, and then falling again, reaching a minimum with an odd number of half-waves. A slight increase in length narrows the radiation pattern and increases the transmission efficiency in the chosen direction, and a significant one leads to a split of the diagram into many petals and is generally inefficient, therefore, in practice, it is usually not used except for multi-band antennas, in which this is a compromise solution.
In general, any increase in the length of the dipole in excess of half of the wave leads to the fact that on the canvas there are areas where the current flows in the opposite direction.This current, of course, also participates in the radiation, but the interference of the field created by it with the field of the conditionally main part of the web causes the radiation pattern to split, which is harmful in most cases: radio communication is usually made in one or several known directions and “Unnecessary” side means simply wasted losses. For example, ground communications are conducted in the direction of the horizon, and radiation into space is wasting power on the transmitter. Therefore, when it is necessary to increase the directivity of the antenna in order to send energy more focused in the right direction, they prefer to use more complex structures based on a dipole, rather than lengthen a single dipole.

When reducing the antenna length from a half-wave dipole (or shortening a quarter-wave core pin), the radiation resistance drops exponentially, which, together with the increasingly complex matching device, makes the shortened antenna extremely inefficient - a small radiation resistance near high resistance means that the matching device with low radiation is in vain./>
That's all that the humanities need to know about antennas.

Source text: Pro antennas for the smallest