The spiral antenna belongs to the class of traveling wave antennas. Its main range of work is decimeter and centimeter. It belongs to the class of surface antennas. Its main element is a spiral connected to a coaxial line. The spiral creates a radiation pattern in the form of two petals radiated along its axis in different directions.
Spiral antennas are cylindrical, flat and conical. If the required width of the operating range is 50% or less, then a cylindrical helix is used in the antenna. The conical spiral increases the reception range by half compared with the cylindrical one. And flat ones already give a twenty-fold advantage. The most popular for receiving in the VHF frequency range was a cylindrical radio antenna with circular polarization and a large gain of the output signal.
Antenna device
The main part of the antenna is a coiled conductor. Here, as a rule, copper, brass or steel wire is used. A feeder is connected to it. It is designed to transmit the signal from the spiral to the network (receiver) and in the reverse order (transmitter). Feeders are open and closed. Open feeders are unshielded waveguides. And the closed type has a special screen from interference, which makes the electromagnetic field protected from external influences. Depending on the frequency of the signal, the following design of feeders is determined:
- up to 3 MHz: shielded and unshielded wired networks;
- from 3 MHz to 3 GHz: coaxial wires;
- from 3 GHz to 300 GHz: metal and dielectric waveguides;
- over 300 GHz: quasi-optical lines.
Another element of the antenna was the reflector. Its purpose is to focus the signal on a spiral. It is mainly made of aluminum. The base for the antenna is a frame with a small dielectric constant, for example, foam or plastic.
Calculation of the main dimensions of the antenna
The calculation of the spiral antenna begins with the determination of the main dimensions of the helix. They are:
- the number of turns n;
- angle of rise of a loop a;
- the diameter of the spiral D;
- step of the spiral S;
- diameter of the reflector 2D.
The first thing to understand when designing a spiral antenna is that it is a resonator (amplifier) of the wave. Its feature was a high input impedance.
The type of waves excited in it depends on the geometric dimensions of the gain loop. Adjacent spiral coils have a very strong effect on the nature of the radiation. Optimal ratios:
D = λ / π, where λ is the wavelength, π = 3.14
S = 0.25 λ
a = 12˚
Because λ is a quantity that varies and depends on the frequency, then the average values of this indicator calculated by the formulas are taken in the calculations:
λ min = c / f max; λ max = c / f min, where c = 3 × 10 8 m / s. (speed of light) and f max, f min - the maximum and minimum parameter of the signal frequency.
λ cf = 1/2 (λ min + λ max)
n = L / S, where L is the total length of the antenna, determined by the formula:
L = (61˚ / Ω) 2 λ cf, where Ω is the directional coefficient of the antenna, depending on the polarization (taken from handbooks).
Range classification
According to the main frequency range, transceivers are:
1. Narrowband. The beamwidth and input impedance are highly frequency dependent. This suggests that the antenna can work without reconfiguration only in a narrow spectrum of wavelength, approximately 10% of the relative frequency band.
2. Wide-range. Such antennas can operate in a large frequency spectrum. But their main parameters (KND, radiation pattern, etc.) still depend on the change in wavelength, but not as much as in narrow-band ones.
3. Frequency independent. It is believed that here the main parameters do not change when the frequency changes. Such antennas have an active region. It has the ability to move along the antenna without changing its geometric dimensions, depending on the change in wavelength.
Most often, spiral antennas of the second and third types are found. The first type is used when you need increased "clarity" of the signal at a certain frequency.
DIY antenna manufacturing
The industry offers a large selection of antennas. A variety of prices can vary from several hundred to several thousand rubles. There are antennas for television, satellite reception, telephony. But you can make a spiral antenna with your own hands. It is not that difficult. Spiral antennas for Wi-Fi are especially popular.
They are especially relevant when it is necessary to amplify the signal from the router in some large house. To do this, you need copper wire with a cross section of 2-3 mm
2 and a length of 120 cm. It is necessary to make 6 turns with a diameter of 45 mm. To do this, you can use a tube of the appropriate size. A shovel stalk fits well (it has about the same diameter). We wind the wire and get a spiral with six turns. We bend the remaining end so that it passes smoothly through the axis of the spiral, “repeating” it. We stretch the screw part so that the distance between the turns is within 28-30 mm. Then proceed to the manufacture of the reflector.
A piece of aluminum with a size of 15 × 15 cm and a thickness of 1.5 mm is suitable for this. From this blank we make a circle with a diameter of 120 mm, cutting off unnecessary edges. In the center of the circle, drill a hole of 2 mm. We insert the end of the spiral into it and solder both parts to each other. The antenna is ready. Now you need to remove the radiation wire from the antenna module of the router. And solder the end of the wire with the end of the antenna emerging from the reflector.
Features of the 433 MHz Antenna
First of all, it must be said that radio waves with a frequency of 433 MHz during their propagation are well absorbed by the earth and various obstacles. For its relay, low power transmitters are used. As a rule, various security devices use this frequency. It is specially used in Russia, with the goal of not creating interference on the air. A 433 MHz spiral antenna requires a larger output ratio.
Another feature when using such transceiver equipment is that the waves of this range have the ability to add the phases of the direct and reflected waves from the surface. This can lead either to amplification of the signal, or to its weakening. From the foregoing, we can conclude that the choice of "best" reception depends on the individual settings of the antenna position.
Homemade 433 MHz Antenna
It’s easy to make a 433 MHz spiral antenna with your own hands. It is very compact. To do this, you need a small piece of copper, brass or steel wire. You can use just a wire. The diameter of the wire should be 1 mm. We wind 17 turns on a mandrel with a diameter of 5 mm. Stretch the helix so that its length is 30 mm. At these sizes, we test the antenna to receive a signal. By changing the distance between the turns, by stretching and compressing the spiral, we achieve the best signal quality. But you need to know that such an antenna is very sensitive to various objects brought close to it.
UHF Receiving Antenna
UHF helical antennas are required to receive a television signal. By their design, they consist of two parts: a reflector and a spiral.
It is better to use copper for a spiral - it has less resistance and, therefore, less signal loss. Formulas for its calculation:
- the total length of the spiral L = 30000 / f, where f is the signal frequency (MHz);
- spiral pitch S = 0.24 L;
- coil diameter D = 0.31 / L;
- spiral wire diameter d ≈ 0.01L;
- reflector diameter 0.8 nS, where n is the number of turns;
- distance to the screen H = 0.2 L.
Gain:
K = 10 × log (15 (1 / L) 2nS / L)
The reflector cup is made of aluminum.
Other types of transceiver equipment
Conical and flat helical antennas are less common. This is due to the difficulty of their manufacture, although they have the best characteristics in the range of transmission and reception of the signal. The radiation of such transmitters is formed not by all the turns, but only by those whose length is close to the wavelength.
In a flat antenna, a helical line is made in the form of a two-wire line coiled into a spiral. In this case, adjacent turns are excited in phase in the traveling wave mode. This leads to the fact that a radiation field is created with circular polarization in the direction of the axis of the antenna, allowing you to create a wide frequency band. There are flat antennas with the so-called Archimedes spiral. This complex form allows you to significantly increase the transmission frequency range from 0.8 to 21 GHz.
Comparison of helical and narrowly directed antennas
The main difference between a helical antenna and a directional one is that it is smaller in size. This makes it easier, which allows installation with less physical effort. Its disadvantage is a narrower frequency range of reception and transmission. It also has a narrower radiation pattern, which requires a “search” for a better position in space for a satisfactory reception. Its undoubted advantage is the simplicity of the design. A big plus is the ability to tune the antenna by changing the pitch of the turn and the total length of the spiral.
Shortened antenna
For better resonance in the antenna, it is necessary that the “elongated” length of the spiral part be as close as possible to the value of the wavelength. But it should not be less than ¼ of the wavelength (λ). Thus, λ can reach up to 11 m. This is relevant for the HF band. In this case, the antenna will be too long, which is unacceptable. One way to increase the length of the conductor is to install an extension coil at the base of the receiver. Another option is to power the tuner circuit. Its task is to coordinate the output signal of the transmitter of radio stations with an antenna at all operating frequencies. Speaking in plain language, the tuner acts as an amplifier of the incoming signal from the receiver. Such a scheme is used in automobile antennas, where the size of the element receiving the radio wave is very important.
Conclusion
Spiral antennas have gained great popularity in many areas of electronic communications. Thanks to them, cellular communication is carried out. They are also used in television and even in long-distance space radio communications. One of the promising developments to reduce the size of the antenna was the use of a cone reflector, which allows to increase the length of the receiving wave, compared with a conventional reflector. However, there is a drawback, expressed in a decrease in the spectrum of the operating frequency. Another interesting example is the “two-way” conical helical antenna, which allows working in a wide range of frequencies due to the formation of an isotropic directivity diaphragm. This is because the power line in the form of a two-wire cable provides a smooth change in wave impedance.