At the mention of the magnetic antenna immediately fill memory design on a ferrite rod, partly right. Variations of one type of device. Called the magnetic loop antenna, the perimeter of which is much smaller than the wavelength. Famous zigzags biquadrate (synonyms) are relatives of the technology in question. It has nothing to do antenna with magnetic base. Easy mounting method. Magnetic base antenna securely holds the unit on the roof of the car. Let's talk today about a special design. Charm magnetic antennas is possible to provide a relatively large gain at relatively long wavelengths. magnetic antenna size small. Let's discuss the title, explain how can be made magnetic antenna with your hands.
The magnetic loop antenna
The theory goes: in the resonant circuit of an inductor, capacitor radiation occurs. Closed wave swinging at the resonant frequency arbitrarily, fading, due to the presence of resistance. circuit elements, inductance, capacitance, are purely reactive (imaginary) impedance. And the size depends on the frequency of uncomplicated law. Something like the product of angular frequency (2 P f) across the inductance value or the capacitance, respectively. At some value of opposite sign imaginary components are equal. As a result, the impedance becomes purely active, ideally zero.
In reality beats fade, each circuit in practice is characterized by the quality factor. Recall that the impedance consists of a purely resistive (active) part (resistors), imaginary. The latter include containers whose resistance and inductance of the imaginary negative with a positive imaginary impedance. Now imagine that the circuit capacitor plates started to breed as long until they were at opposite ends of the inductor. Called vibrator (dipole) Hertz, is a variation shortened half-wave, other types of vibrators.
If a single turn coil in the ring, we get a simple magnetic antenna. Simplified interpretation about right. The signal is taken from the opposite side of the capacitor through the power FET. It provides high sensitivity of the device. Well, on the ferrite rod antenna is considered a kind of magnet, only the rings instead of one host. The name of this kind of devices received a high sensitivity to the magnetic component of the wave. In operation, the transmission is generated, generating an electric field response.
Maximum orientation corresponds to the rod axis. Both directions are equivalent. Due to the small perimeter loop antenna with respect to the wavelength of a sufficiently low resistance. Not just 1 Ohm, Ohm's share. Approximate value estimate formula:
R = 197 (U / λ) 4 ohms.
Under U understood the perimeter, in meters, is similar - the wavelength λ. Finally, R - resistance to radiation, not to be confused with active tester shows. This parameter is used in the calculation for the load matching amplifier. Consequently, for the ferrite antenna, it is necessary to multiply the value by the square of the number of turns.
Properties of the magnetic antennas
Let's see how to make a magnetic antenna yourself. First, determine the circumference and the capacitance of the tuning capacitor. Features a magnetic antenna are: design requires agreement on a mandatory basis. A distinctive feature is the incredible number of options for this operation, looms a separate topic of conversation.
magnetic antenna perimeter length ranges from 0,123 - 0,246 λ. If you want to span the range, you need to choose the right capacitor. In free space, the magnetic antenna radiation pattern in the form of a torus, observe arranging coil parallel to the ground. Polarization is linear horizontal. This variant suitable for receiving television broadcasting. Disadvantage: lobe elevation angle depends on the height of the suspension. It is believed that the distance from the Earth λ figure will be 14 degrees. Impermanence is considered negative quality. For radio magnetic antenna used frequently.
Gain is 1.76 dBi, 0.39 less than a half-wave dipole. The size of the last frequency will be tens of meters - where you will get a whopper. Draw your own conclusions. The magnetic antenna is low (perimeter is 2 m to 20 m wavelength, less than one meter in diameter). For comparison, the frequency of 34 MHz, which is well known truckers due talkies, the wave length is 8.8 meters. It is well known: a good half-wave dipole accommodate rare Kamaz. Incidentally, the above is a description of the loop antenna structure formed by a rubber gasket rear window of the car WHA. When small size device works quite well.
By the way, the design is considered to be more pragmatic than typical blade antennas cars where adjustment is carried out changing the inductance. Losses get smaller. directional pattern covers the high elevation angles, touching vertical. In the case of the rod antenna is not possible.
How to choose the length of the circumference. With the increase of increasing the gain. It must satisfy the conditions set out above, be as large as possible. Sometimes you need to cut off the band. Growth increases the perimeter strip transmission device. In a typical channel width of 10 kHz it is meaningless. Will be automatically truncated adjacent carrying broadcast stations. Optionally, bigger is better. It was conceived for the sake of strengthening the fuss. The antenna is selected perimeter maximum, providing the required selectivity.
Now the big question: to determine capacity. To parallel resonance loop inductance formed by the known formula school. Determination circuit parameters according to the expression:
L = 2U (ln (U / d) - 1,07) nH;
U and d - length of the coil diameter. Trick. U = P d, therefore, instead of relationships can take the natural logarithm of the number Pi. Is the author a mistake, we do not undertake to say. It can be taken into account the fact that the tuning capacitor takes a portion of the length, power capacity... find expression for the inductance of the resonance circuit:
f = 1 / 2P √LC; from whence
C = 1 / 4n2 L f2.
However, the literature is recommended to use an approximate formula for the calculation:
C = 25330 / f2 L,
where f - resonance frequency in MHz, and L - inductance in microhenries.
As a method for removing the signal, it is doing by the tuning capacitor on both sides or on the opposite side a circular loop. In the latter case it is recommended to enter the condenser control by the servo motor in the distance, we believe most readers it seems highly contrived, the world is not so much of radio amateurs, self-made magnetic usefulness own antenna.
What are the magnetic antenna
Not always circular magnetic antenna (ideal form). There are octagonal, square. Readers will have guessed: WiFi biquadrate belongs to the latter category, and double frame. Often more circuits increases the gain in the same plane directivity pattern. Given the fact that the antenna efficiency is calculated by the formula:
Efficiency = 1 / (1+ Rp / R),
We see the need to reduce the resistance Rp losses to a minimum. Otherwise, the device productivity plummets. In practice, few means to make the antenna of gold, silver, catching NTV unrealistic. In said aspect will be used aluminum, copper, preferably the latter. For magnetic antennas suitable capacitor with an air gap, a large plate. Try to perform a quality solder pins.
Example. perimeter length is one-tenth λ, therefore, radiation resistance will be 0.02. Now readers see how much will have to try to adjust the efficiency up to 50%. loss resistance in this case is less than 0.02 ohms. To achieve this result, take a thick copper wire. With an increase in the cross section of the conductor resistivity decreases.
At high Q circuit (low loss) is obtained, the resonance voltage is much higher than when the frequency deviation. Consequently, the magnetic antenna bandwidth is distinguished by great width is required to adjust the device. Done by a condenser. We hope that the answer to the question of how to make a magnetic antenna. Recoup feed: Signal surprise home confident reception in any weather.