The HF band contains a number of radio frequencies( 27 MHz, commonly used by drivers), broadcasting multiple stations. TV shows are not here. Today we consider the amateur series, involved by various radio enthusiasts. Frequencies 3.7;7;14;21, 28 MHz of the HF band, related as 1: 2: 4: 6: 8. It is important, as we will see later, it becomes possible to make an antenna that would catch all the nominal values (the issue of coordination is the tenth thing).We believe there will always be people who use the information, catch radio broadcasts. Today's theme is a do-it-yourself HF antenna.
The design of the HF antenna
Disenchanting many, today we will again talk about vibrators. Objects of the Universe are formed by vibrations( Nikola Tesla's views).Life attracts life, this movement. To give a wave life, hesitation is necessary. Changes in the electric field generate a magnetic response, so the frequency crystallizes, carrying information to the ether. The immobilized field is dead. A permanent magnet will not generate a wave. Figuratively speaking, electricity is the masculine beginning, it exists only in motion. Magnetism quality, rather, feminine. However, the authors went deep into the philosophy.
Considered for transmission, it is preferable to use horizontal polarization. First, the azimuth pattern is not circular( in short), there will certainly be less interference. We know that various objects like ships, cars, tanks are equipped for communication. You can not lose commands, orders, words. The object will turn in the wrong direction, but the polarization is horizontal? Disagree with well-known, respected authors, writing: vertical polarization is chosen as a link for the antenna of a simpler design. Touching the lovers' business, it’s rather a matter of the succession of the legacy of previous generations.
Let's add: with horizontal polarization, the Earth's parameters have less influence on the wave propagation, in addition, when the vertical front suffers attenuation, the lobe rises to 5 - 15 degrees, it is undesirable during transmission over long distances. For antennas( asymmetrical) with vertical polarization, good grounding is important. Directly depends on the efficiency of the antenna. It is better to bury the wires with a length of the order of a quarter wave of the earth, the more, the higher the efficiency. Example:
- 2 wires - 12%;
- 15 wires - 46%;
- 60 wires - 64%;
- ∞ wires - 100%.
Increasing the number of wires reduces the characteristic impedance, approaching the ideal( the indicated type of vibrator) - 37 ohms. Note that the quality should not be brought closer to the ideal, 50 Ohm does not need to be coordinated with the cable( RK-50 is used in connection).Great thing. Let us supplement the information package with a simple fact; with horizontal polarization, the signal is added to the reflected Earth, giving a gain of 6 dB.Vertical polarization shows so many minuses, apply( it turned out interestingly with grounding wires), they reconcile.
The HF antenna device is reduced to a simple quarter-wave, half-wave vibrator. The latter are smaller in size, accept worse, the latter are easier to reconcile. Masts are placed vertically using spacers, stretching. Described the design, hung on a tree. Not everyone knows: at a distance of half a wave from the antenna there should be no interference. Concerns iron, reinforced concrete structures. Wait a while to rejoice, at a frequency of 3.7 MHz the distance is. .. 40 meters. The antenna height reaches the eighth floor. Creating a quarter-wave vibrator is not easy.
It is convenient to build a tower to listen to the radio, we decided to recall the old way of catching long waves. Internal ferromagnetic antennas will find receivers of Soviet times. Let's see if the constructions are suitable for their intended purpose( catching broadcasting).
Magnetic Antenna for the
HF Band Suppose that there is a need to accept the frequencies of 3.7-7 MHz. Let's see if you can design a magnetic antenna. Formed by a core of round, square, rectangular section. Dimensions are being recalculated by the formula:
do = 2 √ pc / π;
do - diameter of a round rod;h, c - height, width of a rectangular section.
Winding is not the entire length, you actually need to calculate how much to wind, choose the type of wire. Take the example of an old design textbook, try to calculate the HF antenna frequency 3.7-7 MHz. Let us take the input cascade impedance of 1000 Ohms( in practice, readers measure the input impedance of the receiver independently), the parameter of the equivalent attenuation of the input circuit, at which a given selectivity is achieved, deep equal to 0.04.
Antenna, the design of which we are engaged in, is part of the resonant circuit. It turns out the cascade, endowed with a certain selectivity. How to solder, think for yourself, just follow the formulas. Conducting the calculation will need to find the maximum, minimum capacitance of the trimmer capacitor, using the formula: Cmax = K2 Cmin + Co( K2 - 1).
K is the subband coefficient determined by the ratio of the maximum resonant frequency to the minimum. In our case, 7 / 3.7 = 1.9.It is chosen from incomprehensible( according to the textbook) considerations, according to the example given in the text, we take equal to 30 pF.Not much wrong. Let Cmin = 10 pF, find the upper limit of the adjustment:
Cmax = 3.58 x 10 + 30( 3.58 - 1) = 35.8 + 77.4 = 110 pF.
Rounded, of course, you can take a variable capacitor of a larger range. The example gives 10-365 pF.We calculate the required inductance of the circuit using the formula:
L = 2.53 x 104( K2 - 1) /( 110 - 10) 72 = 13.47 μH.
The meaning of the formula is clear, let us add, 7 is the upper limit of the range, expressed in MHz. Choose a coil core. At the frequencies of the range at the core, the magnetic permeability is M = 100, we select the ferrite of the brand 100НН.We take the standard core 80 mm long, with a diameter of 8 mm. The ratio l / d = 80/8 = 10.From reference books we retrieve the effective value of magnetic permeability md. It turns out 41.
Find the winding diameter D = 1,1 d = 8,8, the number of winding turns is determined by the formula:
W = √( L / L1) D md mL pL qL;The
formula coefficients are read visually using the graphs below. The figures will show the reference numbers used above. Look for the brand of ferrite, man does not live by bread alone. D expressed in centimeters. The authors obtained: L1 = 0.001, mL = 0.38, pL = 0.9.qL is calculated using the formula:
qL =( d / D) 2 =( 8 / 8.8) 2 = 0.826.
Substitute the figures in the final expression for calculating the number of turns of the ferrite HF antenna, it turns out:
W = √( 13.47 / 0.001) x 0.88 x 41 x 0.38 x 0.9 x 0.826 = 373 turns.
Cascade must be connected to the first amplifier of the receiver, bypassing the input circuit. More, for example, the means of selectivity of the range 3.7–7 MHz have now been calculated. In addition to the antenna turns on the input circuit of the receiver at the same time. Therefore, it will be necessary to calculate the coupling inductance with the amplifier, fulfilling the conditions for ensuring selectivity( we take typical values).
Lcv =( dop - d) Rin / 2 π fmin K2 =( 0.04 - 0.01) 1000/2 x 3.14 x 3.7 x 3.61 = 0.35 μH.
The transformation ratio will be m = √ 0.35 / 13.47 = 0.16.Find the number of turns of the coupling coil: 373 x 0.16 = 60 turns. We wind the antenna with wire FEV-1 with a diameter of 0.1 mm; we wind the coil PELSHO with a diameter of 0.12 mm.
Many, perhaps, are interested in a few questions. For example, the assignment With the formulas for calculating a variable capacitor. The author of the question bashfully bypasses, ostensibly the initial capacity of the contour. Hardworking readers will calculate the resonant frequencies of a parallel circuit in which the initial capacitance of 30 pF is soldered. Slightly mistaken, recommending placing a trimmer of 30 pF near the variable capacitor. The chain is being refined. Beginners are interested in an electrical circuit, which will include a homemade HF antenna. .. A parallel circuit, the signal from which is removed by a transformer, is formed by wound coils. The core is common.
Ready self-HF antenna. You will find such in the tourist receiver( models with dynamo are popular today).Antenna HF range( and especially CB) would be great if you make the design in the form of a typical vibrator. Such designs are not used portable equipment. The simplest HF antennas take up a lot of space. Reception is better. The purpose of the HF antenna is to improve the signal quality. In the apartment, loggia. They told how to make a HF antenna of a miniature size. Use vibrators in the country, in the field, forest, in open areas. Material provided by the design reference. The book is full of errors, and the result seems to be bearable.
Even old textbooks are a typo missed by editors. Concerns not one branch of radio electronics.