|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Steel conductors in antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Theory When choosing a material for the manufacture of antennas, copper or aluminum is usually preferred, since these metals have better conductivity compared to, for example, steel. But steel is cheaper, and sometimes it's easier to make an antenna out of it. The article assesses the loss when replacing copper wires with wires made of steel and other materials, and gives examples of the deterioration in the efficiency of antennas with such a replacement. The causes of high-frequency losses in steel wires are considered, a method for measuring the per unit resistance of wires from a material with unknown properties in the range of 3,5 ... 28 MHz is described, and recommendations are given for computer modeling of steel wire and vibrator antennas. Traditional materials for antennas are copper (wires) and aluminum alloys (tubes). Their advantage is good conductivity. The disadvantages include low mechanical strength and, in recent years, high cost. The experience of using steel structures as secondary elements of antenna systems indicates the possibility of using cheap and durable steels as one of the main materials for the manufacture of antennas. Radio amateurs use weather-resistant bimetallic steel-copper wires (BSM), as well as flexible polyethylene-insulated wire (GSP) [1], which has steel veins along with copper ones. In this regard, it is of interest to estimate the losses when steel replaces traditional copper or aluminum. As a measure of evaluation, the ratio of the active component R of the linear resistance of a wire of round cross section from the material under study at high frequency to the corresponding value RM for a copper wire of the same diameter at the same frequency was taken: R/RM. As is known, the high-frequency electric current is distributed unevenly over the cross section of the wire: it is maximum at the surface and rapidly decreases when moving away from it deep into the material (surface effect). For wires with a diameter of more than 1 mm at frequencies above 1 MHz, the effective thickness of the surface layer in which the current is concentrated (penetration depth) is determined by the formula [2]:
where f - frequency (Hz); δ is the specific conductivity of the material (S/m); μr - relative magnetic permeability of the material; μ0 = 4π 10-7 (H/m). The effective cross section of the wire with a diameter d (m) for the radio frequency current is s = 5πd (m2), and the linear active resistance
In table. 1 shows the values of δ, p and μr of some conductor materials.
For non-ferromagnetic conductors, μr - 1, and formula (2) is sufficient to compare the linear resistance of wires, for example, from aluminum and copper. The desired measure is calculated simply: R/RM = = √δM/δ. So, for example, for aluminum we get: R/RM = √56,6/35,3 = 1,265. For ferromagnetic materials (μr >> 1) everything is much more complicated. The fact is that with increasing frequency, μr rapidly decreases, tending to unity, and losses in the material increase, in particular, eddy current losses increase in proportion to the square of frequency. A decrease in μr leads to a thickening of the surface layer, i.e., to a decrease in resistance, and an increase in losses is equivalent to an increase in resistance. As a result, the losses outweigh and the per unit resistance still increases with increasing frequency. Everything could be taken into account (although not simply) if the chemical composition and structure of the alloy were known exactly. And since this is rarely known, it remains to turn to the old criterion of truth - to practice. The linear resistance of the copper wire RM was determined by calculation according to the formula (2). To determine the linear resistance R of a wire made of any material with unknown characteristics, a high-frequency quality factor meter (kumeter) of the E9-4 type was used. Preliminary preparation of the kumeter consisted in calibrating the level setting on all scales according to the criterion Q = fres / Δf0,707- For this, a vernier capacitor with divisions through 0,1 pF was used. As a result, the device determined the equivalent quality factor Q of the entire measuring circuit, taking into account both losses in the tested inductor coil and other losses (in the device itself, in an additional external capacitor, in the environment and for radiation). For high-frequency isolation of the device housing from the mains and other conductive objects, a shut-off choke is installed, containing 20 turns of a three-wire power cord on a K90x70x10 ring magnetic circuit made of 400NN brand ferrite at the place where the cord is connected to the device. One of the wires of the cord is a protective earth wire (zeroing) of the instrument case. The Kumeter was installed on a dielectric stand 0,5 m high at a distance of at least 2 m from walls and other, especially conductive, large objects. To reduce measurement errors, it is necessary to warm up the device for 60 minutes before measurements, monitor possible zero drift and make several (at least 5-7) measurements of C and Q at each frequency, followed by averaging. When measuring at frequencies above 10 MHz, the result may be affected by the hand of the operator turning the knob of the capacitor. For an accurate reading, the hand should be retracted, and the head should be kept at a distance of no closer than 0,5 m from the device. Suppose it is necessary to determine the linear resistance R of a wire with a diameter d at a frequency f within 3 ... 30 MHz. We take a length of 1 m of this wire and a length of 1 m of copper wire of the same diameter. We make identical short-circuited two-wire lines from these wires with a distance between the wires of 40 mm. We connect these lines alternately to the device as inductors, while the lines must be installed vertically. We measure the quality factors for lines from both materials and the resonant values of the capacitance C on the Kumeter scale. If necessary (for frequencies below 10 MHz), we connect an additional capacitor, preferably mica, but for both materials it is always the same. Its capacity must be known with an error of no more than ± 5%. Next, you need to do some calculations. First, we calculate the value of the total equivalent series resistance of losses req in the measuring circuit (this includes both losses in the wire and other losses). This is done for both materials in accordance with the well-known expression for the oscillatory circuit: req = 1/(2πfCQ). With the same line sizes, with the same additional capacitors and at the same frequency, the above other losses can be assumed to be the same for both materials. And you can find them by measurements on a copper line, since the calculated wire resistance RM is known for it. The resistance of other losses, therefore, is the difference: r pp \uXNUMXd r ppm \uXNUMXd r equiv m - RM. Now it remains to calculate the resistance of a segment of 1 m of wire from the material under test R = r eq - r pp and determine the desired ratio R / Rm. The main error of the kumeter is ±5%. The influence of a possible systematic error is partially compensated due to the fact that the result of determining the value of R contains the difference in the results of measuring the values of req for different materials. From different wires with a diameter of 1 to 4,5 mm and a length of 1 m, short-circuited segments of two-wire lines were made with a distance between the wires of 40 mm, in total - 25 samples. Measurements were made according to the method described above at five frequencies: 3,5; 7; 14; 21; 28 MHz. The results of Rm calculations are shown in the figure.
The results of measurements of the linear resistance R and the calculation of the R / RM ratios for steel and some other wires are summarized in Table. 2.
From Table. 2 shows that for steel wires in the indicated frequency range, the per unit resistance increased by 15,9 ... 24,9 times. For samples with a clean and smooth surface (1, 6, 8), the frequency dependence of R/RM is weak. The contamination of the surface of samples 2, 3 and the significant surface roughness of sample 4 determine a more significant increase in R/RM with increasing frequency. Annealing of steel wires did not have a noticeable effect on losses if the scale was removed and the surface was cleaned. Titanium and non-magnetic stainless steel wires are approximately 2,5 times better than conventional steel wires. Bimetallic steel-copper wire 9 (BSM) at all frequencies loses to pure copper wire by more than 3 times, but 5 ... 6 times better than pure steel wire. Note that with a copper coating thickness of about 0,03 mm, its main purpose is to protect the steel base from atmospheric influences. Lines 10, 11 show data for stranded wires with a cross section of 0,5 mm2 in insulation. The GSP wire has 4 copper and 3 steel wires with a diameter of 0,3 mm. In terms of losses at 28 MHz, it turned out to be at the level of a steel wire with a diameter of 4,1 mm, and in low-frequency bands it is much better. Mounting wire MGShV has 16 tinned copper wires with a diameter of 0,2 mm and is more than 2 times better than GSP. The results for aluminum wire 8 with a smooth and clean surface are in good agreement with the results of calculation by formula (2) and can confirm the correctness of the chosen approach. Computer simulation was carried out using the MMANA program [3]. The peculiarity of the simulation is that as a result of the analysis, the active component of the complex input impedance of the antenna is determined, and not the linear resistance of the wire. And the input impedance depends on the size of the antenna, its configuration and the connection point of the excitation source. This dependence, however, makes it possible, at relatively large wave sizes of antennas, to obtain an almost imperceptible loss when replacing copper with steel. Several loop and dipole antennas of different sizes were taken for analysis. The simulation results are given in Table. 3.
The radiation resistance R∑ is obtained as the active component RA of the input impedance in a lossless analysis. This value of Um was taken unchanged during the transition from copper to iron, since the shape and dimensions of the antenna did not change. Also obtained are the values of RAM and RAzh for antennas made of copper and iron, respectively. The efficiency for copper and iron was calculated as the ratio of R∑ to the corresponding value of RA. The ratio Rzh/Rm was calculated by the formula: Rzh/Rm = (Razh - R∑)/(RAm - R∑) For all considered antennas, it turned out that the ratio Rl/RM is on average close to 27,8, regardless of the frequency. This could happen provided that formula (2) was used for calculations with iron losses, for example, with a table value of resistivity = 0,0918 Ohm mm2/m and a constant μr - 150. By the way, the same results are obtained in the program ELNEC at the specified parameters. Judging from the above experimental data, these simulation results can be used as an estimate of the worst-case steel wire loss in the frequency range up to 28 MHz. For the VHF band, they will, apparently, be closer to the truth. From Table. It can be seen from Table 3 that even with such an assessment for the considered cases, almost all efficiency deterioration coefficients are significantly less than the R/RM coefficients for steel in Table. 2. A smaller loss of the steel antenna will be if the Rh antenna is larger (see, for example, a dipole 2x5,13 m at a frequency of 28 MHz). Electrically small antennas with small R∑ and initially low efficiency for copper are the most sensitive to the replacement of copper with steel. Some wire antenna simulation programs (eg Nec2d, ASAP) do not provide input of material permeability. Apparently, when modeling steel antennas using formula (2), we can assume μr = 1 and introduce the equivalent conductivity δeq (or resistance req) taking into account real losses. For steel in the range of 3,5 ... 28 MHz, you can enter, respectively, δeq = 0,19 ... 0.094 MSm / m (req = 5,3 ... 10,6 Ohm mm2 / m) for rough and contaminated surfaces , or δeq = 0,22 ... 0,17 MSm / m (req = 4,5.-5,9 Ohm mm2 / m) for clean and smooth. The MM AN A program does not allow you to model different wires from different materials, such as copper and steel. To assess the efficiency of the antenna in this case, it is possible to manually enter into each segment of the copper wire, which in fact should be steel, concentrated losses, which are calculated based on the length of the segment, given that the linear resistance of the steel wire at high frequency is 16 ... .25 times more than copper. For example, in each of 10 identical segments of a copper wire 20 m long and 2 mm in diameter at a frequency of 3,5 MHz, you can enter an active load of 16-0,08-20/10 = 2,56 Ohm, where the value of the linear resistance of the copper wire is 0,08 .2 Ohm/m is determined by formula (XNUMX) and can be found from the graphs in the figure. Sometimes, in order to evaluate the efficiency in this situation, it is possible to reduce the diameter of the copper wire in the wire model (also by 16...25 times). However, it must be remembered that this leads to a significant increase in the linear inductive resistance, as a result, the current distribution in the structure and everything connected with it can change dramatically. The change in the efficiency of the antenna when replacing the copper wire with steel depends on the wave dimensions and the initial efficiency of the copper antenna. If the efficiency of a half-wave antenna made of copper is 0,98 ... 0,99, then a steel antenna of the same size can have an efficiency of 0,7 ... 0,85, which is not so bad. However, if the efficiency of an electrically small copper antenna is of the order of a few percent, replacing copper with steel can lead to its deterioration by 15...25 times. The author thanks F. Golovin (RZ3TC) for posing the problem and support in the work, as well as I. Karetnikova for valuable comments. Literature
Author: A. Grechikhin (UA3TZ), Nizhny Novgorod
Artificial leather for touch emulation
15.04.2024 Petgugu Global cat litter
15.04.2024 The attractiveness of caring men
14.04.2024
▪ Surface defrosting in a second ▪ Interaction of radiation with water ▪ LG ltraGear 48GQ900 OLED Gaming Monitor ▪ Test of automatic braking systems in cars ▪ Jabra Elite 4 Active TWS headphones
▪ section of the site Assembling the Rubik's Cube. Article selection ▪ article Summary of works of Russian literature of the XNUMXth century ▪ Article Financial Analyst. Job description ▪ article Primary quartz clock. Encyclopedia of radio electronics and electrical engineering ▪ fuse burn indicator. Encyclopedia of radio electronics and electrical engineering
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page