|
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 High-frequency ammeter for shortwaves. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology For shortwaves, when setting up or testing equipment, it often becomes necessary to measure high-frequency current. A radio amateur usually does not have standard instruments for such measurements. It is easy to measure the high-frequency voltage (diode, capacitor, indicator). There are no problems with measuring voltage in devices. There is a housing against which all voltages are measured. And the wires from the measurement points to the RF voltmeter are usually so short (in terms of the wavelength of the measured voltage λ) that they hardly affect the device under test. But in antenna technology it is more difficult. First, antennas often do not have a "ground" at all (for example, symmetrical antennas). Second, even if there is a ground (say, a GP or Y-matched dipole), the test leads are unacceptably long. Imagine what it would look like to try to measure the voltage in the middle of the GP: after all, from this point to the base of the pin you will have to pull a wire! They actually become part of the antenna, changing its operation and voltage distribution so much that the accuracy and value of such measurements is very low. To study and measure what is happening in the antenna conductors, you need an RF ammeter. It, unlike a voltmeter, is connected at one point, which means it does not have long measuring wires that distort the measurement. The basis of the RF ammeter is the current sensor. This is a special high-frequency transformer on a ferrite ring magnetic circuit. The primary winding of this transformer is the wire in which we measure the current. The secondary winding consists of several tens of turns loaded on a low-resistance resistor. Shown in fig. 1 current transformer works like this. The current in the measured wire through the magnetic circuit induces a current in the secondary winding, which will be less than the current in the primary circuit in relation to the number of turns of the windings. For example, with a ratio of the number of turns of windings of 20 (as in our device), it will be 20 times less. This current, flowing through the load resistor, will create an RF voltage drop across it. The latter can already be measured with any RF voltmeter (there are two points for measurement - the outputs of the secondary winding): from the detector diode to the spectrum analyzer or receiver.
If the resistance of the load resistor R is chosen, for example, 50 Ohm, at a current Ivh in the primary winding of the transformer voltage UO (on its secondary winding there will be Uvyx=(Ivh/20)*50=2,5Iinx. The 50 Ohm resistance was not chosen by chance as a load, but in order to be able to use a receiver or a spectrum analyzer as an RF voltage meter (measurement of very small RF currents). The ratio N of the number of turns of the windings, i.e. the number of turns of the secondary winding (the primary always has one turn), is chosen from compromise considerations. On the one hand, the fewer turns in the secondary winding, the broader the transformer will be. On the other hand, the larger N, the less resistance introduced into the measured wire and the less influence of our transformer on the measured wire. Insertion resistance equals R/N2, i.e. in our case 50/202\u0,125d 0,125 Ohm. Thus, the active input resistance of our RF ammeter is XNUMX ohms, which is acceptable for most measurements. We need a measuring device, not a "display meter". To do this, it is necessary that the magnetic circuit can work in a given band (i.e., the ferrite should not be too low-frequency) and not saturate at significant currents in the measured wire (i.e., the dimensions of the magnetic circuit must be large enough). In addition, the magnetic circuit must be split into two halves, and its frame must be snap-on. Without this, it will be almost impossible to use the device: every time you will not thread the beginning of the measured wire through the magnetic circuit and move the latter to the measurement point. And the last (by mention, but not by importance) requirement for the magnetic circuit of a current transformer: the hole must be large in order to be able to measure the current in the braids of thick cables. Based on the foregoing, the magnetic circuit 28A3851-0A2 was chosen with dimensions of 30x30x33 mm and a hole with a diameter of 13 mm. This is an interference-suppressing snap-on magnetic circuit made of ferrite with an initial magnetic permeability of about 300 at a frequency of 25 MHz. Most likely, many other magnetic cores similar in purpose will do. We wind 20 turns of a thin mounting wire on the magnetic circuit (Fig. 2) and protect the secondary winding with a heat-shrinkable tube (Fig. 3).
We attach it to a small (20 ... 30 cm) dielectric rod with a coaxial instrument connector at the lower end. From the connector to the secondary winding in the rod, we draw a thin coaxial cable with a characteristic impedance of 50 ohms. Now you can check the quality of the manufactured current transformer. To do this, we will carry out measurements according to the scheme shown in Fig. 4.
Let's estimate the expected transfer coefficient. The current through R1 is Uvh/R1. Substituting this for Ivh into the previous formula, we get UO=Uvh/ 20. That is, the transfer coefficient of such a circuit will be 1/20 or -26 dB. This is when the transformer is working perfectly. Let's compare this calculated value with practice. The results of measurements in the band 0,3...30 MHz are shown in fig. 5.
It can be seen that the difference between the transfer coefficient and the calculated one is less than 0,9 dB, i.e., the transformer turned out to be a very accurate measuring sensor. And you can not vouch for the fact that the blockage of the frequency response at the high-frequency edge is associated with the properties of the ferrite, and not with the actual current drop through the transformer. The fact is that the wire passing through the transformer has a non-zero inductance, which increases the load impedance, which causes the resulting SWR to increase slightly (reaching 1,1 at a frequency of 30 MHz) and the load current drops. And it is very likely that the drop in the graph on the frequency response simply shows the truth: the current in the load on the RF drops. In any case, it can be seen that the measurement accuracy is very high (less than 1 dB error) in the frequency band from 0,3 to 30 MHz. The current transformer described above is used in two versions. Firstly, for autonomous operation (for example, on the roof to measure the current in the antennas and study its distribution, or to search for which cables of the radio station the common-mode current from the transmitter spreads), a diode detector with an input impedance of 50 ohms with a switch of measurement limits and a switch is connected to the transformer. device. For example, such as shown in Fig. 6.
Resistors R3-R6 are selected based on the sensitivity of the pointer device according to the following method. With the SA1 switch position "10 A", we supply a constant voltage of 25 V from the power source to the input of the device and, selecting the resistor R6, set the full scale deviation. This must be done quickly, the resistors R1 and R2 get very hot. At the limit of "3 A" we do the same at a voltage of 7,5 V by selecting resistor R5, at the limit of "1 A" - at a voltage of 2,5 V we select resistor R4, at the limit of "0,3 A" - at a voltage of 0,75, 3 V we select the resistor RXNUMX. It turns out a convenient stand-alone RF ammeter, with which you can examine almost any antenna. Almost because the resistance of any ammeter should be many times less than the resistance of the measured circuit. Therefore, to use this RF ammeter in places where the resistance is less than a few ohms (short-circuit loops, magnetic frames, shortened antennas), not only impossible, but unreasonable. Including an ammeter in such places will cause a noticeable change in current, and you will not know its true value. To measure low currents (for example, stray common mode noise currents in various cords and cables), connect the 50 ohm input of the receiver or spectrum analyzer to the transformer. For example, in fig. 7 shows what signals are present in the power cord of the extension cord to which the computer, monitor and digital oscilloscope (also, in principle, a computer) are connected. The amateur band of 160 meters from 1,8 to 2 MHz is being studied.
Such a bleak picture is given by only three switching power supplies. Moreover, these are still good power supplies that meet the standards for parasitic radiation. This, however, does not exclude the fact that they may well interfere with DX reception. The described HF current sensor will help you find the most problematic, in terms of interference, cables and devices. Author: I.Goncharenko
The world's tallest astronomical observatory opened
04.05.2024 Controlling objects using air currents
04.05.2024 Purebred dogs get sick no more often than purebred dogs
03.05.2024
▪ Graphene filter traps everything but water ▪ NVIDIA began to assemble some video cards on its own ▪ Samsung ISOCELL GN2 image sensor
▪ section of the site Your stories. Article selection ▪ article Rules of conduct and methods of protection in case of riots. Basics of safe life ▪ article How many moons does Jupiter have? Detailed answer ▪ article Testing safety belts, portable ladders and ladders. Standard instruction on labor protection
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