Encyclopedia of radio electronics and electrical engineering / Civil radio communications

 Comments on the article

When establishing the equipment and antennas of an amateur radio station, it often becomes necessary to measure the level of electromagnetic radiation in the radio frequency range. Professional equipment for such measurements is rarely available to radio amateurs, but with an accuracy acceptable for practice, it is possible to estimate the electric field strength created by a radio station using simple home-made devices.

Recently, considerable attention has been paid to measures to limit the effects of electromagnetic radiation on humans. These issues are regulated by federal sanitary rules, norms and hygienic standards [see, for example, 1]. In our country, for residential premises, the maximum permissible levels of electric field strength are 10 V / m (for a frequency band of 3 ... 30 MHz) and 3 V / m (30 ... 300 MHz). In a number of European countries, there are similar standards for electric field strength levels. If, by the way, they are not exceeded, then the communications administrations of the country do not accept claims against the transmitting radio station for interference with other electronic devices (for example, audio equipment). In particular, for the frequency band 30...300 MHz, this level is also set to 3 V/m [2]. In other words, if the electric field strength created by a radio transmitting device is considered safe for humans, then this level must also be "tolerated" by electronic household equipment. The foregoing implies that the owner of an amateur radio station must be prepared for controversial situations and be able to at least roughly assess the levels of electric field strength that his radio station creates in residential premises.

In the VHF bands, these levels can be measured using a conventional half-wave dipole. As you know, the voltage U induced in the antenna is equal to its effective height multiplied by the electric field strength of the electromagnetic wave. For a half-wave dipole, the effective height is λ/π where λ is the wavelength [3]. In the amateur VHF band of 2 meters with a field strength of 1 V / m, the voltage U will be 0,66 V for an unloaded dipole and 0,33 V when loaded with a resistor with a resistance equal to the input resistance of the dipole (73 ohms). Such voltages can already be recorded with a conventional high-frequency voltmeter with a diode detector.

The meter turns out to be simple and does not contain a power source. If a high-frequency voltmeter connected to a loaded dipole registers a voltage of 1 V (effective value), then the full-scale deviation of the meter needle will just correspond to an electric field strength of 3 V/m. "Scaling" of the indicator of the device will indicate that at this point the maximum allowable value of the field is exceeded

The scheme of the electric field strength meter for the range of 2 meters is shown in the figure. The halves of the dipole are made of copper wire with a diameter of 2...3 mm. The dimensions in the figure are given in cm. The elements of the high-frequency voltmeter are placed on a small plate of insulating material, to which the halves of the dipole are also attached. The high frequency voltmeter uses a germanium diode because silicon diodes are not suitable for measuring low RF voltages. In addition to the GD508A diode indicated on the diagram, GD507A and D311 can be used here. For germanium diodes of other types (among the common ones), the detection efficiency at frequencies above 30 MHz is noticeably reduced. The values ​​of resistors R1 and R2 are given for a measuring head with a total deflection current of 100 μA and a loop resistance of 2,85 kOhm (M4247).

If the radio amateur has the opportunity to calibrate the high-frequency voltmeter (set the upper measurement limit by selecting resistors R1 and R2, and also remove the dependence of the voltmeter readings on the applied RF voltage), then upon completion of this procedure, the production of the field strength meter ends. Calibration can be done using a VK7-9 voltmeter or similar devices. When selecting resistors, it is useful to observe the condition R1 = R2 for better antenna symmetry.

Of the design features of the device, only one should be noted. To reduce the influence on the measurements of the operator's body, and especially his hands, a small "mast" (not shorter than 0,5 m) must be attached to the antenna with an indicator and the entire structure should be held at arm's length.

If the radio amateur does not have the opportunity to calibrate the RF voltmeter of the field strength meter, then you can use the method below. The total resistance of resistors R1 and R2 is chosen so that the DC voltmeter (these resistors and microammeter) has a voltage measurement limit of 1 V. Their resistance (in kOhm) can be calculated from the ratio

R1 = R2 = (1/iR)/2,

where i is the total deflection current of the RA1 device, mA; R is its internal resistance, kOhm. In this case, the RF voltmeter will also have a measurement limit close to 1 V (effective value), with an error of no more than 20%, regardless of the diodes used in the voltmeter (from among those mentioned above), and the scale of such an RF voltmeter will be a power law with an exponent degree n ~ 1,25. You can read more about this in [4]. For a microammeter with a total deflection current of 100 μA, the correspondence between the instrument readings N and the true values ​​of the RF voltage U (effective value) is shown in the table. For microammeters with other values ​​of the total deflection current, the exponent n changes (but not much, see [4])

The error in measuring the RF voltage with such an RF voltmeter (and, consequently, the electric field strength generated by the transmitter) will not exceed 30%, regardless of the diode instance used. The accuracy is not high, but it is quite sufficient for rough estimates of the electromagnetic environment.

The structure of the electromagnetic field in residential premises can be very heterogeneous due to the reflections of radio waves from metal structures and electrical wiring. For this reason, the indicator must be moved near the measurement point, achieving the maximum of its readings, as well as varying its polarization.

It is impossible to make a similar resonant field meter for lower frequencies due to the large length of the dipole, but for estimates in the KB ranges, the one described above can also be used using it as a Hertzian dipole (very short compared to the wavelength). The effective height of an unloaded Hertzian dipole is -l/2, where I is the total length of the dipole (in our case, about 1 m). Therefore, for example, in the range of 20 meters with an electric field strength of 10 V / m, the induced voltage will be about 5 V. However, the input resistance of the Hertzian dipole has a capacitive character and is large in absolute value. Resistor R3 forms a divider with this resistance, which significantly reduces the voltage across the detector. It can be calculated using the data from [3] or using the MMANA program, but it is still better to calibrate the meter experimentally on each of the used ranges. The resistance of the resistor R3 in this case can be much larger.

Literature

1. cqham.ru/ftp2/55output.txt.
2. Edwin David. Ideas from abroad (Eurotek). - Radio communication. 1996, November, p. 55.
3. Mainke X., Gundlach F. Radio engineering reference book. - M-L .: Gosenergoizdat, 1961, vol. 1.
4. Stepanov B. Measurement of small RF voltages. - Radio, 1980, No. 7, p. 55.56: No. 12, p. 28 (Returning to printed).

Author: Boris Stepanov (RU3AX), Moscow

See other articles Section Civil radio communications.

Read and write useful comments on this article.

<< Back

<< Back

Latest news of science and technology, new electronics:

The world's tallest astronomical observatory opened 04.05.2024

Exploring space and its mysteries is a task that attracts the attention of astronomers from all over the world. In the fresh air of the high mountains, far from city light pollution, the stars and planets reveal their secrets with greater clarity. A new page is opening in the history of astronomy with the opening of the world's highest astronomical observatory - the Atacama Observatory of the University of Tokyo. The Atacama Observatory, located at an altitude of 5640 meters above sea level, opens up new opportunities for astronomers in the study of space. This site has become the highest location for a ground-based telescope, providing researchers with a unique tool for studying infrared waves in the Universe. Although the high altitude location provides clearer skies and less interference from the atmosphere, building an observatory on a high mountain poses enormous difficulties and challenges. However, despite the difficulties, the new observatory opens up broad research prospects for astronomers. ... >>

Controlling objects using air currents 04.05.2024

The development of robotics continues to open up new prospects for us in the field of automation and control of various objects. Recently, Finnish scientists presented an innovative approach to controlling humanoid robots using air currents. This method promises to revolutionize the way objects are manipulated and open new horizons in the field of robotics. The idea of ​​controlling objects using air currents is not new, but until recently, implementing such concepts remained a challenge. Finnish researchers have developed an innovative method that allows robots to manipulate objects using special air jets as "air fingers". The air flow control algorithm, developed by a team of specialists, is based on a thorough study of the movement of objects in the air flow. The air jet control system, carried out using special motors, allows you to direct objects without resorting to physical ... >>

Purebred dogs get sick no more often than purebred dogs 03.05.2024

Caring for the health of our pets is an important aspect of the life of every dog ​​owner. However, there is a common assumption that purebred dogs are more susceptible to diseases compared to mixed dogs. New research led by researchers at the Texas School of Veterinary Medicine and Biomedical Sciences brings new perspective to this question. A study conducted by the Dog Aging Project (DAP) of more than 27 companion dogs found that purebred and mixed dogs were generally equally likely to experience various diseases. Although some breeds may be more susceptible to certain diseases, the overall diagnosis rate is virtually the same between both groups. The Dog Aging Project's chief veterinarian, Dr. Keith Creevy, notes that there are several well-known diseases that are more common in certain breeds of dogs, which supports the notion that purebred dogs are more susceptible to disease. ... >>

Random news from the Archive Black nitrogen, analogue of graphene 06.06.2020 Employees of the University of Bayreuth managed to create "black nitrogen", which disappears when the pressure drops. With their development, scientists have proved that nitrogen is no exception to the rules of the periodic table of Mendeleev. As part of the work, experts were able to refute the special status of nitrogen. Under laboratory conditions, scientists at high temperature and high pressure from nitrogen created a structure that is characteristic of black phosphorus. "Black nitrogen" is similar in conductive properties to graphene. The result of the study is promising for use in high-tech electronics. The main disadvantage of "black nitrogen" is that it turns back into normal nitrogen at reduced pressure.

Other interesting news:

▪ Window to the brain

▪ Chinese Python repairs roads

▪ Red wine speeds up fat burning in the liver

▪ Food of the future

▪ Cheap water treatment

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the site Lecture notes, cheat sheets. Selection of articles

▪ article Nikolai Pirogov. Biography of a scientist

▪ article Why does the moon come in different shapes? Detailed answer

▪ article Supply agent. Job description

▪ article The simplest variable capacitor. Encyclopedia of radio electronics and electrical engineering

▪ article Switchgear with voltage up to 1 kV AC and up to 1,5 kV DC. Installation of switchgears in electrical rooms. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages ​​of this page

Arabic	Hebrew	Polish
Bengali	Hindi	Portuguese
Chinese	Italian	Spanish
English	Japanese	Turkish
French	Korean	Ukrainian
German	Malay	Vietnamese

Home page | Library | Articles | Website map | Site Reviews

www.diagram.com.ua
2000-2024