Field strength meter
When establishing equipment and antennas Amateur radio station often there is a need to measure the level of electromagnetic radiation radio frequency range. Professional equipment for such measurements rarely available for Amateurs, but with an acceptable accuracy rate the electric field created by a radio station, it is possible by using simple home-made devices.
In recent times, considerable attention is paid to measures to limit the impact electromagnetic radiation on humans. These matters are governed by Federal sanitary regulations, norms and hygienic standards [see e.g. 1]. In our country for premises established maximum permissible levels the electric field strength 10 V/m (for band chastot...30 MHz and 3 V/m (30...300 MHz). In a number of European countries have similar standards to the levels the electric field strength. If they are, incidentally, has not been exceeded, then the claim to the transmission station for interference with other electronic devices (e.g., the audio equipment) of the communications Administration of the country do not accept. In particular, for bandwidth 30...300 MHz this level is set too 3 V/m . In other words, if the electric field created by the transmitting the device is considered safe for humans, this level is required to "tolerate" and electronic household equipment. This implies that the owner of an Amateur radio station must be willing to controversial situations and have the opportunity to at least roughly estimate the levels of electric field intensity which creates in the living room of his radio station.
In УSW ranges to measure these levels is possible using a conventional half-wave of the dipole. As is known, the voltage U induced in the antenna is equal to its current the height multiplied by the electric field of the electromagnetic wave. For a half-wave dipole effective height equal to λ/π, where λ is the wavelength . In Amateur УSW range of 2 meters when the field strength 1 V/m voltage U will 0,66 In for the unloaded dipole and 0.33 In with a load resistor with a resistance equal to the input impedance of the dipole (73 Ohms). Such voltage you can register a usual high-frequency voltmeter with diode detector.
The meter turns simple and do not contain a power source. If high frequency voltmeter connected to loaded dipole, registers a voltage of 1 Vrms (effective value), the deflection of the measuring of the device to the full scale to match the strength of the electric field 3 V/m. Peaks in the display device will indicate that at this point the maximum allowable value of the field is exceeded
The scheme of measuring the electric field intensity for the range of 2 meters shown in the figure. Half of the dipole is made of copper wire with a diameter of 2...3 mm. the Dimensions in the figure are given in see Elements of RF voltmeter place on a small plate of insulating material to which is attached and the halves of the dipole. In the high frequency voltmeter used a germanium diode, since silicon diodes are not suitable to measure small RF voltages. Except indicated on the diagram diode GTA, here you can apply GDA and D311. I germanium diodes, other types of (common) efficiency detection at frequencies above 30 MHz is markedly reduced. The values of R1 and R2 are given for transmitter with current total deviation of 100 µa and resistor limits of 2.85 kOhm (M).
If the radio has the ability to calibrate high-frequency voltmeter (set the upper limit of measurement by selection of resistors R1 and R2, and also to remove the dependence of the voltmeter on the applied RF voltage), upon completion of this procedure the fabrication of field strength meter ends. Calibration can be produced using a volt meter VK7-9 or such appliances. In the selection of resistors, it is useful to observe the condition R1 = R2 for best of symmetry of the antenna.
From the constructional features of the device, it should be noted only one. To to reduce the influence on the measurement of the operator's body, particularly his hands, to the antenna with indicator it is necessary to attach a small "mast" (no shorter than 0.5 m) and keep the whole design at arm's length.
If ham is not possible to calibrate the RF voltmeter meter the field strength, you can use the following technique. The total resistance of the resistors R1 and R2 is selected so that the voltmeter DC (these resistors and microammeter) had a limit of measurement voltage of 1 V. the resistance (in kω) can be calculated from the relation
R1 = R2 = (1/(i-R)/2,
where i is the current full deflection device PA1, mA; R is the internal resistance, ohms. When this RF voltmeter will also have a limit of measurement close to 1 In (effective value), with an error of not more than 20 % outside depending on the voltmeter diode (mentioned above), and the scale of such an RF voltmeter will be of a power law with exponent n ~ 1,25. More information can be found in . For micro-ammeter with a current full deflection of 100 μa compliance of the readings of N and the true values The RF voltage U (effective value) in the table. For microammeter with other values of the current total deviation of the exponent n varies (but not much, see )
Measurement error of the RF voltage such RF voltmeter (and therefore the strength of the electric field generated by the transmitter) will not exceed 30% regardless of the instance of the diode. The accuracy is low, but for a rough estimate of the electromagnetic environment is quite sufficient.
The structure of the electromagnetic field in a residential area is likely to be very heterogeneous because of the reflections of radio waves from metallic structures and wiring. For this reason, the indicator needs to be moved near the point measurements, achieving maximum results and also to vary its polarization.
At lower frequencies to make similar resonant field meter cannot because of the length of the dipole, but for estimates in LW bands can be applied as described above, using it as a Hertzian dipole (very short compared to wavelength). Effective height of the unloaded dipole Hertz-l/2, where I is full length of dipole (in our case about 1 m). So, for example, in the range 20 meters when the electric field strength 10 V/m the induced voltage will be about 5 V. However, the input resistance of Hertzian dipole is capacitive the nature and large in absolute value. Resistor R3 forms with this resistance divider greatly reduces voltage detector. It amenable to calculation, using data from  or by using the software MMANA, but it is better to calibrate the meter experimentally used on each of ranges. The resistance of the resistor R3 in this case may be significantly more.
Author: Boris Stepanov (RU3AX), Moscow