Field strength measurement with field strength meter and spectrum analyzer

    Field strength is the abbreviation of electric field strength. It is the size of the electric signal induced by the antenna at a certain point in space, which characterizes the electric field strength at that point. Its unit is microvolt/meter (μv/m). For convenience, dBμv/m is also used (0dB=1μv).

1. Field strength measurement

    The measurement of field strength is shown in the figure. When the antenna is in the same polarization direction as the measured signal in the air, the maximum induced signal is obtained. Generally, an effective value level meter (voltmeter) of radio frequency (RF) can be used for measurement. The measurement principle is shown in the figure.

Schematic diagram of electric field strength measurement (1)

    When the line is well matched, the level value read by the meter is the RF voltage Er (dBμv) obtained at the meter input port (usually 50Ω or 75Ω). Er can be expressed as follows

Er=E+Ga+20lgle-Lf-6---（1）

Er is the reading level of the instrument input port (dBμV);

E is the electric field strength (dBμV/m);

Ga is the receiving antenna gain (dB). If a half-wavelength dipole antenna is used, Ga=0dB;

le is the effective length of the receiving antenna (λ/π);

Lf is the receiving feeder loss (dB);

6 is the correction value (dB) for the open port converted from the terminated value.

The electric field strength E (dBμV/m) can be calculated from formula (1), that is:

E =Er-Ga-20lgle+Lf+6---(2)

Let's take an example to explain: Assume the test frequency is 228.25MHz (λ=1.31m)

Then 20lgλ/π=20lg1.31/π≈-7.6dB;

The receiving antenna is a half-wavelength dipole antenna, Ga=0dB; Lf uses a 10dB/100m attenuation cable, and the attenuation is 1dB when the practical length is 10m; the instrument indicates a level of 15dBμV.

Substituting the above data into formula (2), we get E = Er-Ga-20lgle+Lf+6

  =15-0-(-7.6)+1+6

  =15+7.6+1+6

  =29.6dBμV/m

2. Field Strength Meter

    As we all know, the level meter uses decibel (dB) as the unit, such as dBμV, dBmV, dBm, while the voltmeter uses volt (V) as the unit, such as V, mV, μV, KV, etc. In fact, the level and voltage are the same physical quantity, so in many cases these two units are marked on the same instrument at the same time. In a sense, the voltmeter is also a level meter, and the level meter is also a level voltage meter, but it is customary to call them separately.

    As the name suggests, a field strength meter is an instrument for measuring field strength. The value of a field strength meter is in μV/m, and it contains a length unit m. In principle, the voltage value measured by a level meter (or voltmeter) is at the input port of the meter, while the voltage (or potential) measured by a field strength meter is the voltage induced by the antenna at a certain point in the air. Strictly speaking, a field strength meter consists of a level meter and an antenna.

    As for the current field strength on the market, they also separate the technical indicators of the level meter from the antenna. For example, the main unit of the ML524 field strength meter of Japan's Anritsu Company gives technical indicators according to a level meter, including frequency range, sensitivity, level measurement range, and level test accuracy, while the antennas MP534A and MP666A are optional, giving technical indicators and antenna coefficients according to frequency bands. The PTK3201 field strength meter produced in South Korea, which is currently commonly used in the domestic wireless field, also gives indicators according to the level meter. The frequency range is 0.1 to 2000MHz, and the sensitivity is 0.3mV, which are all given by the instrument input port. It has a whip antenna and no antenna coefficient. It can only qualitatively measure the signal field strength. If you want to measure the dBμV/m field strength, you need to select a measuring antenna.

    It can be seen that there is a big difference between level and field strength, level meter and field strength meter. However, they are often confused in some occasions, especially in the cable TV industry in my country. The cable TV signal is measured by measuring the level of the synchronization head, with dBμV as the unit. It should be called a TV signal level meter, or a level meter. However, since China started to use public antennas, it has been called a field strength meter for some reason. The world's earliest cable TV field strength meter is LFC944 produced by Japan's Lida Company. Lida's original name is TV signal level meter. Since then, many people have called it a field strength meter. The author has repeatedly proposed in the CATV industry that it should be called a level meter. Perhaps it is called too many people, and it is still called a field strength meter. Even foreigners have to accept this name when communicating in China. It can only be a mistake. Regarding the "misnamed" field strength meter in the cable TV range, the author has classified it into TV field strength meter, TV spectrum image field strength meter, CATV analyzer, and spectrum analyzer. It has been recognized by peers and experts and will not be elaborated here.

3. Spectrum Analyzer and Field Strength Meter

   As for the field strength meter, it is closely related to the antenna. If a certain measurement accuracy is required, then from formula (1), it is known that it is directly related to the antenna gain Ga, and then to the working frequency range of the antenna. This is the minimum requirement, so you can't just find an antenna and connect it to the level meter. Therefore, in practice, this kind of antenna is called a test antenna, which has strict technical indicators, such as frequency range, antenna gain, impedance, standing wave ratio, front-to-back ratio, etc. In order to adapt to its frequency range, its shape is very different, including whip antenna, half-wave dipole antenna, logarithmic periodic antenna, loop antenna, etc. The test antenna with high requirements is also quite expensive. For example, the test antenna of Anritsu in Japan is about 1/4 of the host.

    In the past, field strength meters always came with antennas, which means that a field strength meter must be a host (level meter) with an antenna. With the development of electronic technology and electronic measurement technology, especially since the 1980s, spectrum analyzers have been used in large quantities, and single field strength meters have become less and less common, and even single level meters have become less and less common (unless they are used for very professional purposes), because their functions can be replaced by spectrum analyzers. In principle, the basic principle blocks of spectrum analyzers, level meters, and field strength meters (hosts) are the same. The spectrum analyzer itself measures the signal level within the spectrum range. If "zero transconductance" is used, it is a frequency-selective level meter. If a standard test antenna is added to the spectrum analyzer, can't it measure the field strength? A better spectrum analyzer can store the antenna coefficient in the machine, and directly display the field strength value μV/m when in use. For example, most spectrum analyzers of Agilent and Anritsu have the function of storing antenna coefficients.

    As for Japan Anritsu, the sales of field strength meters ML518, HM650, and MH649 produced in the 1980s were relatively large, and China also bought a lot of them. The sales of field strength meters of the ML521/214 series produced in the late 1980s dropped significantly because it produced spectrum analyzers such as MS610 and MS2601 at the same time. Field strength meters and spectrum analyzers can use the same antennas such as MP534, MP651, and MP663 to measure field strength. Their indicators are similar and their prices are similar. Users will of course choose the powerful spectrum analyzer. At present, there are few field strength meters produced in China, and the main reason is that they are replaced by spectrum analyzers.

   The field strength can be measured by using a spectrum analyzer with a test antenna. If the spectrum analyzer can store antenna coefficients, the field strength in μV/m can be directly displayed. If the spectrum analyzer cannot store antenna coefficients, the antenna coefficients must be substituted into the above formula (2) for calculation. If a general antenna for receiving signals without antenna coefficients is used, the field strength can only be measured in space, but the field strength value in μV/m cannot be obtained. In other words, only qualitative test analysis can be performed, not quantitative test analysis.

   In summary, in field strength measurement, the result should be in μV/m. However, due to the development of electronic technology and electronic measurement technology, there are very few single-function field strength meters. Spectrum analyzers are most commonly used. Test antennas should also be selected for more rigorous measurements.