Hengaode analytical field strength meter field strength measurement and differentiation
The field strength meter, as the name suggests, is an instrument for measuring field strength. The value of the field strength meter is μV / m, and it has a length unit m. In principle, the voltage value measured by the level meter (or voltmeter) is at the input port of the meter, and the voltage (or potential) measured by the field strength meter is the voltage induced by the antenna at a certain point in the air. Strictly speaking, the field strength meter is composed of a level meter and an antenna.
A brief explanation of field strength
Field strength is the abbreviation of electric field strength. It is the size of the electrical signal induced by the antenna at a certain point in space to characterize the electric field strength at that point. The unit is microvolts / meter (μv / m). For convenience, dBμv / m (0dB = 1μv) is also useful.
Field strength measurement
The field strength is measured as shown. The maximum induction signal is obtained when the antenna is in the sky with the same polarization direction as the signal under test. Generally, it can be measured with a radio frequency (RF) rms level meter (voltmeter).
When the line matching is good, the level value read by the meter is the radio frequency voltage Er (dBμv) obtained by the meter input port (generally 50Ω or 75Ω). Er can be expressed by the following formula
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 gain of the receiving antenna (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) of the open port converted from the termination value.
The electric field strength E (dBμV / m) can be obtained from equation (1), namely:
E = Er-Ga-20lgle + Lf + 6 --- (2)
Specific examples are now given: set the test frequency 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 selects the attenuation 10dB / 100m type cable, the attenuation is 1dB when the practical length is 10m; the indicator indicates the level is 15dBμV.
When substituting the above data into equation (2), we get E = Er-Ga-20lgle + Lf + 6
= 15-0-(-7.6) + 1 + 6
= 15 + 7.6 + 1 + 6
= 29.6dBμV / m
distinguish
As we all know, the level meter is in units of decibel (dB), such as dBμV, dBmV, dBm, and the voltmeter is in units of volts (V), such as V, mV, μV, KV, etc. In fact, the level and voltage are the same physical quantity. Therefore, in many cases, these two units are marked at the same time in an instrument. In a sense, the voltmeter is also a level meter, and the level meter is also a level voltage. The watch is just used to call it separately.
In terms of field strength on the market, they also separate the technical indicators of the level meter from the antenna. For example, Japan Anritsu ’s ML524 field strength meter host provides technical indicators according to a level meter, frequency range, sensitivity, level measurement range, level test accuracy, and antennas MP534A, MP666A as options, technical indicators are given by frequency band And antenna coefficients. At present, the PTK3201 field strength meter commonly used in the domestic wireless field is produced according to the level meter. The frequency range is 0.1 ~ 2000MHz, and the sensitivity is 0.3mV. It is given by the instrument input port. There is a whip antenna, no The antenna coefficient can only measure the signal field strength qualitatively. If you want to determine the field strength in dBμV / m, you need to choose a measurement antenna.
This shows that the level and field strength, level meter and field strength meter are very different. However, it is often confused in some occasions, especially in the scope of China's cable TV industry. The cable TV signal is measured by measuring the level of the sync head in dBμV. It should be called a TV signal level meter, or level meter. However, starting with the use of public antennas in China, somehow they are called field strength meters. The earliest cable field strength meter in the world is LFC944 produced by Japan's Lida company. Lida's original text is a TV signal level meter. Since then, many people have called it a field strength meter. I have been in the CATV industry many times. It was suggested that it should be called a level meter, perhaps because there are too many people called it, and it is still called a field strength meter. Even foreigners have to recognize this name when they communicate in China. That is only wrong by mistake. Regarding the "misnamed" field strength meter in the scope of cable TV, the author has classified it as a TV field strength meter, a TV spectrum image field strength meter, a CATV analyzer, and a spectrum analyzer, which have been recognized by peers and experts. Recount
Features
Field strength meter
As far as the field strength meter is concerned, it is very closely related to the antenna. If a certain measurement accuracy is required, then from equation (1), it is directly related to the antenna gain Ga, and then to the antenna's operating frequency range, which is At the very least, it cannot be said that it is enough to just find an antenna and connect it to the level meter. Therefore, in practice, this kind of antenna is called a test antenna, and it has strict technical specifications, such as frequency range, antenna gain and impedance, standing wave ratio, front-to-back ratio, and so on. In order to adapt to its frequency range, its shape is very different, there are whip antenna, half-wave dipole antenna, log period 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 main unit.
In the past, the field strength meter always supplied the antenna. That is to say, a field strength meter must be the host (level meter) equipped with the antenna. With the development of electronic technology and electronic measurement technology, especially since the 1980s, a large number of spectrum analyzers have been used, there are fewer and fewer single field strength meters, and even a single level meter is also less and less (Unless highly professional use), because its function can be replaced by a spectrum analyzer agent. In principle, the basic principle blocks of the spectrum analyzer, level meter, and field strength meter (host) are the same. The spectrum analyzer itself measures the signal level within the spectrum range. If "zero transconductance" is used, it is a frequency selection level meter. If the standard test antenna is added to the spectrum analyzer, is it not the measurable field strength? A better spectrum analyzer can store the antenna coefficient in the machine and directly display the field strength value μV / m when used. For example, Agilent and Anritsu spectrum analyzers have antenna coefficient storage function.
As far as Anritsu in Japan is concerned, the field strength meters ML518, HM650, MH649 produced in the 1980s have a relatively large sales volume, and China has also bought a lot of them. At the same time, spectrum analyzers MS610, MS2601, etc. were produced, and the field strength meter and spectrum analyzer can use the same antennas MP534, MP651, MP663, etc. to measure the field strength. They have similar indicators and similar prices. Of course, users choose powerful spectrum analyzers. My domestic field strength meter is also rarely produced. The main reason is also due to the replacement of the spectrum meter.
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