Television signal strength measurement and airborne management

What kind of TV signals are there in the air in a region and what is their field strength is a topic of concern to the radio and television management department and the radio and television commission. This article will discuss this from the perspective of electronic measurement.

1. TV signal and its field strength

As we all know, TV signals modulate video and audio signals on carrier signals in the range of 42-860MHz. To analyze these signal characteristics is the subject of video analysis and audio analysis, but in terms of the strength of the signal, the level of the horizontal synchronization signal of the TV signal is used to characterize it. Since the horizontal synchronization signal is pulsed, the peak value is used as a measure.
The author has explained the field strength measurement and field strength meter in another article. Please refer to "Field Strength Measurement and Field Strength Meter" on the Haoge website www.szheg.com.

There are two points that need to be emphasized here:
1. Field strength is the size of the electrical signal induced by an antenna of unit length at a certain point in the air. Its unit is microvolt/meter (μV/m), not voltage or level units V, μV or dB μV, dBm.

2. The composition of the field strength meter is a level meter (or spectrum analyzer) plus a test antenna. The field strength meter in my country's cable TV industry generally refers to the TV signal level meter, which is measured by peak value.

2. Measurement of TV Signals

The measurement principle of the field strength of TV signals in the air is described by the author in the article Field Strength Measurement and Field Strength Meter. The measurement box is shown in Figure 1. In view of the characteristics of the field strength of TV signals, the level meter should be a TV signal level meter (commonly known as a TV field strength meter), and the antenna should be a measurement antenna within the TV band. The field strength value E is obtained by the following formula (1).

E=E r＋ K + Lf (dBμV/m) —--(1)
Where:
E is the field strength value (dBμV/m)
Er is the level value of the level meter input port (dBμV)
K is the antenna factor (dB)
Lf is the connection cable loss value (dB)

Figure 1: Schematic diagram of the measurement of the field strength of a TV signal in the air

For a TV signal level meter, it is best to have a spectrum analyzer function in addition to measuring the level, so that the signal within the required frequency band can be scanned and observed in full view, and then the cursor can be used to lock the signal you need. However, in addition to TV signals, there are many other signals in the air, such as communication signals, radio signals, and other radio signals. Our task is to observe TV signals, so this instrument should also have a TV image display function. Once the signal search is locked, the image can be seen at the same time. This is to truly search for all TV signals within this frequency band and measure its field strength at the same time. It can also monitor which TV stations are under our jurisdiction and whether there are TV stations outside our jurisdiction.
For antennas, the frequency range of the antenna should be 46-860MHz, and the antenna coefficient K value should be given.

In this way, the field strength meter can be calculated by formula (1). Different antenna manufacturers give positive and negative antenna coefficients. Please pay attention when using them. For example, in this article, Anritsu is -K and domestic 900E is +K.
If the connecting cable is 1 to 2 meters long, it can be ignored. That is to say, the measured field strength is the level meter reading plus the antenna coefficient. That is, E=E r+ K -------- (2)
Figures 2 and 3 are antennas produced by Japan Anritsu. The antenna coefficients K are shown in Figures 4 and 5 respectively. Among them, MP534A is a half-wave oscillator type and MP666A is a logarithmic periodic type. Technical indicators are shown in Table 1.

 

Figure 2: MP534A half-wave dipole antenna Figure 3: MP666A log-periodic antenna

Figure 4: MP534A antenna factor K

Figure 5: MP666A antenna factor K

Figure 6 shows the domestic 900E antenna, of which 900EV is a half-wave dipole type with a frequency of 40 to 220 MHz; 900EU is a logarithmic periodic type with a frequency of 220 to 1000 MHz. The antenna coefficient is shown in Figure 7, and the technical indicators are shown in Table 1.

Figure 6: 900E measurement antenna

Figure 7: 900E antenna factor K

Table 1

model	MP534A	MP666A	900EV	900EU
Antenna Type	Half-wave oscillator	Logarithmic period	Half-wave oscillator	Logarithmic period
Frequency Range	25～520MHz	200～2000MHz	46～220MHz	220～1000 MHz
impedance	50Ω	50Ω	50Ω/75Ω	50Ω/75Ω
Connection type	N-type	N-type	N/BNC	N/BNC
Standing wave coefficient	＜2	＜2	<2.5	<2.5
Average gain	0 dB	5 dB	0 dB	5 dB

For example:

Use PRK3C+ color TV image spectrum field strength meter connected to 900E antenna. After PRK3C+ is turned on, it works in spectrum analysis mode and displays multiple signals. We can move the cursor and search for a signal at 320.25MHz. Then put the instrument in TV receiving mode. While receiving the image, we can also read the level value.

Assuming the level value is 68 dBμV
, check Figure 7. The horizontal axis is at 320.25MHz, and the K value of the vertical axis is 9 dB.
Therefore, the field strength is E=68 dBuV+9 dB=77 dBμV/m

3. Several issues in field strength measurement

1. Field strength measurement accuracy

To measure the field strength in the air, it is self-evident that the antenna induces an electrical signal at a certain point in the air. First of all, the antenna measurement is directional, so the antenna must be rotated to obtain its maximum value.
In terms of its test accuracy, the main sources of error are the instrument and the antenna.

Generally speaking, the error of the level meter is ± (1 ~ 3) dB. I suggest that ± 1.5 dB is appropriate, and ± 1 dB is better. However, we should not pursue high accuracy too much, because the standing wave has a great influence on the accuracy during the measurement process, and it is difficult to achieve an accuracy less than 1 dB. Some manufacturers mark the error below ± 1 dB. In fact, it is obtained by testing under specific test equipment and correcting the error. However, it is impossible to achieve that specific condition at the actual test site, so the actual measurement error will increase.

The antenna coefficient error is less than ±1dB.
In this way, the error of the level meter and the antenna together is ±2.5dB. In fact, there are many random factors, especially when measuring small signals, the noise and other interference are very large. I think it is reasonable to require the field strength measurement error to be 3-4dB.

2. Sensitivity of field strength measurement

An important issue in field strength measurement is sensitivity, that is, the field strength meter is required to measure the weak signal required in the air. For the field strength meter mentioned above, the key is the sensitivity of the level meter. Because no matter what kind of measurement antenna, it is generally impossible to make a high gain. For the TV signal in the air, we also have to have a basic estimate. Generally speaking, the TV can receive the image better at 60 dBμV or above, and can receive the TV image at 50 dBμV. If it is below 40 dBμV, it is difficult to receive, and 30 dBμV will not be able to see anything. This means that at the input port of the TV, if there is a signal below 30 dBμV, it is meaningless. From this, we can know that if the signal from the air field strength search to the level meter port should be measurable at 30 dBμV. In order to ensure the measurement is correct, we should choose a level meter with a sensitivity of 20 dBμV. Perhaps someone might ask whether it could be higher. At present, the sensitivity of TV signal field strength meters is generally between 20 and 30 dBμV. The sensitivity of wireless field strength meters and spectrum analyzers can be a few tenths of a microvolt (i.e., below 0 dBμV), but it cannot display images and cannot meet our proposed test items.

3. Over-the-air management of television signals

For the TV signal air management department, it is very necessary to know which TV signals are in my jurisdiction and how strong their field strength is, but it is not necessarily necessary to pursue high accuracy. In this sense, one of the best ways is to use the current TV field strength meter plus a measuring antenna; another way is to use the TV field strength meter plus an outdoor TV receiving antenna generally used on the market. The latter antenna does not have an antenna coefficient K and cannot correct the instrument readings, but it can also search for all TV signals and measure the level value of the instrument port, which can show the strength of these TV signals in a certain place or direction. We can call the former a quantitative test and the latter a qualitative test.

4. Selection of measuring instruments

In summary, the measurement of TV signal strength and airborne management has its particularity. First, the signal must be searched out and recognized as a TV signal, and its strength must be measured at the same time. The author calls this instrument a TV image spectrum field strength meter. How to choose the instrument and antenna depends on your requirements first, and then on the cost performance. Here are several solutions.

1. High-precision type - PRK7 The instrument has a frequency of 5 to 2050MHz, a dynamic range of 20 to 130dBμV, and an accuracy of ±1dB.
2. Applicable type - PRK3C+ (color image), PRK3+ The instrument has a frequency of 5 to 2050MHz, a dynamic range of 20 to 130dBμV, and an accuracy of ±1.5dB.
3. Economic type - MC377 The instrument has a frequency of 5 to 2050MHz, a dynamic range of 20 to 130dBμV, and an accuracy of ±3dB.
4. Most economical type - EP303 The instrument has a frequency of 46 to 860MHz, a dynamic range of 20 to 130dBμV, and an accuracy of ±2.5dB.

The choice of antenna is very clear. For high requirements, MP534A and MP666A produced by Japan Anritsu Co., Ltd. are selected. They are suitable for matching with PRK7 to obtain high-precision measurement. Generally speaking, the domestic 900E can be selected. In
summary, as a general TV signal air management measurement, as long as any of these instruments is selected and matched with an antenna, the TV signal field strength can be measured.

5. Automated Measurement

The above solutions all require reading on the level meter and then correcting with the antenna factor. It is best to store the antenna factor in the level meter and automatically correct it, so that the field strength value can be read immediately during measurement. The solution is shown in Figure 8.

The aforementioned PRK3C+, PRK3+, and PRK7 all have R232 interfaces. By installing the HEG008 automatic test software developed by Haoge Company in a laptop computer, the antenna coefficients can be stored, automatically corrected during measurement, and the field strength dBμV/m can be read and printed immediately.

Figure 8 Schematic diagram of automated field strength measurement

6. High-precision and high-sensitivity automated measurement

The above measurement solutions are based on practicality and have a good cost-effectiveness. For management departments with high requirements, they can spend a higher price to seek high accuracy and high sensitivity, and then they can use the spectrum analyzer solution, as shown in Figure 9.

Figure 9: High-precision and high-sensitivity automated measurement solution

This solution uses a high-level spectrum analyzer or CATV analyzer as a level meter, such as HP8591C or TEK2715 spectrum analyzer, which is also a CATV analyzer with a sensitivity below 1μV. The antenna also uses a high-precision antenna with good long-term stability. If the HEG-009 automatic test software is installed in the laptop, the TV signal field strength can be measured automatically and accurately at 0dBμV.

The measurement of TV signal strength and its management in the air is a rather complex subject and many issues need further study. Please refer to this article for discussion with the author.