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Technical Trivia by Dr. FB

About the Fresnel zone of the SHF band

Dr. FB

5G and 5 GHz

5G is in the limelight in TV and newspaper reports. This 5G does not mean 5 GHz, which represents frequency. 5G is an abbreviation for Fifth (5th) Generation. In short, starting with the first-generation mobile communication system, 5G means the fifth mobile communication system. Since 5G enables high-capacity high-speed communication, its application is expected used in in various fields.

Since Dr. FB is an amateur radio operator, I am more concerned about radio waves than 5G applications. With the advent of 5G products, I think we will be in an era where it is relatively easy to obtain Super High Frequency (SHF) band components for consumer products. It may be an era where it is relatively easy to go up from 1200 MHz to 2400 MHz and even 5600 MHz, just as it went up from 430 MHz to 1200 MHz a long time ago. Since 5G uses the SHF band, I am also interested in its radio wave propagation, and this time I will explain the phenomenon called the Fresnel zone, that Dr. FB experienced in 2400 MHz operation more than 10 years ago.

Operation on 2400MHz

Thirty years ago, Icom released the IC-970, a VHF and UHF all-mode transceiver. The main body itself is a fixed station radio covering the VHF and UHF bands, but if you install optional units, you can operate up to 2400 MHz as well as 1200 MHz at that time, and I remember being excited as if many amateurs entered an unknown world.

Operation on 2400 MHz is extremely difficult because the number of stations that operate is limited, so it is difficult to find a communication partner station. This is because using a transverter was the mainstream of operation, and it was quite difficult to make an SHF radio by ourselves. I talked with a local station and made a schedule. The local station operator told me “In order to transmit radio waves in the SHF band, it is not enough to just raise the antenna. It is important to raise the antenna as much as possible to earn a wide range of visibility areas.” I thought that there is no problem because I would be operating line-of-sight with the other station. But when I tried raising the antenna height by about 2 meters, I remember that the signals reaching the other station suddenly became stronger.

About Fresnel zone

It is an unfamiliar word, but it is especially important for radio wave propagation in the UHF and SHF bands above the VHF band. Due to the characteristics of propagation, radio waves in the VHF band and above require a line-of-sight path between the two points of communication to achieve stable communications. "Good visibility between two communication stations" in these bands is not same as "simply see each other's antennas physically." As shown in Figure 1, it means that the space required for radio waves to propagate is unobstructed.

Although invisible to the eye, the radio waves radiated from the antenna do not travel in a single line between the two points, but spread from the antenna as an elongated rugby ball shape, as shown in Figure 1. The three-dimensional space drawn by this ellipse is called the Fresnel zone.

Figure 1. Line-of-sight communication with no obstacles between two points

Stable communication can be expected if this Fresnel zone is unobstructed between the two points. On the contrary, even if the antennas are line-of-sight, as shown in Figure 2, if the Fresnel zone is blocked by an obstacle, the communication quality may deteriorate.

Figure 2. Fresnel zone is not sufficiently secured

Theoretically, if 60% or more of the radius of the Fresnel zone is unobstructed, the quality of radio communications does not seem to deteriorate significantly compared to when the Fresnel zone is completely without obstacles.

Formula for finding the radius of the Fresnel zone

If there are obstacles such as mountains, buildings, trees, etc. between the two points of communication, it is necessary to clear the distance between the apex of the obstacle and the line-of-sight at least the radius of the Fresnel zone, hereinafter referred to as the Fresnel radius. Even if there are no obstructions, the distance between the ground surface and the line-of-sight must be equal to or greater than the Fresnel radius. By finding the Fresnel radius, you can find the height of the antenna for stable communication.

Figure 3. Find the Fresnel radius

Example calculation of finding the Fresnel radius in 5600 MHz

Amateur radio is assigned SHF bands such as 5600 MHz band, 10 GHz band, 24 GHz band, and so on. If a 5600 MHz antenna is installed, I will try to find the Fresnel radius at the midpoint when the distance between the two points is 5 km, assuming that the line-of-sight communication is unobstructed.

Using the formula, the ideal antenna height was calculated to be 8.2 m or higher. According to the theory so far, even if 40 % of the Fresnel radius hits an obstacle, it does not affect communication’s quality so much. If you secure a Fresnel zone of 4.9 m (= 8.2 x 0.6) as the minimum height, you can say that it is not necessary to raise the antenna to a height of 8.2 m. However, it is essential to have a line of sight path. If the height of the antenna is gradually lowered from 4.9 m, it is expected that the Fresnel zone will hang on the ground and the communication quality will deteriorate. (Figure 4)

Figure 4. Fresnel radius when operating on 5600 MHz for a distance of 5 km between two points


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