Practical procedure for calculating and constructing an VHF radio path

In this long and tedious article I will try to touch on the topic of calculating and constructing VHF radio paths from a practical point of view, based on my experience of similar work. This order does not claim to be a priori correct and unique, because it is possible for someone it is completely different, more or less thorny. I’ll immediately make a reservation that all radio paths organized using protocols, classes and families such as GSM, WiFi, WiMax, LTE, BWA, RRL, etc., can be attributed to VHF radio channels, as already mentioned in the first article .

So, somehow we needed a channel between two objects and we have a clear understanding that only a VHF radio path will save us. There are 3 different options for how to do this.

1 option.
We rent the necessary channel from a telecom operator that specializes in such activities. It is not a fact that this will be a radio channel, for they proceed from their considerations and resources. This is the easiest option, because it implies that we have only a "bribe" for the installation and rental of this channel. All other difficulties are solved by this telecom operator.

Option 2.
We order the desired radio channel from the contractor. Contractors, of course, are different. They can take on the entire spectrum of work on the organization of this radio channel, can take on only part of the work. Some, for example, are not ready to resolve issues with "supervisory and regulatory authorities." Their services differ, of course, at a price. This option differs from the first in that the radio channel will belong to us, someone will not be required to pay a rental. Well and, naturally, it will be necessary to exploit this channel in the future.

3 option.
We will organize the radio channel ourselves. We get the whole range of difficulties, needs and problems associated with this. We can throw some of them at someone to solve, we can do everything ourselves.

So, we decided to satisfy this need on our own. Where to begin?

1. To begin, no matter how strange and trite it sounds, it is necessary with the definition of these two points. You need to decide either with the exact address, or with specific coordinates on the map. It often happens that only the name of a settlement located in the immediate vicinity is known. Yandex maps , Google maps , of course, help out well here. Although getting the coordinates in them is not so obvious if the address is not known. You can use something else, such as Google Earth . Or some other resources, such as OziExplorer . You can take an ordinary topographic paper map and navigate it.

Once upon a time I was somewhat discouraged when I discovered scans on the Mitinsky market topographic maps of the General Staff of the RF Armed Forces 1: 100000 (1 cm - 1 km) with a shaded inscription “secret”.

The coordinates of points can be determined by the navigator, having arrived at the place.

Somehow, about 5 years ago, we worked together with the guys from the State Reserves Center . I noticed that in the reports they write the coordinates without seconds. A clear answer was given to my question: “We still have laws in force in our country, according to which, the possibility of determining your location with an accuracy exceeding 30 m is prosecuted”.

2. If you have decided on the points, it is worth counting the radio track itself, in order to determine the exact height of the suspension of the radio equipment. The easiest way is to visually determine the presence of visibility. If it is, you do not need to think about additional antenna mast devices. You can view the route by hitting one of the points and determining the direction (azimuth) of the other. You can use binoculars or a telescope, you can take a couple of shots with a camera with a good sensitivity matrix in the direction of another point, and then consider what happened. If visual visibility is limited by some kind of obstacles, weather, or a range of more than 2 km, the route must first be calculated. You can do this either with some kind of on-line calculator for calculating VHF radio paths, at one time manufacturers of radio equipment gave such an opportunity on their websites,this one , or manually. For manual calculation, you can use the following method of calculating the radio path (see Fig. 1):


a) Mark on the map the location points of the transmitting (A) and receiving (B) antenna-feeder devices (AFUs) and measure the distance:

AB = D

b) Draw on the graph the axis of rectangular coordinates:
the abscissa axis - the axis of distances X,
the ordinate axis - the axis of heights N.

c) Construct a "zero level arc" - an arc of the sea level, calculating the coordinates of the intermediate points H = N (X) according to the formula:

N (X) = (D² / 2R) * K * (1-K), km, where
D is the length of the VHF radio link, km;
R - radius of the Earth - 6371 km;
K = X / D - relative coordinate of the intermediate point, 0 X - distance to the intermediate point, km.

For a quick estimate of the maximum height of the “zero level arc”, that is, in the middle of the radio path, you can use the formula:

Nmax = D² / 51

For graphical construction of the "zero level arc" it is enough to calculate 6-8 points.

d) To remove the elevation points along the radio path from the height map, plot their heights in scale on the graph, counting from the “zero level arc” and connect the received points with a line that will be the profile of the radio path.

e) On the profile line of the radio path, on a scale selected in height, we apply local barriers (buildings, forest, etc.), and at the points of deployment of VHF radioelectronic means - the heights of their antenna-feeder devices (the height of the antenna equipment). Next, connect the upper points of the latter with a straight line. This will be the line of sight.

f) Find the minimum distance h between the line of sight and the highest point of the profile of the radio path. Determine its coordinate - the distance X to this point.

g) Calculate the clearance value h1 (the width of the first Fresnel zone) at which at the receiving point B the electromagnetic field strength is equal to the free field strength according to the formula:

h1 = ²√ [1/3 * D * λ * K * (1 - K)], where

h1 is the “clearance”,
D is the length of the radio path in m,
λ is the length of the working radio wave in m,
K = X / D - relative coordinate of the intermediate point, 0 X - distance to the obstacle point, in m.

g) Compare the obtained clearance values ​​h1 with the actual clearance on a given h path. Based on the results of this comparison, a decision is made on the suitability of the track for organizing a radio link in the VHF band.
If the condition h> h1 is met, the path is open.
If h1> h - the path is half-closed and the radio path should have good energy.
If h <0 - the track is closed, you should start searching for the best track, or increase the height of the AFU suspension.
In the conditions of rugged terrain, when the distance between the tracks is less than 25 km, you can limit yourself to calculating the clearance h1 by the formula:

h1 = {[(Ha + Hb) + (ha + hb)] / 2} - (hrep + Hmax), where

Ha, Hb are the elevations at the installation points of both AFUs, determined by the map, m;
ha, hb - the heights of the AFUs themselves (the height of the equipment suspension), m;
hrep - the greatest height of obstacles on the radio path, m;
Hmax - maximum height of the “zero level arc” in the middle of the radio path, m

3. With the points of placement, the heights of the equipment, the profile of the radio paths are determined. Now it’s worth deciding on the equipment and frequencies. This is one of the main parts of the cost. We need to understand the channel of what capacity we need. I will not dwell on this, because when choosing it is worth starting from the statements of the first article . In short, if you need high speeds and high batch performance, you will have to look towards the RRL. If the speed is less than 500 Mbit / s, then you should look in the direction of either the range of 70-86 GHz or broadband wireless access (BWA, the range of 5-7 GHz).

4. We decided on the equipment, frequencies, and everything else that preceded it. It would seem that everything, nothing more is needed. But no. It is necessary. And most often, what they did before seems to be small flowers. Now the most important thing begins - the design of the necessary radio frequencies. This is generally a separate issue. To begin with, it is worth ordering a study of the necessary radio path in the State Reserves Center in order to check your calculations, because the money is not big. But if you are confident in the correctness of your calculations, you should immediately proceed to the procedure for processing these frequencies. I won’t touch on this topic in detail, because it directly depends on the “information exchange” with the State Reserves Center , Ministry of Defense, RosKomNadzorand other regulatory authorities. In short, the State Reserves Center looks at the electromagnetic compatibility (EMC) of our equipment in a particular place and at a specific frequency, and coordinates the frequency with the Ministry of Defense. If the EMC is in order, the frequency is not busy, the military gave the go-ahead for its use, then we get a positive decision on the frequency. Then we pay a bribe and get permission from RosKomNadzor. If someone makes a “hang up” - we return to step 3. The costs of this item are another of the main parts of the costs. I will add that often, getting into this process, many refuse the idea of ​​organizing a radio channel on their own.
Sometimes, a malicious intruderanyone who realistically assesses risks skips this item. What threatens in this case? If we are caught on the “illegal” use of the state’s resource, that is, frequencies, then we will be fined (for the first time not very much) and they will be issued an order to eliminate the shortcomings. If we ignore, they will issue an order again. For the third time - the license will be taken from the operator and the iron will be dismantled. And when can they catch us by the hand? If someone complains or we fall under a routine check of the electro-magnetic situation of the Resource Center. Usually, they complain either of competitors who crossed the road, or a grandmother who “has a headache because of radiation”. How often does this happen or will it be at all? What will we lose if we get caught? Commercial risks, however.
Perhaps that is why mobile operators, including those from the Big Three, are considered “malicious” violators in the State Reserves Center. After all, the larger the network, the greater the very risks and lower the initial profitability.

5. Well, finally, there is permission and the desire to build has not disappeared. Now it is up to the project and its coordination with the regulator.
Often this item and the next are combined in time with item 4. However, then there is a risk of making a mistake with the frequencies.
Often the project is not done and not agreed, it all depends on the capacity of the radio channel, its type and the communication protocol used.

6. Well, finally, we got to the construction site. We’ve built it, everyone’s happy, and they’re throwing bonnets into the air. Now you need to tell the regulator the type of equipment, its mac address, operating frequency, suspension heights, power. In response, a notification about the registration of the route will come. If not built, then the permission for the frequency can be lost after some time. Now our radio path will be periodically checked at the State Reserves Center for compliance with the issued permits. Therefore, if someone mistakenly tuned in the radio equipment and forgot to turn off “adaptive frequency tuning for interference”, wait for the order and a fine. If you changed the mac address due to an accident, for example, then in theory you also need to inform the regulator.

To the uninitiated, this order may seem too cumbersome. So it is in my opinion. However, if you think about it, then this is approximately the whole activity of the Russian telecom.