How much distance does a radio transmitter cover?

How much distance does a radio transmitter cover?


Guide to predict how many kilometers cover an FM Transmitter associated with the antenna system

This post will cover a few key points:

  • Common questions from the customers
  • The formula of the ERP (effective radiated power)
  • How to choose the right connector and cable for transmitter and antenna
  • Attenuation of the Coaxial Cable
  • The distance covered with different height of the antenna system

Common questions from the customers

How to decide how much power I need for an FM transmitter?
What is the best antenna system and cable to use?
How much Power need a radio station to cover a determinate area?
I need an FM Transmitter to cover 90 kilometers.
Please quote to me a complete radio station to cover 150 kilometer
Please quote me a complete radio station for a community radio
We're in the process of starting a community broadcasting and would like to know how much to budget for such venture!
These are some common requests that we receive from our customers. They need a professional advice from industry experts to decide which option is best for them.
With the follow guide we will try to help them to answer this questions.

The most difficult to decide is the power range of the FM Transmitter and the Antenna System type

Most of the time the doubts are not related to Radio Studios.
In these cases it is easier, without being a specialist in the sector, to orient oneself on which type of equipment to buy and most of the times the choice depends exclusively on the budget available.

It is more difficult for the Transmitter System to decide the right power of the FM Transmitter and the type of Antenna to be used.

What follows is simplified guide with some advice, and data, it is a not exhaustive, but useful list of the factors that will determine the coverage or the distance, in kilometers, that can potentially reach the FM signal.

Factors that determine the coverage of an FM Transmission system

The analysis below is based on mathematical calculations, but in summary they demonstrate that the coverage of a transmitting system depends on the power of the transmitter, the antenna system, the height at which the antennas are mounted and the type of area to cover.

The best advice TEKO Broadcast can give you is to build the system with a good transmitter but don't overlook the type of the antenna and the height of the installation point.

With regard to the transmitter power, to make a prediction of the potential distance that it can reach, a memo-technical rule that we can apply is: if we need to double the distance covered we need to quadruple the power of the transmission system.
In other words distance for two, power for four.

There are many factors that determine how far will go the FM signal radiated by the transmitter/antenna system.

A good estimate can be reached by considering four of them:

  • The effective radiated power (ERP)
  • The antenna height
  • The shape of the terrain
  • The area to be covered: rural, urban or large town.

The first parameter we need to know is the ERP, that is the effective radiated power of the total transmitting system.

To calculate ERP you need to know the following factors:

  • The output power of the transmitter
  • The losses of the coaxial cable used to connect the transmitter to the antenna.
  • The length of the coaxial cable.
  • The type of antenna system: dipole vertical polarization, circular polarization, single antenna, systems with 2 or more antennas, etc.
  • The gain of the antenna system in dBb. The gain can be positive or negative.

The formula of the ERP is:

ERP = Transmitter power in Watt x 10^((Gain of the antenna system in dBb - looses of the cable) / 10)

Power of the FM Transmitter = 1000 Watt
Type of antenna = 4 bay dipole vertical polarization with a gain of 8 dBb
Type of cable = low looses 1/2”
Length of cable = 30 meters
Attenuation of the cable = 0,69dB
ERP = 1000W x 10^(8dB - 0,69dB)/10 = 3715W

So the system described on the formula would effectively provide aprox 3 times the transmitter power to 3715 Watts ERP with 152 kilometers coverage.
It must be paid attention, this is only a theoretical calc, to come inside the houses and pass through the obstacles it’s needed much more power to cover this distance.

How to choose the right connector and cable for transmitter and antenna

Alluminium Dipole Antenna
1 800W N RG213
2 1600W 7/16'' 1/2''
4 3200W 7/8'' 7/8''
Stainless Steel Dipole Antenna with Connector 7/16''
1 2000W 7/16'' 1/2''
2 4000W 7/8'' 7/8''
4 8000W 7/8'' 7/8''

Attenuation of the Coaxial Cable

What follows is a table that describes the typical attenuation and the max power allowed of the different Coax Cables typically used

Attenuation at 100MHz Attenuation db/100m Attenuation db/30m Maximum Averange Power Rating
RG213 6,2 1,86 1000
RG214 7,5 2,25 800
RG218 3 0,9 3500
CLX 1/4" (Foam) 4,5 1,35 1500
CLX 1/2 (Foam) 2,3 0,69 3400
CLX 7/8 (Foam) 1,2 0,36 7500
CLX 1 5/8 (Foam) 0,7 0,21 15000

The total power radiated is necessary to win the attenuation of the air on the free space call “Free space attenuation in dB”.
The attenuation of the air on free space is 72,4dB x 1 kilometer.
This attenuation increase of 6 dB each time the distance double, so, for 2km is 78,4dB, for 4km 84,4dB and so on.
It means we have loose 4 times the power each time the distance double.

Table of Free-space attenuation at 100MHz

Free-space attenuation (dB) at 100MHz
Distance (Km) Attenuation (dB)
1 72,4
2 78,4
3 81,9
4 84,4
5 86,6
6 88
7 89,3
8 90,5
9 91,5
10 92,4
15 95,9
20 98,4
25 100,4
30 101,9
35 103,3
40 103,4
50 106,4
100 112,8

Once we know the ERP there are many other factors to consider:

  • The height of the antenna above the area to cover. A way to understand it point is imagine how far the transmitting antenna can effectively see. If you stand in the same point the antenna is mounted and look out with a pair of binoculars, wherever you can see it is possible to transmit to. This can be 5 or 6 kilometers if you are stand up on a flat terrain or up to 30 or 40 kilometer if you are on a mountain top.
  • The height of trees in the area around the antenna
  • The height of buildings around the antenna
  • The type of terrain flat or hills
  • The sensitivity of the receiver
  • The near frequency stations or other radio stations broadcasting in the same frequency, for example the antenna may be able to see 20 kilometer, but if another station is on the same frequency 20 kilometer away, it will block/interfere with the signal.

In general one of the most important points are the topography of the terrain: hills, mountains, large buildings of the city, pass through the windows or walls to come inside the houses....

The C.C.I.R. says the minimum level of signal needed to have a good reception on the different areas is as follows

Rural areas = 48 dBμV

Urban areas = 60 dBμV

Large towns = 70 dBμV

The follows table illustrate the aprox coverage in Kilometers of a rural area with different values of ERP

Watts in ERP Kilometers Rural area
10 8
30 13
50 17
100 24
300 42
500 54
1000 76
2000 108
4000 152
6000 187
10000 241

The follows table illustrate, taking count of the terrestrial curve, the distance covered with different height of the antenna system.

Height (meters) Distance (km)
3 6
10 11
20 16
30 20
60 28
100 36
300 62
500 80
1000 113
2000 160
3000 196

The FM signal will propagate as far as there is optical visibility.

If you look at the horizon with binoculars, the maximum distance we can look at is called the "Line of sight".
The FM signal does not go beyond this distance.
For this reason the height of the antenna is so crucial.

Example of coverage of same FM Transmission sytem installed at different height levels

You could have two different radio station using a 1000 Watt FM transmitter: one of them with a 30 meter tower in a flat terrain will cover 20 kilometer while the other with the antenna on a 500 meters hill will cover 80 kilometer.
I must be known that it can be only estimate the range and can not guarantee results until after a given system has been tested in real practice.

In conclusion, to know the effective coverage of a transmission system we must assume all this factors but mainly the Effective Radiated Power, the Antenna hight from the terrain and the type of area to cover: Rural, Urban, Towns or Large Towns

If the antenna can see 20 kilometer, but 10 Watt ERP are used, it's provably that no more than 8 kilometers will be covered because there is not enough power to propagate the signal 20 kilometer.
If 100 Watt ERP is used, it's very likely that 20 kilometer of range will be reached because 100 Watt ERP is able to propagate a strong signal 20 kilometer.
If 1000 Watts of ERP power is used, it is very provably the signal will reach 20 kilometer, but moreover it will helps to penetrate eventual obstacles.

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