Determining range of a RF communication system

In any wireless project the range of operation between the transmitter and the receiver is the most important performance parameter.  It is well known among the wireless engineers that range of any wireless communication system is dependent upon the transmission power applied at the transmitter side and the efficiency of antenna used.  Like the typical 434 MHz RF modules when used with antenna and transmission power up to 18V have an extended range up to 300 metres compared to their normal range of 50 metres. In this series, configurable RF module are used to monitor performance parameters of RF systems by changing the design factors. With configurable design settings, the RF system can be used for maximum performance and altered to meet variable needs of a RF project. In this tutorial, the factors affecting the range of an RF link are examined and theoretically calculated range is verified in practice.

Let’s take two configurable RF transceivers. In this experiment two RF Pico boards housing Si-1062 wireless microcontroller are taken with an additional LED indicator circuit interfaced to the boards on breadboards. The LED indicator circuits along with the RF Pico boards are supplied 3V voltage from two 1.5 V batteries connected in series at both the transceivers. Both the transceivers operate within ISM band of frequencies. As setup in the previous tutorial, the frequency set for the RF link will be 868 MHz. The LED indicator circuit built on the breadboard is assembled according to the following schematics –

Both the programmable wireless transceivers are kept on a table with antennas removed, so that they can be operated in a comfortable space. A measurement tape is also placed on the table for measuring the range that can be achieved with different RF configurations.

Lets first determine the theoretical range of the RF system. The transmitter power and the losses that occur in the medium or communication channel are the major factors that determine the range of an RF link. If the RF signal strength is plotted on one axis of a graph, the variation in the signal power from the transmission point to the reception point typically have the variation according to the following graph – 

The transmitted power generated by the RF circuitry drops slightly as it reaches the transmitting antenna due to the connector losses. The transmitting antenna amplifies the power significantly and then sends to the transmitting medium. As the signal travels through the transmitting medium or the communication channel, it suffers huge losses in the signal power before it reaches the receiver antenna. The receiver antenna again amplifies the signal to a certain extent so that the receiver circuitry can successfully detect the signal. Before reaching the receiver circuitry the signal again loses power slightly due to connector or cable losses.

All the above mentioned factors need to be considered when calculating the range. The range of an RF communication system in theory is given by the following equation where the range is in meters –

R = (10^{(Pt + Gt + Gr – Pr – Lf + 60 ) / 20} ) / 41.88 * F

Where,

Pt = Transmission Power

Gt = Transmitting Antenna Gain

Gr = Receiver Antenna Gain

Pr = Receiver Sensitivity

Lf = Loss factor

F = frequency of the signal

The different factors influence the wireless range in the following manner –

1) Transmitter Power (Pt) : The transmitter power is the strength with which the radio signals originates from the RF circuitry. This is the power of the signal that can be measured before the antenna, antenna connectors and the matching circuit. The unit of the transmitter power that can be found in the specifications of wireless devices is dBm (Decibel milli-watt). Higher the transmitted power the signals can travel farther distance before it vanishes due to the loss in the medium.

2) Loss Factor (Ls): The loss occurs to a transmitted signal from the point where it originated at the RF circuitry of the transmitter till the point it has reached at RF circuitry of the receiver. It includes all the losses due to the medium of travel (normally air); losses occurred at connectors, cable etc. both at transmitter and receiver end. The Path loss is mentioned using the unit dB (Decibel)

3) Frequency (F):  The transmission frequency is an important parameter in the range calculation as the amount of Path Loss depends on the frequency of the transmitted signal, as the frequency increases more transmitter power is required to achieve a range possible with a lower frequency.

4) Receiver Sensitivity (Pr): The receiver sensitivity is the ability of the receiver to sense the incident radio signals. A receiver device is said to have higher sensitivity compared to another one when it can sense more feeble signals than the other one. It can also be said that it is the minimum power that a receiver can sense and hence its unit is same as the Transmitter Power but always have a value less than 0.

5) Antenna Gain (Gt, Gr): The antenna can be used in both the transmitter device and the receiver device and its job is to amplify the signals from the transmitting device before it suffers Loss and to amplify the feeble signals after suffering Losses, so that the receiver device can sense them. The amount of amplification that can be achieved on a signal by the antenna is called the Antenna Gain of it. The unit for Antenna Gain that is found in the antenna specifications is dBi (Decibel Isotropic).

When using a configurable RF transceiver having a wireless microcontroller, the design factors mentioned above can be programmatically controlled. The values for the various design factors can be set by the Wireless Development Suite for Si-1062 wireless microcontroller based RF Pico board. In this experiment, all the design factors will be kept constant except the transmission power. So the values of the design factors for both the transceivers will be set according to the following table – 

Keeping all the above parameters same the transmitter power is varied to find out the range that can be achieved for each transmitter power. Starting with a transmitter power (Pt) of 13 dBm, and a loss factor (Lf) of 82 for the devices not having antenna connected but have the matching circuitry and antenna connector; the range equation gives a value of 70 cm on substituting the values. Since there is no antenna connected the Antenna Gains can be taken as 0. On testing this 13 dBm transmitter power a range of 64 cm is found to be achievable.

On changing the transmitter power to 7 dBm and 4 dBm, the range is found to be 36 cm and 3 cm respectively in practice. 

From this experiment it can be concluded that the actual range can be achieved in real world situations if the antenna is connected on both the devices. When Antenna is connected, the Loss Factor reduces to a value less than 3 and the Antenna Gain for the 868 Mhz frequency antennas (Gt and Gr) can be substituted by 5 dBi (as per the specifications of antenna used for testing actual range). The actual range in open space LOS is found to be around 600 meters with the given RF parameters for maximum transmission power of 13 dBm. The range achieved with different transmitting powers keeping all other parameters same without use of antenna are as follow –

So it can be seen from the experiment that on increasing the transmission power, the range of the RF link is also increased. In the next  tutorial, the dependency between range and the data rate will be examined. 

Project Source Code

Project Source Code

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