Each radio frequency band has its good and bad features. At WIN, we selected 900 MHz over 2.4 GHz as a practical communication frequency band for the following 7 main reasons:
To demonstrate the basic difference in wave propagation of 900 MHz and 2.4 GHz waves, a quick look at path loss is provided.
As waves propagate out from the transmitter, some attenuation of the signal takes place due to properties of the medium (air in most cases).
Path loss describes this attenuation as a function of the wavelength of the operating frequency and the distance between the transmitter and receiver.
Path loss is derived from the Friis transmission equation and is defined as:
Path Loss = 20 log(4*p*r/λ) dB
where r is the distance between the transmitter and receiver, and λ is the wavelength.The table below shows how path loss differs between 900 MHz
transmitters (λ=0.33meters) and the 2.4 GHz transmitters (λ=0.125 meters).
NOTE: Path loss analysis does not account for effects such as differing TX power outputsand RX sensitivities. See the "Range of 9XStream (900 MHz) and 24XStream (2.4GHz)Modules" section at the bottom of this page for more detailed range information.
|R = 10 Meters||R = 1000 Meters|
|51.527 dB||91.527 dB|
|60.046 dB||100.046 dB|
A link budget analysis can mathematically predict the system range based on the power output, receiver sensitivity, antenna gains, path loss, and fading margin. The path loss equation represents path loss (signal attenuation) as a function of distance between the receiver and transmitter and the wavelength of the operating frequency. This equation is derived from the Friis transmission equation and is given by: Path Loss = 20* log(4*π*r/λ) dB (Eq. 1) where r = distance between transmitter and receiver λ = wavelength The Friis transmission equation can be used to represent the path loss as the sum of the other system factors leading to the following equation: Path Loss = P(t) + G(t) + G(r) - R(s) - F(s) dB (Eq. 2), where P(t) = transmitted power G(t) = gain of transmit antenna G(r) = gain of receive antenna R(s) = sensitivity of receiver F(s) = fading margin, (experimentally determined to be 22dBm)
These two equations can be used to compare the maximum range of the 9XStream and 24XStream RF modules
* For the complete article 900 MHz vs. 2.4 GHz by Digi visit: Digi.com Frequency Comparison article