900 MHz vs. 2.4 GHz – A Practical Selection

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:

1. Longer distance – up to 5,298 feet line of site
2. Better penetration of walls and other structures
3. Less crowded communication frequency
4. Defined by IEEE 802.11 working group
5. License-exempt in North America
6. Provides channel-hopping to avoid interference

Since we provide products for the automation industry, we felt that 900 MHz was the simple, reliable, robust and most practical solution. Each frequency has its strong points so we do not say one is better than the other. For more details, as to what went into our selection, please read the technical information provided by Digi below.

900 MHz versus 2.4 GHz

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
900 MHz
51.527 dB	91.527 dB
71.527 dB
2.4 GHz
60.046 dB	100.046 dB
80.046 dB

Thus, the path loss is +8.519dB more over a given range for the 2.4 GHz modules. Since the range doubles with every 6 dB of reduced path loss, the 900 MHz modules have 2.67 times as much range as the 2.4 GHz modules [2^(8.519/6) = 2.67].

Range of 9XStream (900 MHz) and 24XStream (2.4 GHz) Modules

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

1. Consider the range of the 9XStream RF module:
   
   = 0.33 meters (for f=900 MHz)
   (Eq. 1) Path Loss = 113 dB = 20 * log(4*π*r/λ)
   (Eq. 2) Path Loss = 21dBm + 2dB + 2dB - (-110dBm) - 22dBm= 113 dB
   By setting these equal to each other, a little computation reveals that r=11848 meters, or a little over 7 miles.
2. Now consider the 24XStream RF module:
   = 0.125 meters (for f=2.4 GHz)
   (Eq. 1) Path Loss = 105 dB = 20 * log(4*π*r/λ)
   (Eq. 2) Link Budget = 18dBm + 2dB + 2dB - (-105dBm) - 22dBm = 105 dB
   Once again, setting these equations equal leads to r=1768 meters, or just over 1 mile.

From this example, it is shown that operating at 900 MHz exhibits a significantly longer range than is possible at 2.4 GHz.

* For the complete article 900 MHz vs. 2.4 GHz by Digi visit: Digi.com Frequency Comparison article