900 MHz versus 2.4 GHz in long distance links
It is sometimes assumed that for a long distance radio link, using a radio operating at 900 MHz is better than at 2.4 GHz. This assumption is based on the fact that, for any given distance, the attenuation suffered by the radio waves (free space loss) increases with the operating frequency. However the free space loss is not the only factor that affects the link performance. This application note points out other factors and compares the operation of the link at the two frequencies.
Free space loss
The attenuation over distance favors 900 MHz over 2.4 GHz. At any given distance the free space loss at 2.4 GHz is 8.5 dB larger than at 900 MHz. You can use our RF Link Budget calculator to perform these and other related computations.
In order to cover long distances you will need high gain antennas. The gain of a reflector type antenna goes up as you increase the area of the parabolic surface. But for a given physical size, the antenna gain at 2.4 GHz is significantly higher than an antenna at 900 MHz.
These two antennas have the same exact dimensions - 42 x 24 inches (107 x 61 cm) - but very different gains:
In a point to point link where you use these antennas at both ends of the link, this difference counts twice. In a point-to-multipoint link with an omni antenna at one end the difference can only be counted once.
Therefore from an antenna gain perspective, 2.4 GHz has an advantage of 18 dB in a point to point link and 9 dB in a point to multipoint link.
Atmospheric gases (oxygen and water vapor), fog and rain can add to the free space loss attenuation and their effects are worst at 2.4 GHz. However the total attenuation is still fairly negligible and rarely becomes worst than 0.02 dB/Km. For a 50 Km link this translates to an additional attenuation of 1 dB.
Trees and other obstructions
Both frequencies need "line-of-sight" for proper and predictable
operation. However some type of obstructions are more detrimental to a
2.4 GHz link. Trees with leaves that have dimensions near the
wavelength of 2.4 GHz (but typically shorter than the wavelength of 900
MHz), will cause higher attenuation at 2.4 GHz.
Fresnel Zone clearance
In order to get "free space" propagation conditions you need to clear 60% of the first Fresnel zone . The Fresnel zone is a long ellipsoid between the two end points. The radius of a cross section of this ellipsoid is largest at the mid-point. For a 50 Km link for example, the radius of the 60% Fresnel ellipsoid at the midpoint is:
900 MHz: 38 meters
Effective Transmit Power Limitations
The FCC part 15 rules limits the Effective Transmit Power of transmitters in the ISM bands to 36 dBm. However, for a fixed point-to-point link, there is an exception in the 2.4 GHz band that does not apply to 900 MHz: in the 2.4 GHz band you are allowed, for example, to use an antenna of 24 dBi gain and a transmit power of 24 dBm for a total EIRP of 48 dBm. See our summary of the FCC part 15 rules for a table showing other combinations of antennas and transmit power that are allowed.
On very long distance links several factors contribute to the radio
link performance. Even though the free space loss at 900 MHz is lower
than at 2.4 GHz, when you consider the typical antenna gains and
antenna heights required to clear obstructions, a 2.4 GHz radio often
has the advantage. For a fixed point-to-point link, the FCC rules favor the 2.4 GHz allowing considerable larger transmit power which translates to increased distance.
For any given link we recommend that you perform the complete link path analysis to compare the performance of different radios at different frequencies. You can easily accomplish these computations with our RF Link Budget calculator .