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.

Antenna gains

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.

A very common antenna used in these links is a semi-parabolic die-cast grid.  Pacific Wireless makes these antennas for both 900 MHz (model GD9-DC15 ) and 2.4 GHz (model DC24-HD ).

These two antennas have the same exact dimensions –  42 x 24 inches (107 x 61 cm) – but very different gains: 

900 MHz: 15 dBi
2.4 GHz:   24 dBi

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 Attenuation

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.

It is difficult to quantify the attenuation due to trees in the radio path.  For very long distance links we strongly recommend that you elevate the antennas so that you clear all obstructions.  This is where 2.4 GHz has an additional advantage as explained in the next section.

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
2.4 GHz:    23 meters

The result is that at 900 MHz you would need to elevate the antennas an additional 15 meters (for a 50 Km link) on both sides to clear the ground, or an obstruction, at mid-point.  You may use our Fresnel Zone Calculator to compute the required antenna heights under different conditions.

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.

Conclusions

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 .