Path Loss at VHF, UHF and Microwave frequencies
A free Space graphical representation

From Chris VK3CAE

The following nomogram is a graphical solution of the free space path attenuation between isotropic antennas. It can be found through practical experiments that these figures may be scaled to real world systems where line of site transmission is the case in hand.

Non line of site situations are far more complex, and several variables must be considered such as the terrain and curvature of the earth, for example..

With relatively flat ground and the absence of buildings between each antenna site, we would need a base height of at least 9 metres (29") to obtain line of site transmission over a 20 km path. Other factors such as ground reflections and the possibility of fading due to phase relation are also variables but lets just keep things simple for this exercise.

 

 

Note three additional unbroken lines extend from the 20 km point and cross the attenuation scale to intersect with frequencies of 145, 435 and 1295 MHz.  From this data we can see that you would expect a path loss of at least 102 dB at 145 MHz for the given distance of 20 km. You can also see that at 435 MHz a further 10 dB loss would be expected for the same path distance (112 dB) and at 1250 MHz an extra 20 dB in path loss (122 dB). To find the total signal loss, any additional antenna gain (in dBi) at both the transmission and reception points is subtracted from the given attenuation figure.

The free space attenuation Nomogram and other data such as suggested antenna heights for line of site operation can be found in REFERENCE DATA FOR RADIO ENGINEERS (fifth edition).
 

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Chris VK3JEG 1999