Calculate necessary clearance for the First Fresnel Zone

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Calculate necessary clearance for the First Fresnel Zone(FFZ) for given data: distance of the obstacle from transmitter dt = 20 Km, and the Rx dr = 30 Km measured along the LOS and freuency of transmission is FGHz = 3 GHz ?

Answer:

$$ r _{l} = 17.32 \sqrt{ \frac {d _{t} d _{r}} {(d _{r} + d _{t})f _{GHZ}} } $$ $$ = 17.32 \sqrt{ \frac {20 x 30} {(30 + 20) x 3} } $$ $$ = 17.32 \sqrt{ \frac {20 x 30} {50 x 3} } $$ $$ = 17.32 \sqrt{ 4 } $$ $$ = 17.32 x 2 = 34.64 m $$

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