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As a theoretical rule of thumb, a good decoupling requires a distance of 5 to 10 λ.1)
However, in practive often a distance of 3 λ is sufficient.
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λ = v/f with v∼3×108 m/s.
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At 2.1 GHz (e.g. UMTS) the formula returns 300'000'000/2'100'000'000 = 300/2100 = 0.14m.
Three λ will then be 0.42m i.e. 42cm.
For GSM-R Deutsche Bahn requires a decoupling of up to -60 dBm. According to a simulation of one of our antenna supplier a distance of 350 cm is sufficant to fulfill this requirements
Source: entkopplung_gsm-r_-_v02.pdf