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--- faq:freedom-from-feedback [2015/04/05 17:00] – hess
+++ faq:freedom-from-feedback [2015/04/21 16:21] – hess
@@ Line 1: / Line 1: @@
 ====== Freedom from Feedback ======
-Deutsche Bahn requires a decoupling of up to -60 dBm. According to a simulation of one of our antenna supplier a distanec of 350 cm is sufficant to fulfill this requirements
+As a theoretical rule of thumb, a good decoupling requires a distance of 5 to 10 λ. (([λ] Lambda = Wave length))
+In practive, in most cases a distance of 3 λ is sufficient. \\
+\\
+<html>
+&lambda; = v/f with v∼3×10<sup>8</sup> m/s.
+</html>
+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: {{:faq:entkopplung_gsm-r_-_v02.pdf|}}