Radiation Characteristics of Short Unterminated Transmission Lines

A precise theory is presented for computing the maximum allowable length of an unterminated transmission line and the terminations effect on radiated emissions prediction. This theory is presented to supplant the general "rule of thumb" guidelines that have been used for printed circuit board traces, twisted pair leads and coaxial cables. When transmissions lines are not properly terminated their radiation is difficult to predict. In this theory, an unterminated transmission line is modeled as a monopole antenna in order to predict radiated emissions. The length at which the models radiation gradually transitions from linear to non-linear performance is theoretically derived and experimentally verified. Measured radiated emissions data was collected in a GTEM to verify the models performance for twisted pair, coaxial cables and unbalanced and balanced printed circuit board traces. The results are presented and discussed.