Steven Ring M0SVR delivered a technical talk on the theory and practical uses of radio transmission lines. This was followed by a practical session where simple home-made test equipment was used to measure the characteristic impedance (Zo) of several readily available types of cables. The cable test set-up used is illustrated in the picture. This exploits the use of a quarter-wave length of cable to transform the impedance at the right hand end down to 50 Ohms at the left hand end of the cable under test shown in the picture. A 50 Ohm impedance measurement bridge was used to measure the quality of the 50 Ohm match at the left hand end of the cable under test, whilst a 1k-Ohm variable resistive load (R-load) fitted to the right hand end of the cable is adjusted to achieve a good 50 Ohm match as measured on the bridge. The characteristic impedance (Zo) of the cable under test can then be calculated from the formula: Zo = Square-root(50 x R-load).

Graham
G0MGC