The propagation constants of screened twin cables at frequencies of 200 Mc/s and above are usually measured by a resonance method, the resonating element being a screened air-spaced twin line. Such a system can also resonate in unbalanced modes, and unless it is truly symmetrical these modes are detected on the recording instrument, causing errors in the results obtained for the balanced system. This trouble is avoided by(a) arranging that the bridge, which is moved along the line to bring the balanced system into resonance does not tune the unbalanced one,(b) artificially balancing the system to compensate for any lack of symmetry, or(c) choosing the conditions of measurement such that the unbalanced systems are heavily damped and have a negligible effect on the results.Measurements made with the balanced measuring line will usually detect any lack of symmetry in the cable sample, but will give no quantitative measure of its extent. This is therefore obtained directly by treating the cable as three unbalanced systems, which are measured separately on an unbalanced coaxial-line measuring equipment. The conditions of measurement are so chosen that when the system being measured is in resonance the others are in anti-resonance, and thus have the minimum effect on the results. The results give a direct measure of the balance of the cable and the propagation constants of the unbalanced systems. The constants for the balanced system can be derived from them.Measurements are made in this way on a number of twin cables in the preferred list of the Inter-Services R.F. Cables Technical Committee; good agreement is in general obtained between the results derived from three unbalanced measurements, those obtained from the balanced measuring line and those calculated from the standard formulae.