To derive information about the R.F. impedance (Zt) presented at a port (P) of an electrical network including a source of R.F. signals (for example, an antenna and an antenna tuning unit) without requiring an additional source of R.F. signals, a circuit is coupled to the port (P) successively in two different states. This circuit comprises auxiliary network means (N 1 , N 2 ) and radio indicating means such as a radio receiver (Rx); in the two states, it presents at the port (P) different respective impedances (Z 1 , Z 2 ) and has between the port (P) and the receiver (Rx) different gains (A 1 , A 2 ), the values being selected so that there is no difference between the amplitudes and/or phases of the signals reaching the receiver (Rx) when said R.F. impedance (Zr) lies on a respective locus in the complex impedance plane. The loci may for example be defined so as to intersect at a point representing an impedance with which it is desired to equate said R.F. impedance (Zt). By switching the circuit alternately to one state and the other, amplitude demodulation in the receiver (Rx) can be used to indicate any differences in amplitude and phase. The auxiliary network means may be a hybrid transformer (HT), one port (4) ofwhich is switched between an open-circuit and a short-circuit termination.