4. AN ELECTRICALLY CONTROLLABLE, LINEAR REACTIVE NETWORK COMPRISING, FOUR VARACTORS HAVING INDENTICAL SQUARE LAW CHARACTERISTIC AND CONNECTED IN A BRIDGE CONFIGURATION WITH A PAIR OF INPUT TERMINALS LOCATED RESPECTIVELY AT THE FIRST AND THIRD JUNCTURES BETWEEN ADJACENT VARACTORS, AND A PAIR OF OUTPUT TERMINALS LOCATED RESPECTIVELY AT THE SECOND AND FOURTH JUNCTURES BETWEEN ADJACENT VARACTORS, A SOURCE OF DIRECT CURRENT CONTROL VOLTAGE, A BAND REJECTION FILTER CONNECTED IN SERIES WITH SAID SOURCE OF CONTROL VOLTAGE AND THE INPUT TERMINALS OF SAID BRIDGE CONFIGURATION AND HAVING A HIGH ADMITTANCE AT THE SECOND HARMONIC OF THE OPERATING FREQUENCY, AND MEANS CONNECTING THE OUTPUT TERMINALS OF SAID BRIDGE CONFIGURATION TO A UTILIZATION CIRCUIT.