RF Switch with Pin Diode

This PIN diode rf switch is the ideal antenna switch for VHF and UHF and work with the PIN diodes which are special high frequency switching diodes with very low internal capacitance. The internal resistance of a PIN diode can be remotely variated from 1 to 10.000 Ω by a DC control voltage. The resistance of a PIN diode changes linearly in relation to the current flowing through it. The diode can be used to weaken or totally shut off RF signals and to amplitude modulate a carrier signal.

In this rf switch, the PIN diodes are used to shut on of off several antennas. The current source is a 12 volt circuit composed of a transformer, bridge rectifier and a regulator IC. In order to know which antenna is switched on, 4 LEDs are incorporated into the circuit.

In constructing this rf switch circuit, all wirings must be as short as possible to avoid picking up interferences. Coils L1…L5 are 2 turns magnet wire (1µH) around a ferrite core for VHF range. For UHF range, the coil must be 5 turns (5µH).

The PIN diode switch can be used for antennas with impedances of from 50 Ω up to 75 Ω. The operated diodes block the “disconnected antennas” by about 30 dB. Signal losses at the closed diode is negligible. The signal to noise ration of the receiver deteriorates somewhat up to 1 dB.

RF switch with PIN diodes circuit diagram