When using (expensive) high power amplifiers the use of a RX/TX switching sequencer is highly recommendable. When switching from RX to TX, it switches the antenna relay first and then applies power to the amplifier. From TX to RX, the power is first removed from the amplifier and then the antenna relay is switched off.

12 V power is always applied. The DC-DC converter generates 24 V from 12 V. 24 V is usually enough to safely switch 28 V relais. Note that the diodes across the relay contacts (needed to cut off indution spikes) are not shown.

RX to TX: If PTT-signal (Ch 1) goes high (+12 V when TX) relay A switches first. The SMA relay now gets power thru contact a1 (Ch 2). About 3 ms after power is applied to the SMA relay it switches from RX to TX (Ch 3). Also relay B is powered now (thru contact a2) and, after another 4 ms (risetime of relay B), the amplifier is powered last thru contact b2 (Ch 4). The SMA relay gets power now thru contact a1 and b1.

TX to RX: If switched from TX to RX (PTT goes low, Ch 1) first relay A drops off, contacts a1 and a2 are opened, removing the PA-voltage (Ch 4). Relay B is not powered any more thru contact a2 and falls off. The antenna relais was still powered thru b1 and switches off after settling time of relay B (Ch 2), the TX path is switched over to RX finally (Ch 3), about 7.5 ms after power was removed from the amplifier.

This circuit is used at DF2CK since May 2003 without any problems. Delay times may vary depending on what kind of relays are used.