EME S1 SEQUENCER

The purpose of relay sequencing is to protect associated circuits like low noise preamps and expensive hi power RF relays during the transition time between transmit and receive. The S1 is a full featured sequencer. The S1 sequencer accomplishes this protection by by opening and closing relays so the signal path is well established before and after the transmitter / amplifier power is allowed to flow to the antenna.

The transmit command usually starts in your HF or VHF transceiver. If the transceiver is an HF unit then it will be followed by a transverter. The S1 can accept either a key to ground command or a 2-12 VDC transmit bus command.

The first thing you want to protect is your low noise preamp. The sequencer first activates the RF isolation relay to remove the preamp from the circuit. Then the high power RF antenna relay is closed connecting the antenna to the amplifier and perhaps also removing cutoff bias from the amp. If there is a transverter it is the last to be keyed. Drive from the transceiver or transverter can now be amplified and sent to the antenna. When the transceiver is un-keyed, the relays operate in reverse order and with the same 30 ms time delay between relay openings, In the event that you have a relay that opens when keyed, the S1 sequencer can be modified with a jumper to maintain the correct sequence function.

The sequencer can also key two or relays simultaneously even if they are using different dc voltages. Diode isolation is provided for at each of the 4 outputs. The keying transistors can handle in excess of 40 volts and up to 800 ma each.

One more function has become necessary recently with the use of dual polarized antennas. Generally this capability if referred to a polarity diversity and is very helpful when doing EME (moonbounce) communications. Normally the “polarity relay” is located at the antenna and is controlled manually from the shack. This relay must handle the full power of the amplifier so it cannot be damaged if it is switched during a transmit cycle. The S1 sequencer de-activates the manual control of this relay during transmit thus preventing damage to this critical and expensive relay.

The S1 sequencer also provides 12VDC and 28VDC for external relays and preamp power.