The simplest TXRX sequencer for LNA in the world!

From 50 MHz to 10 GHz I use LNA for any band because there is a certain distance between shack and antennas and the cable loss makes me almost deaf without.

I tried any sort of sequencer over the years and burned a lot of LNA devices during my HAM activity. Most of the time it was my fault during tests and preliminary setup. With this sequencer it will no happen anymore (hopefully!)

The diagram is simple: when the sequencer is supplied with +12V the voltage passes through the Normally Closed relay contact and, if the manual switch “PRE” is turned on, goes to the preamplifier on the roof. At the same time the negative voltage generator is turned on and sends to the transceiver ALC input a negative voltage set by the trimmer R4. This negative voltage completely chokes the RF even if the transceiver is keyed.
When the transceiver goes to TX, the relay switches and removes the supply voltage to preamplifier and negative voltage generator and closes to GND the amplifier’s PTT.
After some time, when the 2.2 uF electrolytic capacitor discharges, the transceiver outputs RF.

The negative voltage generator is a fully insulated ultra-cheap DCDC converter B1212S.

It can be purchased for about 1 euro on the usual Chinese websites.

This DCDC converter can deliver up to about 80 mA, there is lot of headroom for this use.

I realized a prototype that is currently in use on 2m band connected to my IC-910H.

In the picture above the block diagram of my station. With the power supply powered off, the preamplifier is off too and the antenna is connected to an SDR dongle in RX on the FT8 QRG 24/7. When the 12V power supply is turned on, the transceiver starts and it’s ready to be used with or without PRE and PA depending on the manual switches status of the sequencer box.

After some use I can write the pros and cons of this approach.

PROS

• It’s made using a few components of general use: 1 DPDT relay, 1 optocoupler, some diodes, resistances and capacitors.
• No microcontrollers/Arduino needed.
• Failsafe-foolproof: if the relay fails or if the LNA is supplied in some other way, the transceiver does not outputs RF.

CONS

• The transceiver needs a certain time to emits RF after the microphone/key closure. This time can be varied by choosing the value of the electrolytic capacitor on the ALC line (or removing it), but most is due to the ALC decay time of the transceiver and can’t be modified easily. Tested on Icom IC-910H it’s less than 0.5 s that’s ok for FT8 but can be too slow for CW/SSB without training (close PTT, wait, send the message).