TECHNICAL NOTE / PROTECTION / CONTROL
A high-power weak-signal station cannot safely treat an antenna relay and a preamplifier as independent accessories. Their control timing must ensure that transmit power cannot reach the LNA or receiver.
A fail-safe sequence
- Before transmit: disable or bypass the LNA; switch the receive path out; move the antenna relay into its TX state.
- Confirm switching: allow relay contact movement and any required settle time.
- Enable RF: only after the path is safe, permit amplifier drive and transmit RF.
- After RF stops: withdraw drive, let RF decay, return relays to RX and restore LNA power last.
No universal delay fits every system
Mechanical switching times, RF hang-time, amplifier behavior and control-voltage transients differ. Design the sequence around data sheets and direct measurement. Make sure a stuck relay, broken cable or lost DC power cannot silently place the LNA in the transmit path.
Isolation and power handling
Verify relay specifications at the band and power level actually used. DC coil rating, RF connector type, port isolation and insertion loss are separate requirements. A relay adequate on 50 MHz may be unsuitable at 1296 MHz.
This is design guidance, not an approved high-power wiring diagram. Include a positive transmit inhibit, RF exposure assessment and lightning protection appropriate to the installation.
Marshall K5QE: original primary source
Read On the Dark Side of Sequencing — Marshall Williams K5QE (2016, 23 slides). Marshall explains that the sequencer must ultimately control when the transmitting chain is allowed to produce RF. Simply using rig PTT to activate a relay downstream can leave the RF source transmitting before the contacts have settled.
Bench-test both transitions
- Key the sequencer with RF drive inhibited; scope or time-stamp each output and relay-auxiliary contact.
- Confirm receiver/LNA protection and relay settle before the transverter and amplifier are permitted to generate RF.
- On unkey, verify RF is completely removed before the relays return or preamp power is restored.
- Test emergency release, lost-coil supply, partial connector removal and stuck-PTT behavior.
- Use hardware transmit-inhibit and physical interlocks where feasible; software timing alone should not be the only protection.
Do not copy a relay delay or wiring arrangement blindly from an example. Measure timing, power handling, isolation and fail-safe behavior with your actual hardware; high-power RF can damage equipment or cause injury.
Other talks and conference records appear in the K5QE presentations archive.