TR Time-Delay Generator by N6CA ("Sequencer")

Stations which are using high power amplifiers and external mounted switching relays might find several problems occur. When the PTT is pushed, it immediately puts out a ground command (or positive voltage) for relay control. If the amplifier and antenna relays are switched simultaneously, RF can be applied as the relay contacts bounce. In most cases, RF will be applied before a relay can make full closure. This can easily arc contacts on RF relays and cause permanent damage. In addition, if the TR relay is not fully closed before the RF from the power amplifier is applied, excessive RF may leak into the receive side of the relay. This likely result in preamplifier failure.

The TR Time-Delay Generator supplies commands, one after another, going into transmit and going back to receive from transmit, to turn on all station relays in the right order. Most coaxial relays require about 10 ms to change states and stop bouncing. The TR Time-Delay Generator uses a default value of 30 ms delay, which should be an adequate time for all closures to occur. For more technical and construction details refer ro the instructions of N6CA, which was published in the VHF Radio Equipment.

Many RF relays have very poor isolation at VHF frequencies. Many of these have only about 40 dB isolation at 144 MHz. If you are running high power, say 1000 W (+ 60 dBm) at the relay, the receive side of the relay will see +20 dBm (100 mW) when the station is transmitting. This power level is enough to inflict fatal damage on your preamplifier.

One of the more popular antenna changeover schemes uses two coaxial relays - one for actual TR switching and one for receiver / preamplifier protection. Adding a second relay, called the RX (isolation) relay, terminates the preamp in a 50 Ohm load during transmission and increases the isolation significantly. Also, in the event of TR relay failure, this extra relay will protect the receive preamplifier. If high quality relays are used and verified to be in working order, relay isolation can be kept below 0.1dB.

You may take a look at my TX-RX sequence schematic. The schematic shows the unpowered status of the relays and describes the switching order from RX to TX status and back. After power on of the TR Time Delay Generator, the Preamp and RX relay get power on and the system switches into RX status. First, if the PTT is pushed, the Preamp and the RX relay are switched off. Approximately 30 ms later the TX relay switches its state. Another 30 ms later the AMP relay and finally another 30 ms later the PA relay changes its state. If the system status changes from TX to RX again, the switching sequence perfoms in reverse order.

Many thanks must go to Roberto, IZ2FOB (ex IW2DVK), for sending me the technical infos!