Since my IC-746 does not have this feature, I needed to construct a simple relay box. The box would have three coax connectors, a connection to the rig for the send and power, and also a power output to source my active antenna.
The theory of operation is one that takes into account the safety of the transceiver and the active antenna. The failsafe state is TX antenna position. If the circuit fails to power up (or the fuse blows), the TX antenna will stay in circuit. Likewise, if the circuit fails to power up, the active antenna will not get any power. Only when the rig powers the relays will the active antenna attached to RX be connected and powered up. The HSEND output from the radio likewise provides a safety. It disengages the RX active antenna power through the Rp relay, and then subsequently drops out the Rt relay so that the rig may transmit into the TX antenna.
Here are all the parts on the table ready to assemble. I chose an aluminum 6x4x3 BUD box since aluminum is much easier to work with. I also have two 12 volt relays, one small one to switch the loads and one large one to switch the RF. I kept the RF leads as short as possible, and also used shielded cable between the radio and the relay box.
The coax connectors were placed at a certain height so the relay would be very close to them. Keep in mind that the general purpose relay that was employed in this project is not going to be good for anything above HF frequencies. If I could have sourced an economical coaxial relay, that would have been better. This would not work very well as a relay box for using your SDR as an separate RX radio. (That may be a project for the future.)
The fuse protects the rig's ACC2 jack from excessive current draw. The switch disables the operation of the relay box if the receive antenna is not desired. The indicator illuminates when the RX antenna is in circuit and powered up. The lamp goes out when the rig goes into transmit.
The wiring was done point-to-point to keep the leads short. The control and power cable goes out the back bottom of the enclosure through rubber grommets.
The result is good. The relay switches between RX and TX antenna when the rig is keyed and the switch is up. When the switch is left down (in bypass), the RX antenna is not connected at all and all signals go through the TX antenna.
The final touch was to put some magnetic feet on it, so it sticks to the top of the radio. Then apply the labels.
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