Treetop Circuits

Connecting an R-388/51J Receiver with a KWM-2 Transceiver

Having a second receiver connected allows you to work “split” – that is, you can transmit on one frequency and monitor another.

DX stations frequently listen for calls 5 or 10 kHz up or down from their transmitting frequency, so the ability to work split is pretty much a necessity if you want to work them. You can also monitor another frequency or look around for a frequency with less QRM than the one you’re using.

Split operation can be accomplished with a second VFO; however, many operators prefer to use a second receiver because it lets them hear both frequencies at once. It also lets them listen to another band on the second receiver.

The R-388/51J works well as a second receiver for a KWM- 2. The connections, and the reasoning behind them, are as follows. Please note that the following applies to a setup without a linear amplifier:

First, we’ll be sharing the antenna when receiving. But we don’t want to have the R-388/51J connected to the antenna when transmitting (they’re tough, but not that tough). Fortunately, the KWM-2 has a jack on the rear that’s ideal for this purpose. It’s labeled REC ANT (for Receive Antenna). All you need is a piece of RG-58 coax (or similar) with an RCA plug on one end and a PL-259 on the other to connect this to the ANTENNA connection on the R-388/51J. The antenna relay in the KWM-2 then connects the R-388/51J to the antenna when receiving, but not when transmitting.

Next, you should make provision to operate the muting relay in the R-388/51J when transmitting. When operated, this relay disconnects the receiver circuits from the antenna connector and grounds the antenna connection, to provide additional protection to the receiver front end. It also disables some of the circuits in the R-388/51J to mute it and prevent feedback from the receiver into the microphone, so the setup won’t howl.

You’ll need a power supply capable of providing 12 volts DC at 100 milliamps. If you’re using a Treetop Circuits SSB adapter, connect the supply in the adapter according to the owner’s manual. Otherwise, a “wall wart” or similar supply with the correct rating is OK. Connect the power supply to the REMOTE terminal strip on the rear of the R388; positive goes to Terminal 3, and negative goes to Terminal 1 (ground). Now you’ll need an RCA connector on a piece of 2-conductor lamp cord or speaker wire (coax isn’t necessary). Wire it so the shell goes to Terminal 1 on the REMOTE terminal strip, and the center conductor goes to Terminal 2. Plug the RCA connector into the ANT RELAY jack on the rear of the KWM-2 and you’re there!

Since this setup puts the receiver input circuits in parallel when receiving, they share the signal, generally in an unequal fashion, which results in some loss of sensitivity and may affect S-meter readings. In practice, the loss of sensitivity will rarely matter, except possibly on 10 or 12 meters, where quieter band conditions may prevail. If it’s a concern, add a switch in the antenna connection between the radios – but make sure it’s within 2 feet of the KWM-2, so the resulting “stub” doesn’t affect the signal.