Elecraft K4 + fldigi Setup Guide

Complete setup for Olivia and other digital modes. The K4 is Elecraft's high-performance SDR transceiver with integrated USB audio and dual virtual COM ports.

Quick Reference

USB ConnectionUSB-B - single cable for CAT + audio
Rig ControlRigCAT with K4.xml (recommended)
Baud Rate38400 (MANDATORY - do not change)
Audio DeviceUSB AUDIO CODEC
PTT MethodRTS via serial port
COM PortMust be COM9 or lower (Windows)

1. USB Connection Overview

The K4 features an integrated USB interface via a single USB-B connector that provides:

  • Two virtual COM ports (USB-PC1 and USB-PC2) for radio control
  • One USB Audio Codec (virtual soundcard) for TX and RX audio
  • Single integrated connection - no separate cables required
  • Windows identifies audio as "USB AUDIO CODEC"

Key Benefit

Receive audio available at both analog LINE OUT and USB soundcard. Eliminates analog audio cabling, reducing noise and ground loop issues.

2. Driver Installation

Windows 11

Modern Windows includes FTDI drivers built-in. Simply connecting the K4 via USB will result in automatic driver installation.

If manual installation needed:

  1. Download FTDI VCP drivers from ftdichip.com
  2. Install and restart
  3. Verify in Device Manager: Two "USB Serial Port" entries

Critical - COM Port Limitation: Hamlib cannot use COM10 or higher. In Device Manager, right-click USB Serial Port → Properties → Advanced → Change to COM2-9.

macOS

  1. Download FTDI drivers from FTDI macOS download page
  2. Download D2XX driver package and VCP drivers
  3. Install and restart
  4. Verify: ls /dev/tty.usbserial* should show two entries

Linux

Most distributions include the open-source FTDI driver (ftdi_sio) by default.

Verify: lsmod | grep ftdi_sio and ls /dev/ttyUSB*

Permission fix: sudo usermod -a -G dialout $USER

3. Radio Hardware Setup

USB Port Configuration

  1. Press MENU button
  2. Navigate to Serial I/O
  3. Select USB-PC2
  4. Set the following:
    ParameterSetting
    Baud Rate38400 (MANDATORY)
    RTS FunctionPTT
    DTR FunctionFSK
    Data Bits8
    Stop Bits1
    ParityNone

Audio Level Configuration

For RX (K4 to computer):

  1. Press MAIN RX button
  2. Tap LINE OUT
  3. Set level to ~40-50% (avoid clipping in fldigi)

For TX (computer to K4):

  1. Press TX button
  2. Tap LINE IN, select SOUNDCARD
  3. Set appropriately (should not peak above 80%)

4. fldigi Configuration

Audio Device Setup

  1. Open Configure → Audio
  2. Capture Device: Select "USB AUDIO CODEC"
  3. Playback Device: Select "USB AUDIO CODEC"
  4. Sample Rate: 48 kHz
  5. Bits: 16-bit

Rig Control (RigCAT - Recommended)

  1. Download K4.xml from SourceForge
  2. Open Configure → Rig
  3. Select RigCAT
  4. Click "Open" and select K4.xml
  5. Set serial port parameters:
    • Serial Port: Your K4's COM port (COM2-9, NOT COM10+)
    • Baud Rate: 38400 (critical)
    • Data Bits: 8, Stop Bits: 1, Parity: None
    • Handshake: None
  6. Click Initialize to test

PTT Configuration

Recommended: RTS via serial port

  1. In Configure → Rig, PTT section:
  2. Method: "Serial (DTR/RTS)"
  3. RTS for PTT: Checked
  4. DTR for PTT: Unchecked (leave DTR for FSK)
  5. Port: Same as rig control port
  6. Delay: 50ms start, 50ms stop

5. Common Issues and Troubleshooting

fldigi Won't Connect (RigCAT Fails)

  • Verify baud rate is exactly 38400
  • Close all other applications using K4 COM ports
  • Check COM port number is 9 or lower (Windows)
  • Disconnect K4, wait 5 seconds, reconnect
  • Verify K4 is in DATA mode (not SSB)

No Audio in fldigi Waterfall

  • Verify USB AUDIO CODEC is selected as input device
  • Check K4 menu: MAIN RX → LINE OUT enabled with volume >30%
  • In Windows sound settings, verify USB AUDIO CODEC not muted
  • Try different sample rate (44.1 kHz instead of 48 kHz)

PTT Not Keying Transmitter

  • Verify K4 menu: USB-PC2 → RTS set to "PTT"
  • Test PTT method in fldigi: RTS is enabled
  • Check K4 mode is DATA (PTT may be disabled in some modes)

Transmit Audio Too Quiet or Distorted

  • Increase TX LINE IN level in K4 menu (up to 70-80%)
  • Monitor K4 ALC meter during TX (should show some activity)
  • Verify K4 is actually in DATA mode and not SSB

6. Official Documentation

Pre-Flight Checklist

  1. K4 connected via USB-B cable
  2. Device Manager shows two USB Serial Port entries
  3. COM port number is 9 or lower (Windows)
  4. K4 Menu: USB-PC2 Baud = 38400, RTS = PTT
  5. K4: MAIN RX LINE OUT enabled, TX LINE IN = SOUNDCARD
  6. K4 Mode: DATA
  7. fldigi: K4.xml loaded with correct COM port
  8. fldigi: USB AUDIO CODEC for input/output
  9. fldigi: RTS for PTT enabled
  10. Test: Initialize shows rig frequency/mode
  11. Test: PTT keys the K4
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