diff --git a/gui_interface/fyne-ui/mercury_chat_window.go b/gui_interface/fyne-ui/mercury_chat_window.go
index 1fcd0b77..fa7cb8d4 100644
--- a/gui_interface/fyne-ui/mercury_chat_window.go
+++ b/gui_interface/fyne-ui/mercury_chat_window.go
@@ -57,6 +57,7 @@ type chatWindow struct {
 	ip        *widget.Entry
 	arqPort   *widget.Entry
 	bcastPort *widget.Entry
+	bandwidth *widget.Select
 
 	connectBtn    *widget.Button
 	disconnectBtn *widget.Button
@@ -66,12 +67,18 @@ type chatWindow struct {
 	sendARQ       *widget.Button
 	sendBcast     *widget.Button
 	sendBcastWrap *hoverTooltipButton
+	sendCQ        *widget.Button
 	arqMsg        *widget.Entry
 	bcastMsg      *widget.Entry
 
 	// bcastDisabledReason is shown as a hover tooltip on the Broadcast
 	// message button while it is disabled (e.g. during an ARQ session).
 	bcastDisabledReason string
+
+	// cqSending is set while a CQ frame is being transmitted; broadcast and
+	// ARQ connect are blocked until the engine reports the transmission done.
+	// Owned by setCQBusy and touched only from the UI thread.
+	cqSending bool
 }
 
 func (cw *chatWindow) build(app fyne.App, telemetry telemetryState, arqPort, broadcastPort int) {
@@ -88,6 +95,9 @@ func (cw *chatWindow) build(app fyne.App, telemetry telemetryState, arqPort, bro
 	cw.bcastPort = widget.NewEntry()
 	cw.bcastPort.SetText(strconv.Itoa(broadcastPort))
 
+	cw.bandwidth = widget.NewSelect([]string{"2300 Hz", "500 Hz"}, cw.onBandwidthChange)
+	cw.bandwidth.SetSelected("2300 Hz")
+
 	cw.arqMsg = widget.NewEntry()
 	cw.arqMsg.SetPlaceHolder("Type message to be sent...")
 	cw.bcastMsg = widget.NewEntry()
@@ -123,6 +133,8 @@ func (cw *chatWindow) build(app fyne.App, telemetry telemetryState, arqPort, bro
 		return cw.bcastDisabledReason
 	})
 	cw.sendBcastWrap.Disable()
+	cw.sendCQ = widget.NewButton("Send CQ Frame", cw.onSendCQ)
+	cw.sendCQ.Disable()
 
 	cfgForm := widget.NewForm(
 		&widget.FormItem{Text: "My Callsign", Widget: cw.myCall},
@@ -130,10 +142,16 @@ func (cw *chatWindow) build(app fyne.App, telemetry telemetryState, arqPort, bro
 		&widget.FormItem{Text: "IP/Host", Widget: cw.ip},
 		&widget.FormItem{Text: "ARQ Port", Widget: cw.arqPort},
 		&widget.FormItem{Text: "Broadcast Port", Widget: cw.bcastPort},
+		&widget.FormItem{Text: "Bandwidth", Widget: cw.bandwidth},
 	)
 
 	modemRow := container.NewHBox(cw.connectBtn, cw.disconnectBtn)
-	sessionRow := container.NewHBox(cw.arqConnect, cw.arqDisconnect, cw.arqAbort)
+	sessionRow := container.NewGridWithColumns(4,
+		cw.arqConnect,
+		cw.arqDisconnect,
+		cw.arqAbort,
+		cw.sendCQ,
+	)
 
 	controls := container.NewVBox(
 		cfgForm,
@@ -239,6 +257,7 @@ func (cw *chatWindow) setTCP(on bool) {
 			cw.disconnectBtn.Enable()
 			cw.arqConnect.Enable()
 			cw.sendBcastWrap.Enable()
+			cw.sendCQ.Enable()
 		} else {
 			cw.connectBtn.Enable()
 			cw.disconnectBtn.Disable()
@@ -247,6 +266,7 @@ func (cw *chatWindow) setTCP(on bool) {
 			cw.arqAbort.Disable()
 			cw.sendARQ.Disable()
 			cw.sendBcastWrap.Disable()
+			cw.sendCQ.Disable()
 		}
 	})
 }
@@ -260,6 +280,11 @@ func (cw *chatWindow) setARQ(on bool) {
 			cw.sendARQ.Enable()
 			cw.sendBcastWrap.Disable()
 			cw.bcastDisabledReason = "Broadcast is disabled while an ARQ session is active."
+			// A CQ is an unsolicited transmission; firing one mid-session puts
+			// it on the air on top of the session. The engine will not stop us
+			// -- ARQ_CMD_SEND_CQ (arq.c) builds and queues the frame with no
+			// conn_state guard -- so the block has to be here.
+			cw.sendCQ.Disable()
 		} else {
 			cw.arqConnect.Enable()
 			cw.arqDisconnect.Disable()
@@ -268,6 +293,11 @@ func (cw *chatWindow) setARQ(on bool) {
 			cw.bcastDisabledReason = ""
 			if cw.mc != nil && cw.mc.IsConnected() {
 				cw.sendBcastWrap.Enable()
+				// Not while a CQ of our own is still on the air: setCQBusy
+				// owns that case and re-enables when PTT drops.
+				if !cw.cqSending {
+					cw.sendCQ.Enable()
+				}
 			}
 		}
 	})
@@ -307,6 +337,7 @@ func (cw *chatWindow) onConnect() {
 		IP:             cw.ip.Text,
 		ARQPort:        arqPort,
 		BroadcastPort:  bcastPort,
+		BandwidthHz:    bandwidthFromLabel(cw.bandwidth.Selected),
 	}
 	mc := client.New(cfg)
 	if err := mc.Connect(); err != nil {
@@ -316,6 +347,7 @@ func (cw *chatWindow) onConnect() {
 		return
 	}
 	cw.mc = mc
+	cw.cqSending = false
 	cw.setTCP(true)
 	cw.done = make(chan struct{})
 
@@ -337,6 +369,7 @@ func (cw *chatWindow) onDisconnect() {
 	}
 	cw.setTCP(false)
 	cw.setARQ(false)
+	cw.cqSending = false
 	cw.logMsg("Disconnected.")
 }
 
@@ -390,6 +423,64 @@ func (cw *chatWindow) onSendARQ() {
 	cw.arqMsg.SetText("")
 }
 
+func (cw *chatWindow) onBandwidthChange(sel string) {
+	hz := bandwidthFromLabel(sel)
+	if hz == 0 {
+		return
+	}
+	if mc := cw.mc; mc != nil && mc.IsConnected() {
+		if err := mc.SetBandwidth(hz); err != nil {
+			cw.logMsg("set bandwidth: %v", err)
+		} else {
+			cw.logMsg("Bandwidth set to %d Hz.", hz)
+		}
+	}
+}
+
+func (cw *chatWindow) onSendCQ() {
+	mc := cw.mc
+	if mc == nil || !mc.IsConnected() || cw.cqSending {
+		return
+	}
+	if err := mc.SendCQFrame(); err != nil {
+		dialog.ShowError(err, cw.win)
+		return
+	}
+	cw.sendCQ.Disable()
+	cw.setCQBusy(true)
+	cw.logMsg("CQ frame sent.")
+}
+
+// setCQBusy disables broadcast and ARQ connect while a CQ frame is on the air
+// and restores them once the engine reports the transmission finished. It owns
+// the cqSending flag and every widget it touches, all inside fyne.Do, so the
+// flag is only ever accessed from the UI thread.
+func (cw *chatWindow) setCQBusy(on bool) {
+	fyne.Do(func() {
+		if on {
+			cw.cqSending = true
+			cw.sendCQ.Disable()
+			cw.arqConnect.Disable()
+			cw.sendBcastWrap.Disable()
+			cw.bcastDisabledReason = "CQ frame transmission in progress."
+			return
+		}
+		if !cw.cqSending {
+			return
+		}
+		cw.cqSending = false
+		// sendCQ is re-enabled under the same session test as the rest: a
+		// session can come up while our CQ is still on the air, and enabling
+		// it unconditionally here would undo setARQ's block.
+		if cw.mc != nil && cw.mc.IsConnected() && !cw.mc.IsARQConnected() {
+			cw.sendCQ.Enable()
+			cw.arqConnect.Enable()
+			cw.sendBcastWrap.Enable()
+			cw.bcastDisabledReason = ""
+		}
+	})
+}
+
 func (cw *chatWindow) onSendBroadcast() {
 	mc := cw.mc
 	if mc == nil || !mc.IsConnected() {
@@ -482,6 +573,13 @@ func (cw *chatWindow) forwardStatus() {
 				cw.setARQ(true)
 			case "DISCONNECTED":
 				cw.setARQ(false)
+			case "PTT OFF":
+				// The true end of a CQ transmission: an incoming connection's
+				// PENDING/CANCELPENDING never keys the radio, so PTT OFF is the
+				// unambiguous "CQ is off the air" signal. setCQBusy owns the
+				// cqSending flag and only clears it when one is actually in
+				// flight, marshalling the work onto the UI thread.
+				cw.setCQBusy(false)
 			}
 		case <-done:
 			return
@@ -495,3 +593,14 @@ func defaultCall(call, fallback string) string {
 	}
 	return fallback
 }
+
+func bandwidthFromLabel(label string) int {
+	switch strings.TrimSpace(label) {
+	case "500 Hz":
+		return 500
+	case "2300 Hz":
+		return 2300
+	default:
+		return 0
+	}
+}
diff --git a/gui_interface/mercury-client/client/client.go b/gui_interface/mercury-client/client/client.go
index 55ed882b..dbbace23 100644
--- a/gui_interface/mercury-client/client/client.go
+++ b/gui_interface/mercury-client/client/client.go
@@ -20,6 +20,7 @@ type Config struct {
 	IP             string
 	ARQPort        int
 	BroadcastPort  int
+	BandwidthHz    int
 }
 
 // ChatMessage is a single chat line emitted for display.
@@ -62,6 +63,9 @@ func New(cfg Config) *Client {
 	if cfg.BroadcastPort == 0 {
 		cfg.BroadcastPort = 8100
 	}
+	if cfg.BandwidthHz == 0 {
+		cfg.BandwidthHz = 2300
+	}
 	return &Client{
 		cfg:             cfg,
 		LogCh:           make(chan string, 256),
@@ -97,7 +101,7 @@ func (c *Client) Connect() error {
 	mc.SendCommand("LISTEN ON")
 	mc.SendCommand("PUBLIC OFF")
 	mc.SendCommand("COMPRESSION OFF")
-	mc.SendCommand("BW2750")
+	mc.SendCommand(fmt.Sprintf("BW%d", c.cfg.BandwidthHz))
 
 	go c.updateLog()
 	go c.handleIncomingARQ()
@@ -224,6 +228,41 @@ func (c *Client) SendARQFile(filePath string) error {
 	return mc.SendARQFile(filePath)
 }
 
+// SetBandwidth updates the modem's bandwidth and stores it for the next
+// connection. Supported values are 500 and 2300 (Hz).
+func (c *Client) SetBandwidth(hz int) error {
+	if hz != 500 && hz != 2300 {
+		return fmt.Errorf("unsupported bandwidth: %d Hz", hz)
+	}
+	c.mu.Lock()
+	mc := c.modem
+	c.mu.Unlock()
+	if mc == nil || !mc.IsConnected() {
+		return fmt.Errorf("not connected to modem")
+	}
+	if err := mc.SendCommand(fmt.Sprintf("BW%d", hz)); err != nil {
+		return err
+	}
+	c.cfg.BandwidthHz = hz
+	return nil
+}
+
+// SendCQFrame queues a one-shot CQ frame advertising the local callsign and
+// the currently configured bandwidth.
+func (c *Client) SendCQFrame() error {
+	c.mu.Lock()
+	mc := c.modem
+	c.mu.Unlock()
+	if mc == nil || !mc.IsConnected() {
+		return fmt.Errorf("not connected to modem")
+	}
+	if err := mc.SendCQFrame(c.cfg.MyCallsign, c.cfg.BandwidthHz); err != nil {
+		return err
+	}
+	c.LogCh <- fmt.Sprintf("CQ Frame TX: %s (%d Hz)", c.cfg.MyCallsign, c.cfg.BandwidthHz)
+	return nil
+}
+
 // SendBroadcast sends a broadcast message. The local callsign is prefixed to
 // the payload because the Mercury broadcast plane carries no callsign, so
 // receiving stations can attribute the message.
diff --git a/gui_interface/mercury-client/modem/modem.go b/gui_interface/mercury-client/modem/modem.go
index bb37760e..3ccd362d 100644
--- a/gui_interface/mercury-client/modem/modem.go
+++ b/gui_interface/mercury-client/modem/modem.go
@@ -219,6 +219,12 @@ func (mc *ModemClient) ListenOff() error {
 	return mc.SendCommand("LISTEN OFF")
 }
 
+// SendCQFrame queues a one-shot CQ frame advertising the source callsign and
+// the given bandwidth (Hz).
+func (mc *ModemClient) SendCQFrame(src string, bwHz int) error {
+	return mc.SendCommand(fmt.Sprintf("CQFRAME %s %d", src, bwHz))
+}
+
 func (mc *ModemClient) ConnectARQ(src, dst string) error {
 	mc.mu.Lock()
 	if mc.ARQControlConn == nil {
@@ -475,6 +481,12 @@ func (mc *ModemClient) dispatchControlLine(line string) {
 
 	case strings.HasPrefix(upper, "CQFRAME"):
 		mc.StatusCh <- "CQFRAME"
+
+	case strings.HasPrefix(upper, "PTT ON"):
+		mc.StatusCh <- "PTT ON"
+
+	case strings.HasPrefix(upper, "PTT OFF"):
+		mc.StatusCh <- "PTT OFF"
 	}
 
 	mc.mu.Lock()