patrickrb · Oct 6, 2026
diff --git a/docs/clear-cq-slot-selection.md b/docs/clear-cq-slot-selection.md
@@ -79,6 +79,17 @@ tunables below are constructor parameters with the defaults shown.
 - Off by default (config key `autoClearTxFreq`); the manual waterfall tap
   always wins — it just becomes the new "current" offset that the keep-current
   bias protects.
+- **Minimum auto TX audio frequency** (config key `minAutoTxFreq`, Hz, 0 =
+  off; choices 500/1000/1500/2000). A rig that clips or squares the TX audio
+  radiates harmonics of the tone, and from a low offset they still fall inside
+  the ~3 kHz TX filter (3rd harmonic below 1000 Hz, 2nd below 1500 Hz). With a
+  floor set, the picker scans only from the floor up, tie-breaks toward the
+  middle of the remaining range, and at CQ start lifts a current offset that
+  sits below the floor even when it is clear or nothing has been decoded yet.
+  The mid-CQ re-check still fires only on "occupied", so a deliberate waterfall
+  tap below the floor survives until the next CQ start. The same floor stops
+  the TX=RX follow from moving onto a station below it (we answer split from
+  our own offset). Fox/Hound QSYs are protocol-mandated and ignore it.
 
 ## Validation
 

diff --git a/ft8af/app/src/main/java/com/k1af/ft8af/GeneralVariables.java b/ft8af/app/src/main/java/com/k1af/ft8af/GeneralVariables.java
@@ -177,6 +177,12 @@ public static boolean isConfiguredUsbAudioOutput(int vid, int pid) {
     //the decode-delivery thread inside recordBandActivity.
     public static volatile boolean autoClearTxFreq = false;
 
+    //Floor (Hz) for automatically chosen TX audio offsets — the clear-CQ picker
+    //and the TX=RX follow. 0 = off. For rigs whose TX audio is clipped/squared,
+    //where low offsets put harmonics inside the TX passband. One of
+    //ClearFrequencyFinder.minAutoTxChoicesHz(). volatile: same threads as above.
+    public static volatile int minAutoTxFreqHz = 0;
+
     //Tune button (issue #408): hard cap on a single tune carrier in seconds
     //(clamped by TuneController), whether the tune level is independent of the
     //FT8 drive, the global independent level (0..100), and the per-band

diff --git a/ft8af/app/src/main/java/com/k1af/ft8af/database/DatabaseOpr.java b/ft8af/app/src/main/java/com/k1af/ft8af/database/DatabaseOpr.java
@@ -2984,6 +2984,17 @@ protected Void doInBackground(Void... voids) {
                 if (name.equalsIgnoreCase("autoClearTxFreq")) {//Auto-select clear CQ offset (issue #418)
                     GeneralVariables.autoClearTxFreq = "1".equals(result);
                 }
+                if (name.equalsIgnoreCase("minAutoTxFreq")) {//Floor for auto-picked TX offsets, 0 = off
+                    //Defensive parse: settings import (#382) can feed anything here.
+                    if (result != null) {
+                        try {
+                            GeneralVariables.minAutoTxFreqHz =
+                                    com.k1af.ft8af.ft8transmit.ClearFrequencyFinder.clampMinAutoTxHz(
+                                            Integer.parseInt(result.trim()));
+                        } catch (NumberFormatException ignored) {
+                        }
+                    }
+                }
                 if (name.equalsIgnoreCase("tuneMaxOnSeconds")) {//Tune carrier hard cap (issue #408)
                     //Defensive parse: settings import (#382) can feed anything here.
                     //Null/non-numeric keeps the default; TuneController clamps the range.

diff --git a/ft8af/app/src/main/java/com/k1af/ft8af/ft8transmit/ClearFrequencyFinder.java b/ft8af/app/src/main/java/com/k1af/ft8af/ft8transmit/ClearFrequencyFinder.java
@@ -30,6 +30,40 @@ public final class ClearFrequencyFinder {
     public static final float PREFERRED_CENTER_HZ = 1500f;
     public static final long DEFAULT_MOVE_HOLDDOWN_MS = 45_000L;
 
+    /**
+     * Choices for the "minimum auto TX audio frequency" setting, in Hz; 0 is
+     * off (no floor beyond {@link #DEFAULT_MIN_OFFSET_HZ}). A rig that clips
+     * or squares the TX audio radiates harmonics of the tone: with a ~3 kHz
+     * TX filter the 3rd harmonic is rejected above 1000 Hz and the 2nd above
+     * 1500 Hz, so the floor keeps automatic picks out of the low offsets
+     * whose harmonics would still land in the passband.
+     */
+    private static final int[] MIN_AUTO_TX_CHOICES_HZ = {0, 500, 1000, 1500, 2000};
+
+    /** A copy of the setting's choices (index 0 = off). */
+    public static int[] minAutoTxChoicesHz() {
+        return MIN_AUTO_TX_CHOICES_HZ.clone();
+    }
+
+    /**
+     * Index of the largest choice not above {@code hz} — maps any stored or
+     * imported value (negative, off-grid, oversized) onto a valid choice.
+     */
+    public static int minAutoTxChoiceIndex(int hz) {
+        int index = 0;
+        for (int i = 0; i < MIN_AUTO_TX_CHOICES_HZ.length; i++) {
+            if (MIN_AUTO_TX_CHOICES_HZ[i] <= hz) {
+                index = i;
+            }
+        }
+        return index;
+    }
+
+    /** Snap a stored/imported setting value onto one of the choices. */
+    public static int clampMinAutoTxHz(int hz) {
+        return MIN_AUTO_TX_CHOICES_HZ[minAutoTxChoiceIndex(hz)];
+    }
+
     /** One observed signal: base offset + the slot UTC it was decoded in. */
     private static final class Observation {
         final float freqHz;
@@ -148,14 +182,28 @@ public synchronized boolean isOccupied(float offsetHz) {
      *     current offset's clearance.
      */
     public synchronized Float selectClearOffset(float currentOffsetHz) {
-        if (history.isEmpty()) {
+        return selectClearOffset(currentOffsetHz, minOffsetHz);
+    }
+
+    /**
+     * {@link #selectClearOffset(float)} with an operator floor: no candidate
+     * below {@code floorHz} is considered, and a current offset below the floor
+     * always warrants a move (even with no history, and even when it is clear)
+     * — the floor exists because the low offsets are unusable on this rig, not
+     * because they are busy. A floor at or below the grid minimum is no floor.
+     */
+    public synchronized Float selectClearOffset(float currentOffsetHz, float floorHz) {
+        float lo = Math.max(minOffsetHz,
+                Math.min(floorHz, maxOffsetHz - SIGNAL_BANDWIDTH_HZ));
+        boolean belowFloor = lo > minOffsetHz && currentOffsetHz < lo;
+        if (history.isEmpty() && !belowFloor) {
             return null;
         }
-        float currentClearance = clearanceAt(currentOffsetHz);
+        float currentClearance = belowFloor ? -1f : clearanceAt(currentOffsetHz);
         if (currentClearance > 0f) {
             return null; // already clear: a gratuitous QSY loses spotted callers
         }
-        Candidate best = bestCandidate();
+        Candidate best = bestCandidate(lo);
         if (best == null || best.clearance <= currentClearance) {
             return null; // whole band as bad or worse: stay put
         }
@@ -173,19 +221,24 @@ private static final class Candidate {
     }
 
     /**
-     * Best candidate on the grid: max clearance, ties broken toward the band
-     * centre (1500 Hz, flat in every rig's passband), then toward the lower
-     * offset for determinism.
+     * Best candidate on the grid from {@code lo} up: max clearance, ties broken
+     * toward the band centre (1500 Hz, flat in every rig's passband), then
+     * toward the lower offset for determinism. With a floor raised above the
+     * grid minimum the tie-break targets the middle of what is left instead,
+     * so the pick doesn't hug the floor it was told to stay clear of.
      */
-    private Candidate bestCandidate() {
+    private Candidate bestCandidate(float lo) {
+        float preferred = lo > minOffsetHz
+                ? (lo + maxOffsetHz - SIGNAL_BANDWIDTH_HZ) / 2f
+                : PREFERRED_CENTER_HZ;
         Candidate best = null;
-        for (float f = minOffsetHz; f + SIGNAL_BANDWIDTH_HZ <= maxOffsetHz; f += stepHz) {
+        for (float f = lo; f + SIGNAL_BANDWIDTH_HZ <= maxOffsetHz; f += stepHz) {
             float clearance = clearanceAt(f);
             if (best == null
                     || clearance > best.clearance
                     || (clearance == best.clearance
-                        && Math.abs(f - PREFERRED_CENTER_HZ)
-                            < Math.abs(best.offsetHz - PREFERRED_CENTER_HZ))) {
+                        && Math.abs(f - preferred)
+                            < Math.abs(best.offsetHz - preferred))) {
                 best = new Candidate(f, clearance);
             }
         }

diff --git a/ft8af/app/src/main/java/com/k1af/ft8af/ft8transmit/FT8TransmitSignal.java b/ft8af/app/src/main/java/com/k1af/ft8af/ft8transmit/FT8TransmitSignal.java
@@ -676,7 +676,8 @@ public void setTransmit(TransmitCallsign transmitCallsign
         // Same-frequency mode moves our TX offset onto the station we answer — unless
         // "Hold TX freq" is on (WSJT-X "Hold Tx Freq"), in which case we keep calling
         // on our own offset. Tester reported the auto-move felt inverted.
-        if (shouldFollowTargetFreq(GeneralVariables.synFrequency, GeneralVariables.holdTxFreq)) {
+        if (shouldFollowTargetFreq(GeneralVariables.synFrequency, GeneralVariables.holdTxFreq,
+                transmitCallsign.frequency, GeneralVariables.minAutoTxFreqHz)) {
             setBaseFrequency(transmitCallsign.frequency);
         }
 
@@ -700,10 +701,16 @@ public void setBaseFrequency(float freq) {
      * Whether to move our TX offset onto the frequency of the station we're about to
      * answer. True only in TX=RX (synFrequency) mode AND when "Hold TX freq" is off —
      * holding TX freq (WSJT-X "Hold Tx Freq") keeps us calling on our own offset.
+     * A target below the "minimum auto TX audio frequency" floor (0 = off) is not
+     * followed either: we answer split from our own offset instead.
      * Pure decision logic so it can be unit-tested without the Android runtime.
      */
-    static boolean shouldFollowTargetFreq(boolean synFrequency, boolean holdTxFreq) {
-        return synFrequency && !holdTxFreq;
+    static boolean shouldFollowTargetFreq(boolean synFrequency, boolean holdTxFreq,
+                                          float targetFreqHz, int minAutoTxFreqHz) {
+        if (!synFrequency || holdTxFreq) {
+            return false;
+        }
+        return minAutoTxFreqHz <= 0 || targetFreqHz >= minAutoTxFreqHz;
     }
 
     /**
@@ -2625,7 +2632,8 @@ public void recordBandActivity(ArrayList<Ft8Message> messages) {
                 && clearFrequencyFinder.shouldRelocate(
                         GeneralVariables.getBaseFrequency(), newestUtc)) {
             Float pick = clearFrequencyFinder.selectClearOffset(
-                    GeneralVariables.getBaseFrequency());
+                    GeneralVariables.getBaseFrequency(),
+                    GeneralVariables.minAutoTxFreqHz);
             if (pick != null) {
                 applyClearCqOffset(pick, newestUtc, "occupied");
             }
@@ -2663,7 +2671,8 @@ private void maybeSelectClearCqOffset() {
             return;
         }
         Float pick = clearFrequencyFinder.selectClearOffset(
-                GeneralVariables.getBaseFrequency());
+                GeneralVariables.getBaseFrequency(),
+                GeneralVariables.minAutoTxFreqHz);
         if (pick != null) {
             applyClearCqOffset(pick, UtcTimer.getSystemTime(), "cq-start");
         }

diff --git a/ft8af/app/src/main/kotlin/radio/ks3ckc/ft8af/ui/settings/TransmissionSettings.kt b/ft8af/app/src/main/kotlin/radio/ks3ckc/ft8af/ui/settings/TransmissionSettings.kt
@@ -22,6 +22,7 @@ import androidx.compose.ui.unit.sp
 import com.k1af.ft8af.GeneralVariables
 import com.k1af.ft8af.MainViewModel
 import com.k1af.ft8af.R
+import com.k1af.ft8af.ft8transmit.ClearFrequencyFinder
 import com.k1af.ft8af.ft8transmit.MeterProtectionController
 import com.k1af.ft8af.ft8transmit.TuneController
 import com.k1af.ft8af.ft8transmit.TuneMethod
@@ -93,6 +94,7 @@ fun TransmissionSettings(
 ) {
     var synFrequency by remember { mutableStateOf(GeneralVariables.synFrequency) }
     var holdTxFreq by remember { mutableStateOf(GeneralVariables.holdTxFreq) }
+    var minAutoTxFreqHz by remember { mutableIntStateOf(GeneralVariables.minAutoTxFreqHz) }
     var clearOnBandModeChange by remember { mutableStateOf(GeneralVariables.clearOnBandModeChange) }
     var watchdogMs by remember { mutableIntStateOf(GeneralVariables.launchSupervision) }
     var noReplyLimit by remember { mutableIntStateOf(GeneralVariables.noReplyLimit) }
@@ -123,6 +125,7 @@ fun TransmissionSettings(
     var showWatchdog by remember { mutableStateOf(false) }
     var showStopAfter by remember { mutableStateOf(false) }
     var showMax73 by remember { mutableStateOf(false) }
+    var showMinAutoTxFreq by remember { mutableStateOf(false) }
     var showTuneMethod by remember { mutableStateOf(false) }
 
     // Index == TuneMethod.AUTOMATIC/INTERNAL/TONE
@@ -209,6 +212,30 @@ fun TransmissionSettings(
         )
     }
 
+    // -- Minimum auto TX audio frequency picker --
+    if (showMinAutoTxFreq) {
+        val choices = ClearFrequencyFinder.minAutoTxChoicesHz()
+        val labels = choices.map { hz ->
+            if (hz == 0) stringResource(R.string.common_off)
+            else stringResource(R.string.settings_hz_format, hz)
+        }
+        ListPickerDialog(
+            title = stringResource(R.string.settings_min_auto_tx_freq),
+            items = labels,
+            selectedIndex = ClearFrequencyFinder.minAutoTxChoiceIndex(minAutoTxFreqHz),
+            onDismiss = { showMinAutoTxFreq = false },
+            onSelect = { index ->
+                showMinAutoTxFreq = false
+                val hz = choices[index]
+                GeneralVariables.minAutoTxFreqHz = hz
+                minAutoTxFreqHz = hz
+                mainViewModel.databaseOpr.writeConfig(
+                    "minAutoTxFreq", hz.toString(), null,
+                )
+            },
+        )
+    }
+
     // -- Max 73 Sends Picker --
     if (showMax73) {
         val max73Options = mutableListOf(stringResource(R.string.settings_max_73_auto))
@@ -267,6 +294,15 @@ fun TransmissionSettings(
                         },
                     )
                     SectionDivider()
+                    SettingsRow(
+                        label = stringResource(R.string.settings_min_auto_tx_freq),
+                        description = stringResource(R.string.settings_min_auto_tx_freq_desc),
+                        value = if (minAutoTxFreqHz == 0) stringResource(R.string.common_off)
+                        else stringResource(R.string.settings_hz_format, minAutoTxFreqHz),
+                        showChevron = true,
+                        onClick = { showMinAutoTxFreq = true },
+                    )
+                    SectionDivider()
                     SettingsRow(
                         label = stringResource(R.string.settings_clear_on_change),
                         description = stringResource(R.string.settings_clear_on_change_desc),

diff --git a/ft8af/app/src/main/res/values-ar/strings_compose.xml b/ft8af/app/src/main/res/values-ar/strings_compose.xml
@@ -600,6 +600,8 @@
     <string name="cq_offset_moved">تم نقل إزاحة CQ إلى موضع خالٍ: %1$d هرتز</string>
     <string name="settings_auto_clear_tx">اختيار إزاحة إرسال خالية تلقائيًا</string>
     <string name="settings_auto_clear_tx_desc">عند نداء CQ يختار موضعًا خاليًا وفق نشاط النطاق الأخير وينتقل إذا أصبح مشغولًا</string>
+    <string name="settings_min_auto_tx_freq">أدنى تردد صوتي للإرسال التلقائي</string>
+    <string name="settings_min_auto_tx_freq_desc">أدنى إزاحة صوتية يختارها التطبيق تلقائيًا (اختيار الإزاحة الخالية، متابعة TX=RX). ارفعها إذا كان جهازك يقصّ أو يربّع صوت الإرسال لتقع التوافقيات خارج مرشح الإرسال</string>
     <string name="tune_content_description">Tune: يبدأ موالف الجهاز أو يرسل موجة حاملة مستمرة لضبط الهوائي</string>
     <string name="tune_stopped_timeout">توقف Tune بعد %1$d ث</string>
     <string name="tune_blocked_tx">أوقف CQ/QSO قبل استخدام Tune</string>

diff --git a/ft8af/app/src/main/res/values-cs/strings_compose.xml b/ft8af/app/src/main/res/values-cs/strings_compose.xml
@@ -592,6 +592,8 @@
     <string name="cq_offset_moved">Offset CQ přesunut na volné místo: %1$d Hz</string>
     <string name="settings_auto_clear_tx">Automaticky volný TX offset</string>
     <string name="settings_auto_clear_tx_desc">Při volání CQ vybere volné místo podle nedávné aktivity v pásmu a uhne, pokud se obsadí</string>
+    <string name="settings_min_auto_tx_freq">Minimální automatický TX audio kmitočet</string>
+    <string name="settings_min_auto_tx_freq_desc">Nejnižší audio offset, který aplikace sama zvolí (výběr volného offsetu, následování TX=RX). Zvyšte jej, pokud rádio TX audio ořezává nebo převádí na obdélník, aby harmonické padly mimo TX filtr</string>
     <string name="tune_content_description">Tune: spustí tuner rádia nebo vysílá souvislou nosnou pro ladění antény</string>
     <string name="tune_stopped_timeout">Tune zastaven po %1$d s</string>
     <string name="tune_blocked_tx">Před použitím Tune zastavte CQ/QSO</string>

diff --git a/ft8af/app/src/main/res/values-es/strings_compose.xml b/ft8af/app/src/main/res/values-es/strings_compose.xml
@@ -545,6 +545,8 @@
     <string name="cq_offset_moved">Desplazamiento de CQ movido a un hueco libre: %1$d Hz</string>
     <string name="settings_auto_clear_tx">Offset de TX libre automático</string>
     <string name="settings_auto_clear_tx_desc">Al llamar CQ, elige un hueco libre según la actividad reciente de la banda y se aparta si se ocupa</string>
+    <string name="settings_min_auto_tx_freq">Frecuencia de audio TX automática mínima</string>
+    <string name="settings_min_auto_tx_freq_desc">Offset de audio más bajo que la app elegirá por sí sola (selector de offset libre, seguimiento TX=RX). Súbelo si tu equipo recorta o cuadra el audio de TX, para que los armónicos queden fuera del filtro de TX</string>
     <string name="tune_content_description">Tune: inicia el sintonizador del equipo o transmite una portadora continua para ajustar la antena</string>
     <string name="tune_stopped_timeout">Tune detenido tras %1$d s</string>
     <string name="tune_blocked_tx">Detén el CQ/QSO antes de usar Tune</string>

diff --git a/ft8af/app/src/main/res/values-fr/strings_compose.xml b/ft8af/app/src/main/res/values-fr/strings_compose.xml
@@ -545,6 +545,8 @@
     <string name="cq_offset_moved">Décalage CQ déplacé vers un créneau libre : %1$d Hz</string>
     <string name="settings_auto_clear_tx">Décalage TX libre automatique</string>
     <string name="settings_auto_clear_tx_desc">En appelant CQ, choisit un créneau libre selon l\'activité récente de la bande et se déplace s\'il devient occupé</string>
+    <string name="settings_min_auto_tx_freq">Fréquence audio TX automatique minimale</string>
+    <string name="settings_min_auto_tx_freq_desc">Décalage audio le plus bas que l\'application choisira d\'elle-même (sélection du décalage libre, suivi TX=RX). Augmentez-le si votre poste écrête ou met en créneaux l\'audio TX, afin que les harmoniques tombent hors du filtre TX</string>
     <string name="tune_content_description">Tune : démarre le coupleur de la radio ou émet une porteuse continue pour l\'accord d\'antenne</string>
     <string name="tune_stopped_timeout">Tune arrêté après %1$d s</string>
     <string name="tune_blocked_tx">Arrêtez le CQ/QSO avant d\'utiliser Tune</string>

diff --git a/ft8af/app/src/main/res/values-in/strings_compose.xml b/ft8af/app/src/main/res/values-in/strings_compose.xml
@@ -580,6 +580,8 @@
     <string name="cq_offset_moved">Offset CQ dipindahkan ke tempat kosong: %1$d Hz</string>
     <string name="settings_auto_clear_tx">Offset TX kosong otomatis</string>
     <string name="settings_auto_clear_tx_desc">Saat memanggil CQ, memilih tempat kosong dari aktivitas band terkini dan berpindah jika terisi</string>
+    <string name="settings_min_auto_tx_freq">Frekuensi audio TX otomatis minimum</string>
+    <string name="settings_min_auto_tx_freq_desc">Offset audio terendah yang akan dipilih aplikasi sendiri (pemilih offset kosong, ikuti TX=RX). Naikkan jika radio Anda memotong atau mengubah audio TX menjadi gelombang kotak, agar harmonik jatuh di luar filter TX</string>
     <string name="tune_content_description">Tune: memulai tuner radio atau memancarkan gelombang pembawa kontinu untuk penalaan antena</string>
     <string name="tune_stopped_timeout">Tune dihentikan setelah %1$d dtk</string>
     <string name="tune_blocked_tx">Hentikan CQ/QSO sebelum menggunakan Tune</string>