diff --git a/pm3py/core/hf.py b/pm3py/core/hf.py
index 2d118ac..03305e5 100644
--- a/pm3py/core/hf.py
+++ b/pm3py/core/hf.py
@@ -16,9 +16,17 @@ class HFCommands:
         self.mfc = None  # type: ignore
 
     async def tune(self, on_progress: ProgressCallback = None) -> dict:
-        """Measure HF antenna voltage."""
-        resp = await self._t.send_ng(Cmd.MEASURE_ANTENNA_TUNING_HF, timeout=10.0)
-        voltage = struct.unpack_from("<I", resp.data, 0)[0] if len(resp.data) >= 4 else 0
+        """Measure HF antenna voltage.
+
+        CMD_MEASURE_ANTENNA_TUNING_HF is a START/MEASURE/STOP state machine that
+        requires a 1-byte mode arg (firmware rejects length!=1 with EINVARG):
+        mode 1 = START (spins up the HF FPGA), mode 2 = MEASURE (returns uint16 mV),
+        mode 3 = STOP. Voltage is a uint16_t, not uint32_t.
+        """
+        await self._t.send_ng(Cmd.MEASURE_ANTENNA_TUNING_HF, bytes([1]), timeout=10.0)   # START
+        resp = await self._t.send_ng(Cmd.MEASURE_ANTENNA_TUNING_HF, bytes([2]), timeout=10.0)  # MEASURE
+        voltage = struct.unpack_from("<H", resp.data, 0)[0] if len(resp.data) >= 2 else 0
+        await self._t.send_ng(Cmd.MEASURE_ANTENNA_TUNING_HF, bytes([3]), timeout=10.0)   # STOP
         result = {
             "voltage_mV": voltage,
             "voltage_V": round(voltage / 1000.0, 3),
diff --git a/tests/test_hf.py b/tests/test_hf.py
index 281aa86..d700a37 100644
--- a/tests/test_hf.py
+++ b/tests/test_hf.py
@@ -7,7 +7,12 @@ from pm3py.core.hf import HFCommands
 def test_hf_tune():
     t = AsyncMock()
     hf = HFCommands(t)
+    # MEASURE (mode 2) returns the voltage as a uint16 mV; 15485 mV = 0x3C7D
     t.send_ng.return_value = PM3Response(cmd=Cmd.MEASURE_ANTENNA_TUNING_HF, status=0,
-                                         reason=0, ng=True, data=b"\x00" * 4)
+                                         reason=0, ng=True, data=b"\x7d\x3c")
     result = asyncio.get_event_loop().run_until_complete(hf.tune())
-    assert "voltage_mV" in result
+    assert result["voltage_mV"] == 15485
+    assert result["voltage_V"] == 15.485
+    # Firmware requires the START(1)/MEASURE(2)/STOP(3) mode-byte sequence
+    modes = [call.args[1] for call in t.send_ng.call_args_list]
+    assert modes == [bytes([1]), bytes([2]), bytes([3])]