diff --git a/quisk.py b/quisk.py
index 24d2ee5..644e187 100644
--- a/quisk.py
+++ b/quisk.py
@@ -39,6 +39,7 @@ from filters import Filters
 import dxcluster
 import configure
 import quisk_conf_defaults as conf
+import quisk_utils
 import quisk_wdsp
 
 for name in configure.name2format:      # Add defaults. These may be overwritten by a user config file.
@@ -2584,7 +2585,7 @@ class GraphScreen(wx.Window):
     else:
       self.mouse_is_rx = False
     if self.mouse_is_rx:
-      freq = application.rxFreq + self.VFO + wm * event.GetWheelRotation() // event.GetWheelDelta()
+      freq = application.rxFreq + self.VFO + quisk_utils.wheel_step(event.GetWheelRotation(), event.GetWheelDelta(), wm)
       if conf.freq_spacing:
         freq = self.FreqRound(freq, 0)
       elif freq >= 0:
@@ -2594,7 +2595,7 @@ class GraphScreen(wx.Window):
       tune = freq - self.VFO
       self.ChangeHwFrequency(self.txFreq, self.VFO, 'MouseWheel', event=event, rx_freq=tune)
     else:
-      freq = self.txFreq + self.VFO + wm * event.GetWheelRotation() // event.GetWheelDelta()
+      freq = self.txFreq + self.VFO + quisk_utils.wheel_step(event.GetWheelRotation(), event.GetWheelDelta(), wm)
       if conf.freq_spacing:
         freq = self.FreqRound(freq, 0)
       elif freq >= 0:
diff --git a/quisk_utils.py b/quisk_utils.py
index 4abe8a3..cbf70a0 100644
--- a/quisk_utils.py
+++ b/quisk_utils.py
@@ -1,6 +1,29 @@
 from __future__ import print_function
 from __future__ import division
 
+
+def wheel_step(rotation, delta, step):
+  """Convert signed wheel rotation into configured frequency steps.
+
+  A measured downward event reported rotation=-180 with delta=120. Scaling
+  that by a 50 Hz step gives -75 Hz; flooring the resulting absolute frequency
+  made downward tuning jump -100 Hz while upward tuning moved only +50 Hz.
+  Round the wheel units symmetrically first: +/-180 now gives +/-50 Hz, while
+  +/-240 still gives +/-100 Hz rather than being collapsed to one step.
+  """
+  if delta == 0:
+    return 0
+  rotation = int(rotation)
+  delta = int(delta)
+  step = int(step)
+  direction = -1 if rotation * delta < 0 else 1
+  whole_steps, remainder = divmod(abs(rotation), abs(delta))
+  if remainder * 2 > abs(delta):
+    whole_steps += 1
+
+  return direction * whole_steps * step
+
+
 class SplineInterpolator:	# From Numerical Recipes in C
   """Interpolate a table of [x, y] values."""
   def __init__(self, table, Xmin=None, Xmax=None):
diff --git a/quisk_vna.py b/quisk_vna.py
index cd23f57..b004e7b 100644
--- a/quisk_vna.py
+++ b/quisk_vna.py
@@ -26,6 +26,7 @@ import threading, webbrowser
 import _quisk as QS
 from quisk_widgets import *
 import configure
+import quisk_utils
 
 DEBUG = 0
 
@@ -630,7 +631,7 @@ class GraphScreen(wx.Window):
       mouse_x, mouse_y = self.GetMousePosition(event)
       self.SetTxFreq(index=mouse_x - self.originX)
   def OnWheel(self, event):
-    tune = self.display.tune_tx + event.GetWheelRotation() // event.GetWheelDelta()
+    tune = self.display.tune_tx + quisk_utils.wheel_step(event.GetWheelRotation(), event.GetWheelDelta(), 1)
     self.SetTxFreq(index=tune)
 
 class HelpScreen(wx.html.HtmlWindow):
diff --git a/test_wheel_steps.py b/test_wheel_steps.py
new file mode 100644
index 0000000..db80470
--- /dev/null
+++ b/test_wheel_steps.py
@@ -0,0 +1,26 @@
+import quisk_utils
+
+
+def test_wheel_step_rounds_half_steps_toward_zero():
+    assert quisk_utils.wheel_step(120, 120, 50) == 50
+    assert quisk_utils.wheel_step(-120, 120, 50) == -50
+    assert quisk_utils.wheel_step(180, 120, 50) == 50
+    assert quisk_utils.wheel_step(-180, 120, 50) == -50
+
+
+def test_wheel_step_preserves_whole_multiple_steps():
+    assert quisk_utils.wheel_step(240, 120, 50) == 100
+    assert quisk_utils.wheel_step(-240, 120, 50) == -100
+    assert quisk_utils.wheel_step(180, 120, 1) == 1
+    assert quisk_utils.wheel_step(-180, 120, 1) == -1
+
+
+def test_wheel_step_handles_zero_delta():
+    assert quisk_utils.wheel_step(180, 0, 50) == 0
+
+
+if __name__ == '__main__':
+    test_wheel_step_rounds_half_steps_toward_zero()
+    test_wheel_step_preserves_whole_multiple_steps()
+    test_wheel_step_handles_zero_delta()
+    print('wheel_step ok')
\ No newline at end of file