diff --git a/src/Nonemm.App/Dialogs/ChangeOperatorDialog.axaml b/src/Nonemm.App/Dialogs/ChangeOperatorDialog.axaml
new file mode 100644
index 0000000..346f787
--- /dev/null
+++ b/src/Nonemm.App/Dialogs/ChangeOperatorDialog.axaml
@@ -0,0 +1,21 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        x:Class="Nonemm.App.Dialogs.ChangeOperatorDialog"
+        Title="Change Operator" Width="380" SizeToContent="Height"
+        WindowStartupLocation="CenterOwner" CanResize="False">
+  <DockPanel Margin="14">
+    <StackPanel DockPanel.Dock="Bottom" Orientation="Horizontal" HorizontalAlignment="Right"
+                Spacing="6" Margin="0,12,0,0">
+      <Button Content="Cancel" Click="OnCancel" IsCancel="True" />
+      <Button Content="Ok" Click="OnOk" IsDefault="True" />
+    </StackPanel>
+    <StackPanel Spacing="6">
+      <TextBlock Name="AboutText" TextWrapping="Wrap" />
+      <Grid ColumnDefinitions="*,6,150">
+        <TextBox Name="CallBox" />
+        <ComboBox Name="KnownBox" Grid.Column="2" HorizontalAlignment="Stretch"
+                  SelectionChanged="OnPickedKnown" PlaceholderText="known" />
+      </Grid>
+    </StackPanel>
+  </DockPanel>
+</Window>
diff --git a/src/Nonemm.App/Dialogs/ChangeOperatorDialog.axaml.cs b/src/Nonemm.App/Dialogs/ChangeOperatorDialog.axaml.cs
new file mode 100644
index 0000000..60d955b
--- /dev/null
+++ b/src/Nonemm.App/Dialogs/ChangeOperatorDialog.axaml.cs
@@ -0,0 +1,49 @@
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+
+namespace Nonemm.App.Dialogs;
+
+/// Which callsign to write against the contacts picked in the log. It closes
+/// with the callsign, or with null when nothing is to change. The operators
+/// already known to the program are in the drop-down; any other callsign is
+/// typed.
+public sealed partial class ChangeOperatorDialog : Window
+{
+    public ChangeOperatorDialog(int contacts, string callsign, IReadOnlyList<string> known)
+    {
+        InitializeComponent();
+        AboutText.Text = contacts == 1
+            ? "Operator callsign for the contact picked in the log"
+            : $"Operator callsign for the {contacts} contacts picked in the log";
+        CallBox.Text = callsign;
+        KnownBox.ItemsSource = known;
+        // the callsign is all this dialog asks for, so the cursor starts in it
+        // and the operator types over what is there
+        Opened += (_, _) =>
+        {
+            CallBox.Focus();
+            CallBox.SelectAll();
+        };
+    }
+
+    private void OnPickedKnown(object? sender, SelectionChangedEventArgs e)
+    {
+        if (KnownBox.SelectedItem is string callsign)
+        {
+            CallBox.Text = callsign;
+        }
+    }
+
+    /// An empty box leaves the dialog open: clearing the operator of a contact
+    /// is not what this dialog is for.
+    private void OnOk(object? sender, RoutedEventArgs e)
+    {
+        string callsign = (CallBox.Text ?? "").Trim().ToUpperInvariant();
+        if (callsign.Length > 0)
+        {
+            Close(callsign);
+        }
+    }
+
+    private void OnCancel(object? sender, RoutedEventArgs e) => Close(null);
+}