diff --git a/tests/test_nmea/test_protocols.py b/tests/test_nmea/test_protocols.py
new file mode 100644
index 0000000..e2abac6
--- /dev/null
+++ b/tests/test_nmea/test_protocols.py
@@ -0,0 +1,44 @@
+"""Unit tests for NMEA structural protocols."""
+
+from typing import Any
+
+from nmea.protocols import NMEASentenceHandler
+
+
+class ConformingNMEADecoder:
+    """A dummy class that implements the NMEASentenceHandler protocol."""
+
+    def decode(self, sentence: str) -> dict[str, Any]:
+        return {"talker": "GP", "sentence_id": "RMC"}
+
+    def checksum(self, sentence: str) -> str:
+        return "00"
+
+
+class NonConformingNMEADecoderMissingChecksum:
+    """A dummy class missing the checksum method."""
+
+    def decode(self, sentence: str) -> dict[str, Any]:
+        return {"talker": "GP"}
+
+
+class NonConformingNMEADecoderMissingDecode:
+    """A dummy class missing the decode method."""
+
+    def checksum(self, sentence: str) -> str:
+        return "00"
+
+
+def test_nmea_sentence_handler_conforming() -> None:
+    """Test that a compliant class is recognized by the protocol."""
+    decoder = ConformingNMEADecoder()
+    assert isinstance(decoder, NMEASentenceHandler)
+
+
+def test_nmea_sentence_handler_non_conforming() -> None:
+    """Test that non-compliant classes are rejected by the protocol."""
+    missing_checksum = NonConformingNMEADecoderMissingChecksum()
+    assert not isinstance(missing_checksum, NMEASentenceHandler)
+
+    missing_decode = NonConformingNMEADecoderMissingDecode()
+    assert not isinstance(missing_decode, NMEASentenceHandler)
diff --git a/tests/test_protocols.py b/tests/test_protocols.py
index f352054..38034b6 100644
--- a/tests/test_protocols.py
+++ b/tests/test_protocols.py
@@ -2,7 +2,6 @@
 
 from ais.protocols import AISMessageHandler
 from aisutils.protocols import DatabaseBridge, GISExporter
-from nmea.protocols import NMEASentenceHandler
 
 
 class DummyAISDecoder:
@@ -13,14 +12,6 @@ def encode(self, params, validate=True):
         return None
 
 
-class DummyNMEADecoder:
-    def decode(self, sentence):
-        return {"talker": "GP"}
-
-    def checksum(self, sentence):
-        return "00"
-
-
 class DummyDBBridge:
     def sql_create_table(self, outfile, db_type="sqlite", table_name=None):
         pass
@@ -39,11 +30,6 @@ def test_ais_message_handler_protocol():
     assert isinstance(decoder, AISMessageHandler)
 
 
-def test_nmea_sentence_handler_protocol():
-    decoder = DummyNMEADecoder()
-    assert isinstance(decoder, NMEASentenceHandler)
-
-
 def test_database_bridge_protocol():
     bridge = DummyDBBridge()
     assert isinstance(bridge, DatabaseBridge)