Designing a generic associative table

Question

I need the ability to associate two entities of the same or different types. Take the concept of planes and trucks that have the ability to radio to each other.

+----------------------+
|        Airplane      |
+----------------------+
| AirplaneId           |
| Wingspan             |
| MaxTakeoffWeight     |
| etc...               |
+----------------------+

+----------------------+
|         Truck        |
+----------------------+
| TruckId              |
| CargoCubicFt         |
| IsResidential        |
| etc...               |
+----------------------+

+----------------------+
| CommunicationBetween |
+----------------------+
| [AirplaneOrTruckId]  |
| [AirplaneOrTruckId]  |
+----------------------+

What's the best way to accomplish these associations?

Notes

I can't combine or abstract out the two entities (e.g. can't combine planes and trucks into a Vehicles table) because each entity has many unique properties that the other doesn't have.

The only other idea I have is to create a table like this:

+----------------------+
| CommunicationBetween |
+----------------------+
| Entity1              | -- "Airplane" or "Truck"
| Id1                  | -- AirplaneId or TruckId
| Entity2              | -- "Airplane" or "Truck"
| Id2                  | -- AirplaneId or TruckId
+----------------------+

... but that's not an elegant solution.