To be fixed under the bike saddle.
Great just because it allows the movement on three axes, so there's complete freedom while biking.
Step 1: What you need
A cross bracket for antenna poles
Step 3: Assembling
Lock the bolt with the self-locking nut.
If you want, you can cut the exceeding bolt thread with a grinder before assembling.
Using the grinder, shape a piece of square 30x30mm iron pole as shown in the picture, then drill two passing holes, one for each axle, and fix the piece to the wheel frame by the horizontal axle.
The iron piece will allow the vertical movement, the trailer bar that is going to be attached will allow the horizontal movement and the bearing of the wheel frame the torsional movement.
A simple steel pin will be used to fix the trailer bar to the hitch.