Yes it does have an antenna port.
It is on top same side as power switch. The cover is a little difficult. Just lift straight up.
I glued a 12 inch pie plate with dicor on the roof of my trailer.
The pie plate was grounded to the trailer frame.
Thus the pie plate serves as a magnetic base for my magnetic antenna and a ground plane for the antenna.
It does work with a significant increase in signal strength.
My 5th wheel has the groilla cage aluminum frame which is not good for radio signals inside the trailer.
Search for antenna adaptors and antennas for the 6620L.
