Swap the scanner antenna onto the Kenwood temporarily and if it still works fine, you know the antenna is good.The base radio kenwood tk 7180 coming off the other antenna works fine.
I do use a MCA204M
VHF/UHF Receiver Multicoupler - 25 MHz to 1 GHz
For the scanners
Same thing with the multicoupler...try with and without it to make sure it's not the problem. (or being overloaded...I don't have one, but if it is amplified, it could be overloaded too).