help with ground loop on antena

[imos]

i have a 48 inch half wave 27mhz CB antenna mounted outdoors with the lead in made with rg58 coax with an approx length of 54 feet it has terrible reception on all bands except for the Motorola trunking control frequencies 475.3750-947.3500mhz im using it with a pro 164 tri trunking scanner and when i hook my home theater system to it for an external speaker i geta ground loop that puts approx 21.3 volts ac on the shield of the coax and this is on a non grounded antena system and the reciver i use for an external speaker has a two prong polorized plug hard wired to it so there is no source of grounding from that to account for a ground loop help me figure this one out

[N_Jay]

Find a good ground, the measure the audio ground of the TV system, the shield of the antenna and the case of the scanner each independently.

Report back.