Might be your gain is set too high.
Thoughts on optimizing gain - #2 by wiedehopf
Other possibility: Interference, you might need the FA filter in addition to the integrated filter.
RG58 isn’t good cable, but for 3 ft it’s not too bad.
With such a short cable i would assume an indoor antenna?
Depending on the materials of the house around the antenna, results can vary dramatically.
Above the house is always best, if that’s not possible on the side of a house is the 2nd best option for a good 180 degrees of view.
Also depends on other houses nearby of course.
ADS-B is mostly line of sight.