Total posts : 45366
The input for your transmitter can be A-OK and your PC Sound card output can be working great, but there might be an impedance mis-match between the output and input that distorts the audio without overloading the input level.
One of the Talking House transmitters I use has a somewhat peculiar input impedance and I had to use a matching transformer (8-1000 Ohm, CT) to get clear audio. You could also try running it through a mixer for a better impedance match and some pre-amplification.
There also might be some PC crud (a PC is a nightmare of RFI) sneaking up the audio cable. Perhaps try an isolation transformer in the audio chain.
The telescoping antenna should be fine, can’t imagine how that would gaak the audio.