Sprint Galaxy S4 Problems with dropped calls and data connection

Hello. I have been having problems with my Sprint S4 model SPH-L720 This is the S4 that has the extra red antenna on the left side.

At my home I do not get any data network connection and it drops calls frequently. The signal indicator bars go up and down in a matter of seconds and my battery is draining fast. Sprint swears it is not my phone, they just did tower upgrades in town, and I have spoken to them multiple times about this issue.

The only odd thing is, I activated a friends Galaxy S3 and I get great cell and 4G data connection! So my question is, what part(s) may be causing this problem?

Thanks so much in advance for any and all help! Jason