I have a table top HD Radio, the NS-HDRAD2. I am having problems with getting many of the local radio stations to come it. I'm in a cement building and have it as close to the window as I can in my office. Is there a way to connect an external antenna to this radio? It doesn't have the plug in the back like the NS-HDRAD. Is there some type of converter that can be plugged into the AUX or the headphone jack?