Matthew&Wendy wrote:
I have a simple scanner/shortwave receiver and have recently installed a
wire antenna. I have been able to pick up stations over 500 miles away.

OK. You should be able to hear stations thousands of miles away on
shortwave. Where are you located?

At night, listen 5800 to 6200 kHz, 7100 to 7600 kHz, 9400 to 10000 kHz.

During the day, try 11,500 to 12,100; 15,000 to 15,600; 17,500 to 18,000
kHz.

However, it seems as though the stations are grouped in very narrow bands.

They are. See above.

Is there a way to make the antenna "look" longer or shorter electronically
so that I have a greater range of frequencies?

Yes, but that's not really necessary. What Make/Model radio are you using?
Is it a small handheld or portable unit? If so a "good" antenna may very
well overload it with strong signals.

Art N2AH