FIRST thing VERY FIRST THING you need to do is FIND OUT where the FM stations are...miles and terrain. alternatively, or in addition, can you receive these from your car while pulled away from the shop? How about the house? Does a system in the house work well?
You have two potential problems.........low signal strength, and HIGH NOISE LEVEL
Lots and lots of "stuff" generates noise, carried over the power system. Microprocessor in your micorwave, stove, washer dryer, computers, modems, you name it, generates noise, even when "off." ONE OF THE worst noise makers in my house is the tiny "switching" charger for my cell phone
A GOOD outdoor antenna is a good idea, fed with coax just like TV, but if the stations are too far away, or mountainous territory, you might not gain enough
WHY your present antenna may not work:
May well be noise generated by "something" in the shop / over power lines, and that antenna is picking up that noise and since your makeshift antenna is "just a wire" it is picking up the noise right from the radio
JUST USING COAX to isolate a makeshift antenna may now work, either. There is a situation do to mismatch between the antenna / coax (swr) that will cause the coax to what is called "radiate." This means that the coax outer shield becomes part of the "radiating" system, and will pick up and conduct noise into the radio. You must have a DESIGNED matched system. On something like a TV antenna (similar) which is 300 ohms, you must have a 300 ohm--to 75 ohm balun transformer.l (Balun means BAlanced to UNbalanced), that is twinlead to coax
IT IS IMPORTANT TO UNDERSTAND that many home made "internet" design antennas do NOT PROPERLY ADDRESS the situation of properly matching coax to the antenna and creating proper shield isolation so that the coax does not pick up noise
If stations are close enough you don't need a "gain" antenna, or if they are from several directions, a turnstile or "S" antenna might be a good choice
FM Antennas