If you find the FM radio performance lacking, don’t bother ordering a signal performance booster, as the OEM antenna already has an integrated powered amp. As others have noted, you should check to insure that the antenna mast is snugly screwed into the antenna mount. You probably shouldn’t get a ‘stubby’ antenna either, as your radio performance may suffer some more.
Ideally, a FM antenna length should always be 31 inches, and that is why you’ll notice that a wire is spiraling along the shaft of the shorter 16″ OEM antenna to replicate that. You might not like the look, but if you are really out in the sticks, you might actually try a ‘true’ 31″ antenna.
If a FM station offers HD, you’ve probably noticed that the sound quality is best when HD is displayed. When the digital signal strength is inadequate, you should notice a degradation in the sound quality as the radio switches back to the regular analog signal.
The FM antenna also serves as your AM radio antenna, but your GPS Navigation antenna (behind your instrument cluster) and your XM radio antenna (sharkfin on your trunk lid) are separate.
Regarding the popular aftermarket 5″ rubber ‘stubby’ antennas, they too have a 31″ wire coiled internally around a much shorter shaft, but they will never pull in stations as well as OEM or true 31″ antennas, but if you spend most of your time near a large city, you may find the performance to be fine. This ‘stubby’ that is also marketed for the Mustang fits very nicely on the 124 Spider, as shown ⇓.
21ˢᵗ Century Fiat / Abarth 124 Spider 
It would be interesting to see if there is a powered extendable antennae available on the market that would also be an installation option. I have a factory installed version of one of these on my Mitsubishi Montero (yes, it’s old… 2005 model, but with 80,000km or 50,000 miles on the odometer… it’s been spoiled). Anyway, to get back on track, what I love about it that that when it’s not in use is fits seamlessly into a socket mounted flush to the body of the vehicle. When I need it, I press the panel switch…. and up it comes. It used to do this automatically whenever you started the vehicle, but an underground garage incident in which I left the antenna in the grips of some overhead duct-work had me install an override switch hat controls when it goes up and down. I don’t really use FM very much as I’m an XM fan, but if I were…. I’d be trying to see if I could install a telescoping FM antennae.
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Mark Booth has a great article in the miata forum on modifying the antenna for CB use (also read the comments), but he has observed that on our vehicle (and the ND):
so it may make it difficult to make any changes. Amazon has a few Motorized AM/FM Fully Automatic Power Antennas, but I’m quite sure the power wires would be inadequate and would need to be retrofitted. Also,these antenna were always activated with the radio power switch, which doesn’t exist in the Spider.
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