Antennacraft FM-6 is 70-in long with medium Gain (5-6 dBd = 7-8 dBi): [dBd + 2.15 dB = dBi]
So pretty much anything you chose from Beasley's chart would be an improvement.
Unfortunately, some of the best performers are either no longer in production (APS) or are
likely to be available only ($$$$) from overseas sources (Korner).
You didn't say what would be an "acceptable" boom length.....
K7MEM's On-Line (Narrowband, 50 or 200-ohm) Yagi Calculator results for 103 MHz.
Gain & dimensions are completely determined by the frequency....there are no in-betweens:
103 MHz, 5 Elements, 7.8 dBd = 10.0 dBi, 77.8-in long
103 MHz, 6 Elements, 8.9 dBd = 11.1 dBi, 105.5-in long
103 MHz, 7 Elements, 9.8 dBd = 12.0 dBi, 137.5-in long
103 MHz, 8 Elements, 10.6 dBd = 12.8 dBi, 172-in long
There are very few High Gain FM Antennas on the market (A-D, AntennaCraft & C-M are all low-gain).
Fortunately there are several choices that are only slightly longer than the obsolete A-C FM-6:
Winegard HD6055P, up to 9 dBi Gain, 82-in: http://www.winegard.com/kbase/upload/HD6055P.pdf
Wade C-55-FM, 9.5 dBi Gain, 79-in: http://www.wade-antenna.com/Wade/CSeries.pdf
Wade J-55-FM, 10 dBi Gain, 79-in: http://www.wade-antenna.com/Wade/jseriesyagi.pdf
I'm not sure what the length is for this one (148-in???)...their spec chart is very confusing:
Wade C-101-FM, 10.5 dBi Gain, ???-in: http://www.wade-antenna.com/Wade/CSeries.pdf
You might want to select one of the DIY projects on Beasley's website, perhaps
using the dimensions of one of the (too big???) high performance commercial antennas:
Or rescale ALL of the dimensions of one of the Hi-VHF Yagi's I analyzed here:
[Dimensions found in the 4nec2 Files.]
Note Gain curves for most Yagi's slope upwards, reaching max on the highest freq.
Hi-VHF 10-Element Folded Dipole Yagi - Lorkoe's Antique: 11.5 dBi Gain, ~2*78-in long
Hi-VHF 8-El Ch11-13 Yagi (K7MEM On-Line Calculator): up to 13 dBi Gain, ~2*87-in long.
After 2:1 Rescale, these are probably too long for you....
To Rescale from say a Hi-VHF antenna with max Gain on Ch13 (216 MHz) to max Gain
on say 108 MHz), enlarge ALL dimensions by the ratio 216/108 = 2.0.
When I say ALL, I also mean the element diameters...although smaller, "standard" size
elements (e.g. 3/8-in) should work just fine in the FM Band....with a slightly higher SWR.
You can Rescale a 4nec2 File to make sure by multiplying all of the X,Y,Z parameters
by the above Rescale Factor (I define a Rescale Factor "F" via a SYmbol statement).
If there is an FM Antenna thread on this forum, I couldn't find it. Here is
"EV's Best Top Rated FM and HD Radio Antenna Guide & Reviews":
Worldwide TV-FM DX Association (WTFDA) Forum also has a lot of useful info re FM Antennas, et. al.: