Strangely, although the J-pole is supposedly a narrowband antenna and the OCFD is supposed to be wide bandwidth, I've found that a J-pole made out of 300 ohm twinlead performs better than an off center fed dipole over a wide range of freqs.
Specifically, I've built twinlead J-poles and Slim Jim antennas cut for 124MHz (VHF airband) and 157MHz (marine band) and find they peform better than an OCFD. For the OCFD I used 18" and 48" elements.
I also found the J-pole significantly better than either a directly fed dipole or folded dipole fed with 4:1 balun. The dipole and folded dipoles were also built for 124MHz and 157MHz.
Theoretically, the dipoles should have had the same performance as the J-pole and Slim Jims, but the dipoles are noticeably less sensitive.
One thing that may be causing the abnormal results is that I'm using all of these antennas indoors, sometimes taped against a wall, sometimes hanging from a window sill. This probably lowers the impedances a bit.
The J-poles cut for 157MHz do a pretty good job in the 42MHz VHF low police band, and also in the 460-485MHz band. They are a bit less sensitive in the VHF airband than is the airband J-pole, but both are lightyears ahead of the rubby ducky antennas and even an outside mounted RadioShack 20-176 scanner antenna.
I don't have any test equipment, so my tests and trimming are based solely on comparing the reception on weak signals, preferably always on, constant strength signals such as VORs and ATIS for the VHF airband, and NOAA Wx radio just above the marine band.
It doesn't take much to build these simple antennas. Why don't you just build both the J-pole and OFCD and tell us the results.
My next little mini-project will be a twinlead J-pole and a twinlead Slim Jim that are fed not by 50 ohm coax, but instead are fed by the same 300 ohm twinlead that I use for the matching stub. Then I'll use a 4 to 1 balun right at the scanner.
The only change in the design is a slight change in the total stub length, and movement of feedline connection point to about 2.5" up from the short. For 156.8MHz, and assuming 0.8 velocity factor for the stub and 0.95 for the dipole, the lengths I'll be using are 51" total; 35-3/4" for the dipole (or folded dipole for the Slim Jim) and 15-1/4" for the bottom stub. The 300 ohm twinlead feed will attach to the stub at about 2.5" above the short. (Calculations are 2" for the J-pole, 3" for the Slim Jim. I split the difference and try the antenna in both the J-pole/dipole and the Slim Jim/folded dipole modes. The only difference is whether the two wires at the top of the antenna are open or shorted together).
When I build the antennas, I will probably make the the feedline and the upper part of the tuning stub as one continous piece of twinlead, and then solder on the lower shorting stub where normally the feedline coax is soldered on.
has an excellent analysis of the J-pole antenna and some useful basic and excel calculators.