You could use NEC or MININEC to model the antenna and expected impedance. And why yes, I have an app for that:
http://www.blackcatsystems.com/software/mininec-antenna-analysis-modeling-software.html I'll be honest and say that while it will give you a rough idea of how the impedance is going to vary over the HF bands, there's a lot of "known unknowns" as they say, such as the fact that the actual antenna geometry is not going to match what you put into the program (exact distances, heights, etc). Throw a few unknown unknowns into the mix, and it gets worse. You can tell how likely the model is to match reality by making small changes to your geometry (length, height etc) and see what happens to the output. If it varies a little, it is stable, and may be close to reality. But if you make small changes and the output varies wildly, then it's not stable, and probably just generating random numbers.
Yes, if you could make your loop a little longer and get to around a wavelength for 43 meters, that would be better, but 100 ft is probably not going to be to too bad for a receive only antenna.
For reference, my sky loop is about 670 ft, or 206 meters. Down on the medium wave band, it does receive all the way down to 530 kHz, where it is only 0.36 wavelengths long. Sure, the signals are not as strong as up around 1500 kHz, but still quite good.
This is where I add a shameless plug for The Squid universal matching transformer
http://blackcatsystems.com/rf-products/matching-transformer-unun-balun-beverage-longwire-k9ay-flag-ewe-dipole-antenna-shortwave-ham-radio.html Try the various impedances and see which works best for the band(s) of interest. Plus if you ever change the antenna (add additional wire, re-route it, etc) you can re-try the impedances to see if there is a better match, without buying a new balun.