Hi Baz and Geoff,
Re: Capture area is a function of wavelength that is why physically small HF antennas work as well as large antennas. The main issue with small HF antennas is the very low radiation resistance. For these small antennas to work efficiently the "ohmic losses" must be held much lower than the radiation resistance hence the thick conductors - much greater attention must be paid to construction. Smaller antennas have a major drawback in that they have mucher lower bandwidth (high Q) and must be tuned to each freq. for best performance.
eg. A half wavelength vertical has a radiation resistance of many thousands of ohms a short vertical (DDRR antenna) may be 0.1 Ohms. Both these will perform the same except that the half wavelength antenna can be made much more efficient than the short antenna.
OK on the choice of antennas except that I would steer well clear of helical antennas at the frequency in question. The helical is a wonderful antenna at 400MHz and above where you would normally use them in groups of 2, 4, 8 etc. to act as 1 hi-gain antenna.
Suggestion: As you have increased the range of antennas to choose from and noting that the signals you are interested in go upto 40MHz. You may consider a log periodic antenna that will operate on all freqs from 20 to 40MHz.
Cheers and Beers,
Jerry.
