Why is it that my full loop antenna for 40m with ladder line not working on 40m?

My full loop antenna with 40m of wire for the loop and 10m homebrew ladder line has been working well on 20m and higher bands with ATU-100 tuner, but not on the 40m band. Lowest SWR on a NanoVNA sweep was noted at 23.25 MHz! This NanoVNA sweep was taken with a homebrew 4:1 balun, common mode choke and 183 cm of HLF 200 coax in series with the ladder line-antenna system. Read an article by WB6BYU on theory of full wave loops, forwarded to me by VU3CDK. The first few lines have the answer probably. Traditional formula for length in meters has been given as 306/MHz. For the center frequency of the 40m band in this region of 7.1 MHz, it comes to about 43m.

That means my ‘full wave’ loop is shorter by 3m! Unlike the dipoles, larger diameter conductors need larger circumference for resonance. My antenna wire has 2.5 sq mm of multi-stranded insulated copper wire meant for electrical wiring. As the antenna wire is continuous with the ladder line, with original total wire length of 60m of which 20m has gone into two sides of the ladder line, only way to lengthen the antenna is to remove some of the ‘steps’ of the ‘ladder’ and test.

Full wave loop antenna is like two dipoles connected together at the ends or like a folded dipole with increased separation between the elements. Radiofrequency current on the full wave loop has two maxima, one at the feed point and another at the point on the loop opposite to the feed point. There are two points of minimum current at the midpoint between the two points mentioned. If the loop is mounted in four quarter wavelengths, current in parallel regions of the loop are in phase. This is probably the reason for broadside gain of such loops. In my case, the antenna does not conform to any particular shape and has feed point at one side at present. Feed point impedance may vary from 115 to 135 ohms depending on the shapes and location of feed point. So that is another source of mismatch with ladder line. My ladder line has a calculated impedance of about 466 ohms.

Remaining discussion in the video only as it was extempore!