NanoVNA sweep of loop antenna with ladder line

The NanoVNA has been connected to the antenna system using a small SMA to SO 239 jumper cable. PL 259 on the 183 cm long HLF 200 coaxial cable in connected to the SO 239 connector. The coaxial cable is soldered to the terminals of 10 m long ladder line just outside the shack. 10 m long ladder line continues as the 40 m long loop antenna at the feed point, without any joint in between as it was originally a 60 m long wire, of which 20 m was utilized for the ladder line.

NanoVNA start frequency was set at 3 MHz and stop frequency at 28 MHz. Five major dips in the SWR pattern was visible. One at 14.5 MHz where the marker has been kept, shows SWR of 4.92.

Here the dip is at 23.5 MHz and the SWR seen is 2.628.

This image shows SWR dip at 19.75 MHz and the value is 1.701.

Here the SWR is 4.771 at 5.25 MHz. As you would have noted, none of these dips correspond to amateur radio bands. Hence it not possible to use this antenna-feedline combination without a tuner. That was anticipated as the ladder line has a high impedance compared to the radio and loop antenna. The loop antenna mounting pattern is quite irregular and does not correspond any of the known patterns like folded dipole, delta loop or circular loop. There are no suitable locations for any standard mounting at my location and hence the compromises. As the ladder line is continuous with the loop antenna, there is no scope for shortening or lengthening the loop antenna. It can only be matched using tuners or transformer.

I am using ATU-100 EXT and the internal antenna tuner of FT-710 to finally tune to an SWR of 1:1 on 7, 14, 21 and 28 MHz bands. It does not tune to the WARC bands at all. A homebrew common mode choke is in place between the feedline and the ATU-100.