Back on 80m dipole with good SWR

I had a feeling that loading coils were eating away the power output of my radio on 80m. I presumed that from the relatively lower SNR noted on reverse beacon network. Hence I decided to go back to standard dipole for 80m, though mounting constraints were still there. Took total of 20.5m of insulated copper wire on either side and mounted with apex at about 6m from ground. One end of the antenna was at 3m and another end at about 2m. That is due to lack of tall supports in my compound. The height of the antenna is much below the recommended quarter wave length which comes to 20m for 80m band. Initial resonance was at 3.4 MHz. When I reduced 1m of wire on either side by folding back and twisting, resonance moved to 3.5 MHz. I accepted that for time being as the minimum SWR in the radio is usually slightly above the resonance seen on NanoVNA in my setup.

For more precise reading, the NanoVNA stimulus frequency range was changed to 3-4 MHz from the initial 1-11 MHz. You may be aware that NanoVNA checks SWR by sending tiny signals at 101 frequency steps within the set range for checking SWR. So the frequency steps will be wide when the range is kept between 1-11MHz. That was why lower range was set on repeat measurement for getting a more accurate picture of the SWR pattern. Here the minimum SWR of 1.469 is seen at 3.480 MHz, slightly below the 80m amateur radio band. SWR noted on VU 80m band is tunable with the built-in antenna tuner of my FT-710 radio.

This tracing on NanoVNA shows initial third harmonic resonance at 10.4 MHz, above the 30m amateur radio band.

After shortening the antenna to improve resonance on 80m band, third harmonic resonance was noted at 10.9 MHz, well above the 30m amateur radio band. Hence I will not be able to use this antenna to work on 30m with the built-in antenna tuner in my FT-710 radio. If I get an external antenna tuner, it might still be possible to work 30m band with this antenna on third harmonic resonance.