New blunder or wonder with fan dipole antenna?
My complex fan dipole was having reasonable SWR on 40, 30, 20, 15, 10 and 6m. I wanted to simplify the antenna a bit and improve performance on 30. I thought of removing the loading coils on 20m elements and folding back the 40m elements instead. My hope was that 30m dipole would perform better than the loaded coil antenna which could waste a bit of power in the coils. I was ready to forego 40m and third harmonic 15m for the time being. I folded back the 40m elements by 2.5m on both sides. Loading coils on 20m elements were removed along with the extension section. As soon as I had finished the work came the painful realization that 30m will have third harmonic on 10m. Basic principle of fan dipole is that elements should not have odd harmonic relationship. That would mean that both the 30m elements and 10m elements would resonate on 10m. My simple mental maths was that it would reduce the antenna impedance from 50 ohms to 25 ohms on 10m. That would mean high SWR on 10m. But to my pleasant surprise, SWR on 10m band was very low, near 1!
Previous clip had shown the low SWR on 28.010 MHz. This one shows low SWR on 28.075 MHz.
SWR was low on 28.150 MHz as well.
The same pattern continued up to 28.200 MHz, beyond which it was slightly more. As expected, SWR on 40 and 15m were very high. Unfortunately SWR on 20m also was very high and I was perplexed, because I had not expected that. Previously the fan dipole had very good SWR on 20m.
As usual analysis with NanoVNA made the picture quite clear. SWR dip at 28 MHz was 1.322, explaining the SWR pattern noted in the radio.
Next dip in SWR was at 25 MHZ: 3.085. Luckily, it was tunable with the auto-tuner on FT-710 on 12m amateur radio band, which is just below that.
Dip in SWR at 14.750 MHZ: 1.209 explained why 20m amateur radio band has high SWR. But I can’t explain why the point of resonance has gone beyond the upper limit of the band. I had not shortened the previously functioning elements. Hardly 5 cm has been folded back at the ends. My option now is to lengthen that part and see if the resonance returns to 20m amateur radio band.
Next dip in SWR at 8.5 MHZ: 2.338 explained why 30m band has very high SWR. If I have to bring the resonance to 30m band, I will have to shorten the elements a bit. Then most likely interaction with 10m elements will occur and I will lose the excellent SWR on 10m band. Now the elements meant for 30m are not affecting 10m band because the third harmonic of 8.5 MHz will be 25.5 MHz. Recall that we saw an SWR dip at 25 MHz on NanoVNA.
For completion sake, I checked SWR on 6m band as well. It was 2.433 at 50.300 MHz. That is tunable in my FT-710 radio. So I still have access to 6m band, whereas I have lost access to 40m, 30m, 20m and 15m. For time being I wish to try my luck with the best ever SWR I have on 10m for a few days before I make any further changes to the antenna.
As a bonus, SWR is tunable in 12m amateur radio band. I will also try my luck on that band as I do not have any contacts on that band so far. The elements originally meant for 30m may be giving resonance on 12m as discussed from the NanoVNA tracings.
Finally, I could have quick QSOs on 12m with 4Z5PT at 4797.8 km, UT3QW at 5854.0 km and SM4IED at 7401.9 km . So this antenna experiment is not all of blunder, but there is some wonder as well, in giving me a new band!