Did I spoil my nice multiband fan dipole?
Those of you who have been following my posts would know that I have been trying to come up on 80m for the past few weeks. Initially I tried a 20/80m loaded coil dipole antenna, which never resonated on 80m. It did resonate on 20m, 30m and 12m intermittently while was trying various combinations of coil inductance and trimming/lengthening of antenna elements before and after the coil. I could also get spotted on various bands, even at far away places, on Reverse Beacon Network on CW. Next I removed the loading coils and added extra wire to make it a linear loaded dipole for 80m. That attempt also failed to reach 80m, though I had partial success with tunable SWR on the higher end of VU 80m band.
Today I thought of trying out loaded coil 40/80m antenna. Calculated that I need to add 1.5m of wire on either side along with my 120 microhenry loading coils. As I did not have space to accommodate the extra length, I added the coil and kept the extra wire at almost 90 degrees to the inner antenna elements in the horizontal plane, hoping that it might work. SWR prior to the alteration on 40m was excellent, 1.1 from 7010 to 7100 kHz. It became HI SWR through out the 40m band after adding the loading coil and pigtail to either side. So that effectively cuts me off from my favourite 40m ragchews! Fortunately changes on 20, 15 and 10m were only mild and acceptable. In fact, it was better on 21010 kHz, where it came down from 4 initially to 2.2. Upper end of 20m continued to be at an SWR of 1.1, which is excellent for me.
What would have caused this catastrophe on 40m? I am considering three possibilities. I had unfolded the terminal end of 40m elements, may be by 30cm or more while connecting the loading coils, which would shift the resonance off the 40m amateur radio band. Secondly, the loading coils would have changed the resonance point of the 40m elements. Thirdly, the terminal pigtail which is almost at right angles to rest of the antenna elements, in the horizontal plane, could also be interacting to offset the point of resonance. Anyway, I have ordered for a NanoVNA a couple of days back and is expecting to get it after a few days. As further alterations would mean trimming previously perfectly tuned 40m elements, I wish to wait for the NanoVNA and check the current resonant points before making any trimming. I have learned it the hard way during my 20/80m loaded coil experiments with repeated trimming and lengthening and finally abandoning the project!