Simple Analysis of a Complex Antenna with NanoVNA

I have a multi-band fan dipole antenna with elements for 40, 20 and 10m, which can also work on 15m (21 MHz) as the third harmonic of 40m (7 MHz). This antenna has four regions of resonance on NanoVNA as posted earlier. Now I have added two loading coils of 122 microhenries each, to the 20m elements to get additional resonance on 80m. After the loading coils, an extra length of 2.5m wire has also been added.

The first picture is SWR pattern as seen on NanoVNA for the original multi-band fan dipole antenna. Second picture shows the change after adding loading coils and extra pigtail segment to the 20m elements. At one look there are five major dips in the SWR. But there is a caveat, first picture shows frequency range from 6.5 MHz to 28 MHz at the bottom of the screen. Second picture shows a range of 3 to 28 MHz.

Thought of looking at the SWR pattern in a little more detail and analysed from 1 to 11 MHz. Here you can see a minor dip in SWR at 1.2 MHz with SWR 3.453.

The next minor dip in SWR is seen at 4.5 MHz with an SWR of 3.781. Cursor in NanoVNA is moved using the rotary function switch at the top, which can be moved to either side.

Next bigger dip is seen at 7 MHz with SWR 2.828, followed by another dip of similar magnitude nearby. Though these values are not ideal, they can be very well tuned with the auto-tuner in my FT-710 radio. There will be minor differences in SWR seen on NanoVNA and that seen in the SWR meter of the radio.

The second dip is actually at 7.5 MHz, above the 40m amateur radio band, with SWR 2.204 and hence not useful for me! NanoVNA analyzes the SWR on 101 frequencies in the chosen range to show these patterns. It sends out a tiny signal on each frequency and reads the forward and reflected power.

There is a still bigger dip at 10.4 MHz with SWR 1.560, again above the 30m amateur radio band.

Now I have changed the stimulus frequency range. It is from 3 to 28 MHz. SWR at 7 MHz is 2.664. I have seen slight fluctuations in SWR with time earlier also. I presume that it is because the antenna is swaying in wind and the relative position to nearby wires and other structures is changing.

Next dip is seen at 10.5 MHz with SWR 3.514, slightly different from the 10.4 MHz noted initially.

Moved the cursor further and reached the next dip at 13.750 MHz with SWR 2.986. Earlier the fan dipole had SWR of nearly 1 around 14.2 MHz. Adding the loading coils and pigtails has shifted the SWR below the amateur radio band and made this antenna of no use for 20m band in the present configuration. To bring it back to the amateur radio band, I will have to trim the 20m elements. That is a laborious process, as I learned it the hard way, from my previous experience. As it is the element before the coil, folding back and twisting may not be fine, though I have not tried it yet.

Though the next SWR dip is seen as 1.825 at 21.750 MHz, on testing in my radio, I found the 15m band tunable with the auto-tuner of FT-710 radio. Again, slight disparity between what is seen on NanoVNA and the radio.

Final dip in the range of frequencies selected is at 27.500 MHz with SWR 1.443, just below the 10m amateur radio band.

Moved the cursor to the extreme end of the range and noted SWR 2.287 at 28 MHz, the lower end of 10m amateur radio band. This was also tunable with the auto-tuner of FT-710. Interestingly, auto tuning was also possible on 50 MHz or 6m band. In conclusion, my modified multi-band fan dipole antenna can be used on 40m, 15m, 10m and 6m in the current configuration. I have not been able to hear anything on 6m yet. From this analysis of the complex antenna, next step would be to get back the resonance on 20m band. After that I will have to see if lengthening of the pigtails beyond the loading coils can bring the 4.5 MHz resonance to 80m band (3.5 MHz). Alternatively I could try that first as it is easier. I have enough folded and twisted wire in that segment as I had reused a longer wire cut for a previous experiment. I have not gone through the Smith Chart on NanoVNA as it will be very complicated for this highly complex antenna.