Last week I wrote about recent work that identified where my G3TPW Cobwebb antenna was resonating on each of its five bands (20m, 17m, 15m, 12m and 10m).  Following on from that work G0MGM and myself spent a day recently adjusting my Cobwebb so that it was resonant around the SSB sections of each band and this article summarises that work.

The instructions supplied by G3TPW for his Cobwebb are excellent and identify the tuning effect of shortening/lengthening each dipole leg.  For reference I have identified these below:

What the instructions omit is whether the dipoles interact, whether they should be adjusted in any sequence (e.g. 20m before 17m) and the impact of extending/reducing the gap between each dipole leg (spanned by the string) upon resonance.  These were all questions that were going through my mind prior to starting adjustment work and which drove the approach adopted.

The reactance, resistance and impedance data presented in Part 1 was collected using my MFJ-259b antenna analyser.  The MFJ-259b is a basic analyser and does not have any capability for data logging or data export necessitating the collection of data at multiple manually sampled frequencies.  This process was laborious and constrained the number of samples it was practical to collect and thus the accuracy of the overall result.  As I foresaw the need to resample each of the five bands for each single adjustment, it clearly, was not going to be practical to use the MFJ analyser.  Fortunately, a good friend, G0MGM, has a miniVNA analyser that can auto-sweep a band, log the results and export them in a CSV formatted file, which we later imported into Microsoft Excel. This capability made it practical to capture data samples, visualise and analyse the impact on each band of every change.

All graphs in this article may be enlarged by ‘clicking’ upon them.

Baseline

Because a different analyser was used, two new sets of baseline data were captured with the antenna at 3m and 8.5m above ground.  The lower height represents the  height of the antenna when my mast is retracted.  The baseline data presented in this article is that sampled at 8.5m.  Measurements were taken in the radio room at the end of the RG-213 coax feeding the antenna.

It is interesting comparing the baseline results above with those captured previously using the MFJ analyser and presented in Part 1.  It should be noted that the comparison was performed with the antenna at the same height, with the same coaxial feeder, but on different days, that the weather was similar and that on both occasions the antenna and its surroundings were completely dry.  Furthermore it should be noted scales and colours vary thus some interpretation is required.

Adjustment

Based on experience of adjusting a Butternut HF-6V antenna, now made by DX Engineering, and a need to start somewhere, the decision was made to sequence adjustments from 20m, progressing to 10m.

After analysing baseline results it was decided to adjust the 20m, 17m and 15m elements.  Starting with the 20m element we reduced the length of each leg by 2cm and resampled data across each of the five bands.  Results showed that the change had a positive impact, raising the 20m resonant frequency by the amount expected with little or no change on any of the other bands.  This was good news and was the first indication that there was little interaction between the five elements. We then repeated the process removing a further 2cm from each leg.  Again the results were the same. i.e. the change on 20m was that expected and there had been little or no change on the others.  The graph below is the final result of the two changes.

Next the 17m dipole element was adjusted, reducing each leg by 2cm.  The process of sampling across all five bands was repeated and again it was found that the change had no significant effect upon the frequency of resonance for the other bands.  This really gave confidence that each element could be adjusted independently and that no sequence of adjustment was necessary.

Only one change was necessary and it raised the resonance point to the frequency required.

Sticking with the original plan, although it was almost certain by now, the 15m element was adjusted reducing each leg by 1cm.  This raised the resonant frequency to that required.  This time some minor changes were noted to the point of resonance on the other bands, however, the change was very small.

The changes to 20m, 17m and 15m had necessitated the retying of the string between there individual leg elements.  The string between the unchanged elements had remained unchanged and it was now observed that there was noticeably more slack in the wire at the leg ends for those elements.  It was thought that this additional slack may be causing the very small changes observed.

With changes complete the results from sampling each of the five bands were analysed and found to be acceptable so again the antenna was lowered and the length of string on the unchanged elements reduced so as to tighten the wires slightly.  The change in string length was small but afterwards it was noted the resonant frequency had raised a little on those band elements.  This was expected as reducing the gap between each leg end adds capacitance.

The results for 12m and 10m are shown below.

Conclusion

Adjusting the Cobwebb proved to be much simpler than expected.  Results showed that each dipole element could be adjusted without impacting other elements and that adjustment need not take place in any particular sequence.  It was also found that frequency change per cm as specified by G3TPW in his instructions was accurate.

In writing this article it was realised that it would have been useful to capture the length between the ends of each element leg.  i.e. the string length.  When these can next be measured I will update this article with the information.  Describing the tautness of the elements is difficult.  They are neither taut nor slack, but ‘just right’.  i.e. there is a little movement of the wire.  Perhaps it is better to describe by stating that their tautness does not deform the cross shape of the fiberglass spreading arms.

Key to the success of the adjustment work was the miniVNA analyser and its ability to visualise and log sampled data.  Without it, what took approximetly 5hrs would have taken much longer.

Finally, thanks to Rob, G0MGM, for his assistance and his miniVNA and enjoy the bottle of sake 😉

Andrew
G0RVM

I’ve had a G3TPW Cobwebb for almost a year but recently its moved location and is now on the top of a Total Mast Solutions 11m pneumatic mast.  Because previously it was in a temporary installation I didn’t think much about ensuring that resonance on its five HF bands (20m, 17m, 15m, 12m & 10m) was where I wanted it to be.  I just used a manual coupler (aka ATU) to ensure its match to the transceiver was close to 50 Ohms.

The antenna has been performing very well considering its very small size and has yielded worldwide contacts but I decided recently it was time to do some investigative work to see just where it was resonating and thus whether it could be further improved.  I have an MFJ-259b antenna analyser which lets me identify the resistive and reactive components at a given frequency.  However, and annoyingly, it doesn’t support any sort of automated band sweep or result logging capability.  Therefore its necessary to take and record multiple individual measurements then manually enter these into a graphing tool (MS-Excel in my case) to visualise the results.

To provide a reference baseline I captured the resistive and reactive components both with the antenna at 3m and 10m above ground.  As the results were similar I’ve included only those results when at the greater height and these are shown below.

Select each for a larger size.

In the coming days a good friend G0MGM who has a MiniVNA analyser has agreed to help.  His analyser has two of the key capabilities my MFJ-259b lacks: Band sweep and result logging.  The intention is to adjust each dipole individually to achieve resonance just where I want it.  However, I’m not clear how the five dipole’s interact, I assume they must to some degree as they are closely spaced, or whether its best to adjust them in any sequence.  i.e. 20m before 17m.

In part 2 of this post I hope to report what we found, the adjustment methodology and importantly the results.

Andrew
G0RVM