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\u00a0each\u00a0dipole leg\u00a0(spanned by the string) upon resonance.\u00a0 These were all questions that were going through my mind prior to starting adjustment work and which drove the approach adopted.<\/p>\n
The reactance, resistance and impedance data presented in Part 1<\/a> was collected using my MFJ-259b<\/a> antenna analyser.\u00a0 The MFJ-259b<\/a> is a basic analyser and does not\u00a0have any capability for\u00a0data logging or data export\u00a0necessitating the\u00a0collection of data at multiple manually sampled frequencies.\u00a0 This process was laborious and constrained the number of samples it was practical to collect\u00a0and thus the accuracy of the overall result.\u00a0 As I\u00a0foresaw the need to\u00a0resample each\u00a0of the five bands\u00a0for each single\u00a0adjustment, it clearly, was not going to be practical to use the MFJ analyser.\u00a0 Fortunately, a good friend, G0MGM<\/a>, has a miniVNA <\/a>analyser that can auto-sweep a band, log the results and export them\u00a0in a CSV<\/a>\u00a0formatted file, which we later imported into Microsoft Excel<\/a>. This capability made it practical to capture data samples, visualise and analyse the impact on each band of\u00a0every change.<\/p>\n All graphs in this article may be enlarged by ‘clicking’ upon them.<\/p>\n Because a different analyser was used, two\u00a0new sets of baseline data were captured with the antenna at 3m and 8.5m above ground.\u00a0 The lower height represents the\u00a0 height of the antenna when my mast is retracted.\u00a0 The baseline data presented in this article is that sampled at 8.5m.\u00a0 Measurements were taken in the radio room at the end of the RG-213<\/a> coax feeding the antenna.<\/p>\n It is interesting comparing the baseline results above with those captured previously using the MFJ<\/a> analyser and presented in Part 1<\/a>.\u00a0\u00a0It should be noted that the comparison was performed with the antenna at the same height, with the same coaxial feeder,\u00a0but on different days, that the\u00a0weather was similar\u00a0and that on both occasions the antenna and its surroundings were completely dry.\u00a0 Furthermore it should be noted scales and colours vary thus some interpretation is required.<\/p>\n Based on experience of adjusting a Butternut HF-6V<\/a> antenna, now made by DX Engineering, and a need to start somewhere, the decision was made to sequence adjustments from 20m,\u00a0progressing to 10m.<\/p>\n After analysing baseline results it was decided to adjust the 20m, 17m and 15m elements.\u00a0 Starting with the 20m element we\u00a0reduced the length of each leg\u00a0by 2cm and resampled data across\u00a0each of the five bands.\u00a0 Results showed that the change had a positive impact, raising the 20m resonant frequency by the amount expected\u00a0with little or no change on any of the other bands.\u00a0 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.\u00a0 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.\u00a0 The graph below is the final result of the two changes.<\/p>\n Next the 17m dipole element was adjusted, reducing each leg by 2cm.\u00a0 The process of\u00a0sampling 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.\u00a0 This really gave confidence that each element could be adjusted independently and that no sequence of adjustment was necessary.<\/p>\n Only one change was necessary and it raised the resonance point to the frequency required.<\/p>\n Sticking with the original plan, although it was almost certain by now, the 15m element was adjusted reducing each leg by 1cm.\u00a0 This raised the resonant frequency to that required.\u00a0\u00a0This time some minor changes were noted to the point of resonance on the other bands, however, the change was very small.<\/p>\n The changes to 20m, 17m and 15m had necessitated the retying of the string between there individual leg elements.\u00a0 The string between the unchanged elements had remained unchanged and it was now observed that there was noticeably more slack\u00a0in the wire at the leg ends for those elements.\u00a0 It was thought that this additional slack may be causing the very small\u00a0changes observed.<\/p>\n 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.\u00a0 The change in string length was small but afterwards it was noted the resonant frequency had raised\u00a0a little on those band elements.\u00a0 This was expected as reducing the gap between each leg end adds capacitance.<\/p>\n The results for 12m and 10m are shown below.<\/p>\n Adjusting the Cobwebb proved to be much simpler than expected.\u00a0 Results showed that each dipole element could be adjusted without impacting other elements and that adjustment need not take place in any\u00a0particular sequence.\u00a0 It was also found that frequency change per cm as specified by G3TPW in his instructions was accurate.<\/p>\n In writing this article it was realised that it would have been useful to capture the length between the ends of each element leg.\u00a0 i.e. the string length.\u00a0 When these can next\u00a0be measured I will update this article with the information.\u00a0 Describing the tautness of the elements is difficult.\u00a0 They are neither taut nor slack, but ‘just right’.\u00a0 i.e. there is a little movement of the wire.\u00a0 Perhaps it is better to describe by stating that their tautness does not deform the cross shape of the fiberglass spreading arms.<\/p>\n Key to the success of the adjustment work was the miniVNA<\/a> analyser and its ability to visualise and log sampled data.\u00a0\u00a0Without it, what took approximetly 5hrs would\u00a0have taken much longer.<\/p>\n Finally, thanks to Rob, G0MGM<\/a>, for his assistance and his\u00a0miniVNA<\/a>\u00a0and enjoy the bottle of sake \ud83d\ude09<\/p>\n Andrew<\/em> Last week I wrote about recent work that identified\u00a0where my G3TPW Cobwebb antenna was resonating on each of its five bands (20m, 17m, 15m, 12m and 10m).\u00a0 Following on from that work\u00a0G0MGM and myself spent a day recently\u00a0adjusting my Cobwebb … Continue reading Baseline<\/span><\/h1>\n
![\"20m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/20m-00.jpg?resize=300%2C188\")
<\/a>![\"17m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/17m-00.jpg?resize=300%2C186\")
<\/a>![\"15m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/15m-00.jpg?resize=300%2C186\")
<\/a>![\"12m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/12m-00.jpg?resize=300%2C187\")
<\/a>![\"10m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/10m-00.jpg?resize=300%2C187\")
<\/a><\/p>\nAdjustment<\/span><\/h1>\n
![\"20m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/20m-05.jpg?resize=300%2C185\")
<\/a><\/p>\n![\"17m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/17m-05.jpg?resize=300%2C186\")
<\/a><\/p>\n![\"15m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/15m-05.jpg?resize=300%2C186\")
<\/a><\/p>\n![\"12m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/12m-05.jpg?resize=300%2C192\")
<\/a>![\"10m](\"https:\/\/i0.wp.com\/www.tsgarc.uk\/wp-content\/uploads\/2015\/08\/10m-05.jpg?resize=300%2C194\")
<\/a><\/p>\nConclusion<\/span><\/h1>\n
\n G0RVM<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"