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
The only way to log data from the MFJ is to manually record each sample. Its a basic analyser. Its an MFJ 😉
Personally, if I where to buy an analyser today I would opt for one that can auto sample across a defined RF spectrum and log the results so that further analysis can be performed. My recommendation: spend the extra money on something like a miniVNA, it is much better.
If you really want an MFJ-259b make me a sensible offer. 😉 I’m going to upgrade.
Andrew would it be possible or could you export the data from the MFJ analyser (assuming an interface of some kind) or is it a case of logging results manually the inputting the data (probably!!). Will be purchasing that analyser soon so preparing for some time consuming work mind you its interesting work (have always liked tuning).