Sunday, 2 December 2007

Theres a hole in my sealant

I finally got around to resizing the doublet from 102ft, as was for G5RV, to 88ft top section. I took the opertunity whilst scared stiff up the ladder to extend the lanyard considerably, taking the end of the antenna well clear of the buildings, and hopefully putting an end to any RFI problems triggering the neighbours security light.

As soon as i had it back in the air and went to test, disaster! nothing would tune right, and 20m (my favourite band) wouldnt work, giving an SWR greater than 6:1 if i ran high power. So, i thought, the line lengths are now wrong, so i went through the calculations and added 6ft of feeder to ensure no band was an odd quarterwave. No change. Hmm, strange. So i inspected all the joints and found one that was corroded. This being the point where the original G5RV matching section line joins the ladder line back to the balun. So, i cut the bad joint out. Still no better. Closer inspection showed one side of the ladder to have black copper oxide on it, hmm, its been wet. I cut it back, and back, and back. I cut out 4ft of line and it was still oxidized. Now, i had assumed that the water had got in at the bad joint, but now the oxidation was on a section that went vertical. So i lowered the whole antenna and had a look at the feedpoint. Nasty. Theres a little hole, right above the end on one of the lines, where the water can get in. And guess what - yes, thats the side thats corroded. All the way down the line.

So it looks like the poor tuning and performance is down to this water ingress. So i have to replace that section of line. Unfortunately i only have 13ft of line left, and the section if much longer, so unless i come up with a better way of routing the feeder, i'll have to buy another roll of ladderline.

All this has come at an awkward time, just as my friend Andy G7PZL is finally equiped for HF and wanted to do on air tests. I couldnt hear him at all, and hes only 25miles away or so. Im considering a Cobwebb antenna for the higher bands, this at least is coax fed. It all depends on what the XYL will let me have. Shes already not happy with me wanting to put two VHF beams up. It would still mean i would need antennas for 40m & 80m, maybe even topband 160m, but i could perhaps get away with a fan dipole for these and coax feed.

Either way, i need to finalise the antenna system before spring, for both HF and VHF, after which no further work can go into them except normal maintenance, likewise the shack. It has to become operational only, no engineering.

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