Recent posts have been all about trying to fathom out a series of problems that sprung from nowhere with my delta loop for 15m.
The delta has worked through hurricane winds for more than six years, worked global DX, and never complained a jot.
I started the investigations when I could hear the received signals breaking up intermittently, revealing a problem somewhere.
Having eliminated the audio from the rig as the problem, it was time to follow the coax!
Throughout this story, the rig itself - an FT-450 - and two standard SWR meters all reported perfect happiness at 1:1.15.
My SARK-110 analayser, though, had other ideas. It showed a good curve from time to time, but had lots of noise. The noise initially appeared to be down to the MCX connector, but eventully proved to be fine.
Anyhow, to cut a long story short, I found plenty of problems when I started to dig:
(1) badly corroded coax, which I replaced.
(2) a somewhat damp 4:1 balun, though it was only condensation and in fact, seems to work fine
(3) a supplied patch lead from the SARK to the outside world was defective and lost continuity.
(4) a metal fatigue fracture had developed in the antenna's hard-drawn wire, which was simply cut away and a new join made.
The fifth and final problem I discovered, when I just couldn't get the impedance and SWR curves to make any sense despite all the renovation work, was sorted out at past midnight last night!
It turned out that the 4:1 balun was the wrong step-down for the delta, which is corner fed for mechanical simplicity in the 15m antenna's case. Bypassing the 4:1 with a pair of crocodile clipped leads and connected to a 2:1 yielded the perfect curves for both.
So, I guess the take-home messages are:
(1) a good SWR at an analogue meter can fool you into false security if the impedance is wrong at the antenna
(2) an analyser, or certainly the SARK, can reveal problems that otherwise you would either not know about, or not find out about until they might pose a risk to your rig.
(3) it seems you can always trust a SARK analyser
(4) RG58/U is not a good coax to use (well, we all start somewhere!) and may have such losses even in 15m-runs that it can mask mismatch problems.
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