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Ο λόγος στασίμων 1:1 ενός κεραιοσυστήματος, δεν συνεπάγεται τον 1:1 λόγο μεταφοράς ισχύος, προς την κεραία!

Tuesday, 19 April 2016

RG-58 coaxial cable bulged!

Noun[edit]

bulge ‎(plural bulges)
  1. Something sticking out from a surface; a swelling, protuberant part; a bending outward, especially when caused by pressure.


It was not particular obvious at first glance and it was very confusing measuring continuity and reading low loss(with a pinch of salt) with the AA. Not to mention that I still had an excellent reception !
But it was malfunctioning, and I will tell you the whole story.
There was no problem working with 5 or 10 Watts last Sunday during the QRP Spring Gathering. Turning rig's power high it was a come and go situation and using the Lin Amp to clear the QRG for the fellow qrpers, it was almost infinite swr.
First I check for electrical continuity between both PL-259 and strange enough there it was. (There it was the problem too!)
Then I redone the solders at my homebrew balun just to be on the safe side but the malfunction persisted.
After that, I used my MFJ-259 Antenna Analyser to see the loss of the 1 meter of coax cable I am using to feed the output of my tuner to the current balun outside of the shack. The losses was 0.6db. Sounds reasonable but for a length of only 1 meter of coax, it was a bit odd.
So as a last resort I started running my finger tips onto the cable trying to feel something odd and there it was. A couple of centimetres from the balun side, I felt something that had to go for a thorough inspection even if it meant a scalpel, a pair of cutters and a vice!


And there it was ladies and gents.
An RF bulge, created from an RF voltage peak due to excessive resistance and reactance at a given frequency.

I cut and discard the damaged piece of coax cable and things are back to normal now!

This method of feeding a balun and ladder-line is described by Doug W1FB in his book,  W1FB's Antenna Notebook, where he propose the connection of the in-shack tuner with the balun outside, using a length of coaxial cable no more than 8 feet. Mine was 3ft!