The story of the VK5SRP 6 Metre J-pole antenna failure early November 2016

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The 10 Metre tower outside the radio shack at my QTH supports the centre of my G5RV antenna and on top of it is a 6 Metre J-Pole extending a further three Metre into the sky. Recently I turned on the IC-756 to listen to the WIA Sunday broadcast at 9.00AM and then to take part in our 53.1 MHz AM chat after the broadcast. It was soon obvious all was not well with the antenna. There are two or three local fellows who usually bend my S meter with plus 30 or 40 signals but they were not even making S10. At first I guessed there may be water in the cable and when this has happened in the past a short burst of 30 Watt of RF fixed the problem. As the signal was much lower than it had ever been, I lowered the RF output to about 10 Watt and monitored the SWR as I pressed the PTT when it was my time to check in. Output power was very low and the SWR was very high and stayed high (off the scale) even after a 30 Second transmission !!! If there had been water in the cable it would have gone.

An inspection of the tower outside the shack showed the RG213 cable that went up the tower to the 6 Metre J-pole was blowing in the wind, all the cable ties that held the top half of the cable in place had snapped. Lesson, make sure your cable ties are UV stable. Sure looked like the tower had to be folded down with the winch and the problem investigated. This is a job for the "Nericans" after our Wednesday coffee club. After we had the "un-erection party" I started to look at the coax cable to the 6M J-Pole. My first assumption was very wrong. My first through was the wind blowing the un-supported cable around in the air had caused the centre conductor on the end of the RG213 cable that runs up the tower, to pull back into the N-type connector and not make contact with the mating connector on the "ugly balun". The J-pole has an ugly balun consisting of 10 turns of RG58 on a 50mm former and that is terminated in an N-type connector. Looking into the male N-type had me doubting my first diagnosis. The pin looked to be in the right place.

Next step was to put a 50 Ohm dummy load on the 6M J-pole lead where it came into the shack and start measuring for 50 Ohm from the top of the tower. Open circuit at the top, so time to undo all the self amalgamating tape on the joints and measure at each joint. The cable is not in one piece, it comes down the tower to a surge protector near ground level and then into the shack in another length of RG213. The Surge protector is a Revex or MFJ device with coax connectors each end and a gas discharge cartridge in the middle. Measuring for the 50 Ohm at the top side of the surge protector a found an open circuit still, so it was not the cable running up the 10M tower !!! Measured for 50 Ohm on the cable that run into the shack and all was well. Took the surge protect out of circuit and put it on the bench. To cut a long story short, found the two bodies (outer threads) of the PL259 connectors on opposite ends of the nicely machined body of the protector were not connected to each other or to the body of the protector. How can that be, the body is a machined aluminium block with threads each end for the PL259 connectors ???

I then cut the protector in half and started to formulate a theory, the inside looked discoloured at the top end where the cable ran up the tower. The paint on the top end of the body had been hot, it was bubbled. See the second photo. The 6M J-pole was not earthed at the top of the tower, it found an earth though the 10M of RG213 cable down to the surge protector and then via an earth strap, to a ground at the base of the tower.

My conclusion is the 6M J-pole has been discharging passing clouds for five or six years via the body of the surge protector and the current produced a good insulation layer of Aluminium oxide between the PL259 outer body and the body of the surge protector. To add more evidence to the story, about 300mm of the inner conductor of the RG213 cable to the top was blackened, probably by the heat that was generated as this fault progressed.

This proves to me the fault current was coming down the cable, not from the equipment in the radio shack. I suspect what did the final damage would have been a near-by lightening strike, not on the tower/antenna but some distance away. If the tower/antenna had been struck there would have been nothing left of the antenna or the surge protector

Selected pages from the Technical stuff on this web site
Introduction to the Squid Pole Antenna
The Balun and the UnUn
Intoduction to HF Radio Propagation - Australian Government IPS Radio and Space Services
Understanding LF and HF Propagation - free ebook from Steve Nichols (G0KYA)

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Written by and Copyright, Phil. Storr © 31st December 2016