Having recently taken delivery of a Palstar AT4K manual tuner, I was keen to get her into production to replace my CG5000 in the attic.
Problem: the route to the attic from the shack is complex but I have a number of spare coax runs going that way including a couple of RG58 cables that I installed about 10 years ago as backups. Actually I originally installed three RG58 lines but I’ve been using one of them to send 12V up the line to the ATU.
After MUCH research, I finally used about 20 feet of parallel coax feeders, connecting ladder line to both ends. To clarify, I run about 12 feet of ladder line from the ATU to the parallel RG58 cables. I soldered the ladder line to the inner core of the RG58 coax and shorted the braid-to-braid. My 20 feet of RG58 runs to the attic, through walls, up ceilings etc and in reverse, I connected the ladder line to the RG58. Again, I shorted the braids of each line to each other with a solder blob. My ladder line then has another run to the feedpoint of a large 60m loop that runs through the attic and around the garden.
The results have been quite amazing. Comparing my 40m reference dipole to the the CG5000 (SG230 type) ATU feeding the 60m loop has always shown that the loop was about an S point lower than my reference dipole for most stations.
Now the auto ATU at the feedpoint has been removed (along with about 30m of Westflex, 10mm coax), I can report that the loop now outperforms the reference dipole by around 1 S point – which is what the modelling always suggested.
I reason therefore that the ATU had some significant losses which the AT4K doesn’t have – even though I’m using parallel coax feeders for maybe 30% of this feeder arrangement.
If you stumble upon this via Google because you are also wondering how to use parallel / dual coax in place of ladder line, these were the problem areas of research that I had:
Do I join the braids together at the top and the bottom of the run twin coax feed arrangement? I did both.
Do I have to ground the braid to anything (including the tuner)? I didn’t. They effectively float.
Is cheap RG58 an acceptable coax to use in this arrangement? It’s all I had that was already installed that was spare else I’d have used RG213 or similar. But amazingly, even at 400w key-down on all the lower bands, I don’t appear to have any radiation, leakage or losses to worry about.
Finally, on the old arrangement, that is feeding an SG230 or CG3000 type ATU with coax from the shack, the burglar alarm would bleep, pop and splutter on 40m on high power. This is actually the reason I put up a different 40m dipole, to keep what I thought was radiation off the antenna away from the house only on the 40m band. It appears it wasn’t the regular radiation that caused the issue, it was either the coax bleeding RF with common mode currents or the ATU itself. Tonight after doing some RTTY at 400w on 40m with the ladder line and the AT4K, I have noticed the burglar alarm was silent. Quite amazing.
— NOTE (edit Feb 2017) . I raised a query on eHam about this topic and there is very little known about this topic. The interested reader might be advised to go here: http://www.eham.net/ehamforum/smf/index.php/topic,92627.0.html