Posted: 9/12/2014 11:07:25 PM EDT
I recently swapped out the coax fed OCF dipole for a ladder line fed 134' doublet (non-resonant). Thought I'd post a pic for any wire antenna afficionados. The sweeping line going to the top left is a belaying line used to keep the wire out of a pear tree when raised or lowered. Pull points are the same tree hung lines used for the OCF.
I am using the same 4:1 current balun that was used for the OCF feedpoint for the transition from ladder line to coax for the final run to the manual antenna tuner (the ladder line for the 160m full wave loop occupies the balanced line connection on the tuner). Still no RFI issues noted to date. 
(old pic from OCF feed point) The doublet tune is a bit "twitchy" compared to the OCF, with fairly sharp dips at frequency as opposed to the broader transitions with the OCF. The big benny for the black on black ladder fed wire is that it pretty much disappears when viewed from a distance against the tree filled background. This is especially true when contrasted against the white OCF wire fed by a light grey coax & a red balun! I tried ladder feed with the OCF, but there were some frequency ranges that did not want to tune to 2:1 or below with any of the baluns tried (1:1, 4:1, 9:1), so I went with the doublet which looks to be usable on all bands tried so far with SWR <= 2.0 achievable & many areas 1.2:1 or less. The big loop is still the favorite here, though. Nick |
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Cool stuff! I'm currently using a doublet for 80m and up. Runs from the back eave of my house, over a pine tree and to our back privacy fence. Wish I could get it up higher, but HOA restrictions and all... Feeding it with 450 ohm ladder line all the way back to the shack and have it hooked up to an MFJ 993B (with the built in 4:1 balun) autotuner. Works well!
Josh |
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The ladder line is homebrew. 14 awg THHN ($42 for 500 foot spool at Lowes) with spreaders made from Zabera fence post insulator tubes ($13 for 200, $2 for 25 at TSC). I drill wire sized holes across the diameter at each end & then split the holes out to the tube end with a knife tip. The wire gets inserted into the holes via the split ends and ultimately secured with a shot of hot glue into both ends of the tube.
The assembly is done with both wires under tension (my work space lets me do about 30 feet at a time). I put a spreader in at roughly 18 inch intervals but that is probably more frequently than really needed for long straight runs. Finished width of wire separation is just under 4 inches (3.75?). Guesstimated impeadance is 550 ohm... The coax starts right after entry to the crawl space so it is probably 30 + feet. I may extend the ladder line farther towards the tuner at some point but the coax was already in place so I just removed the exterior portion and used the remainder. Nick |