My HF Doublet at G4SPE
My HF Doublet at G4SPE
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I have been using a doublet as the main HF antenna for many years. This antenna has been repaired on many occasions after storm damage and general wear and tear, so I thought it was time it was replaced.
This plan is not to scale but shows the general layout of the house and garden. The main house has two stories, but the kitchen, garage and conservatory are single story.
You will see the position of three poles. The centre pole is mounted on the house above the end of the kitchen roof, and is also above the position of the radio room. The other two poles are at the bottom of the garden and at the end of the garage. This is not quite a straight line, but near enough to be usable.
As a doublet is basically a dipole which is not resonant on any particular frequency, and fed with a balance feeder into an ATU which matches the rig to the antenna, the builder has a choice of the way the antenna is fed . Over the years I have used open wire feeders, but also commercially made 450 and 300 ohm slotted ribbon cable. All are low loss, much lower loss than coax, but I find that the best of all is home made ladder line, which is what I am using this time.
I made about forty spreaders to make up the ladder line, using left over vinyl floor tiles, glued face to face and cut into strips about four inches by one inch, with two holes drilled in each end. The holes are there to thread small zip straps through to anchor the spreaders onto the wires.
This picture shows a test piece I made up to check that my spreader system
works. I chose to use a gap of 12 inches between spreaders.
This shows the centre piece that I made out of a nylon chopping board, with a plastic dog bone insulator mounted at the top to take the strain of the the two dipole elements. The brass fittings are the connectors removed from a Choc-Block connector strip, and to be used to clamp the wire to the insulators.
I purchased 50 Metres of Poly coated Flex Weave cable, and two ceramic insulators for the ends of the elements. The plan is to have no joins at all between the elements and the ladder line, all made of one continuous wire, so less chance of corrosion or breakage.
This is the start of the assembly. I first measured and marked out the positions of the parts on the wire, by folding it in two and stretching it out across the garden, marking each wire together so that everything was equal. I even marked the spreader positions with coloured nail varnish to ensure that I positioned everything correctly. Each element was to be 48 feet, which fits nicely between my poles. As you will see in the picture, lots of small zip straps were used.
The station dog thought this was very tiring work !
I then carried on fitting the spreaders to make up about 30 feet of ladder line.
This shows my method of securing the cable to the insulators using the brass connectors. I used sections of glue - filled heat shrink to hold the free ends securely. Anything folded back like this is not counted as part of the radiating element.
The centre of the doublet was pulled to the top of the pipe on the house, and the feeder well secured so it cant flap about in the wind. I have a pulley on the top, and a rope looped through to make this an easy job. Note my 10M vertical photo-bombing the picture ! The shack is the window on the right of the picture. (not the bird box).
The ladder line was fed through the soffit of the kitchen roof. The two wires were fed through air vent slots, then fitted with the spacers within the loft. This continued through the loft until it was fed into the shack through an adjoining wall.
Here are the two end poles. They are not too visible, they have been there for many years and I've not had any complains yet.
The ladder line terminates in a 4:1 balun on the wall inside the shack directly where it comes through the wall. I have tried many different balun designs over the years, but I find that a 4:1 balun on T200-2 toroid works very well. This goes into a 1:1 common mode choke, wound on an
FT240-31 toroid just before it terminates on the mAT-Y200 ATU and into the FTdx10.
Its a very quiet antenna. The noise floor is always very low, helped by the balanced feeder and the care taken to ensure that everything is as symmetrical as I could make it.
So, I'm very pleased with the end result. It will tune all bands from 80M up to 10M, and is a pleasure to use. I hope it will remain maintenance free for some time.
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