Full credit for the design of the antenna described here must go to Steve Blackmore and John Boyer. Click HERE to download their instructions.
What follows is how I built it, but you may wish to improve on my method, if so remember the closer the dimensions are adhered to the better it will be. Click HERE should you wish to download these construction details.
Tall Narrow Quadrifilar Helix Antenna designed by Steve Blackmore & John Boyer. ( Method of construction, developed by Keith Owen ).
Parts list and instruction notes
1 off 20 mm x 4 outlet PVC Junction Box ( J Box )
8 off 20 mm PVC end plugs to suit J Box.
1 off PVC Flange to take 32 mm PVC Waste Pipe.
8 off 90 º Elbows to suit Copper tube Dia. ( If not available, see how I made them later in instructions. )
1.5 m x 32 mm PVC Waste Pipe ( mast ).
5 m x 8mm or 10mm OD Copper Pipe. ( will depend on what elbows are available.)
From the tube cut the following :-
2 x 190 mm Bottom horizontal Radial Elements.
2 x 920 mm Short Helix Elements. *
2 x 1020 mm Long Helix Elements. *
4 x 85 mm Top horizontal Radial Elements.
* The Helix Elements sizes allow for trimming during assembly.
Suitable length of RG58 for Balun and Feed.
First prepare the J Box.
1. Drill a hole in the centre of each 20mm end plug to produce a neat sliding fit on the radials you have cut. This is best done in a lathe but, failing that, carefully mark and drill as accurately as possible.
2. Take 4 of the end plugs and cut them 5mm to 8mm in length. Remove any burrs so as each one may be reversed and pushed into a separate J Box outlet. ( see diagram ). Make sure they go all the way in. A little PVC cement or contact cement helps. The remaining 4 plugs will be fitted later.
3. Drill a neat hole the size of your COAX in the centre of the J Box bottom, then fit the Flange using suitable screws and some cement. Drill a hole in line with the centre hole if necessary.
4. Take the 4 remaining end plugs and feed one on to each of small radial elements, then carefully enter each into the J Box so as the inner ends are visible looking down on it. Use some sealer to keep water out. Before the sealer sets align the 4 ends so they are 20mm apart and all are central. When the cement sets, drill holes and fit the CS Self tapping screws to hold radials permanently in place.
5. Drill a hole to suit your COAX 10-15 cm from top of mast and ease the outer edges so as to soften the bend in the COAX when winding the balun. Do not wind it yet.
6. Feed a piece of COAX from the outside into the hole in the mast and carefully pull it about 20 cm through.
7. Cement the mast to the J Box and make the connexions as detailed in the diagram.
You may use screws or solder direct to the radials, either way is ok, but watch the heat if using solder. I used a 100 Watt Scope Iron without any problems.
8. Next mark and drill the holes for the lower radials. Follow diagram and instructions regarding this, taking care that they are on the same centre lines as the top radials and 100 mm apart centre to centre vertically. Cement the lower radials in place making sure they are plumb with the upper radials and the correct distance apart and central to the mast. Here I used 5 minute Araldite. You may at this point wind the ballun exactly as detailed and tape it, using plenty of tape, then lightly tape what's left to the mast.
11. Shape the Helix Elements. I used a piece of 4 inch PVC pipe as it was all I could find, and adjusted them on assembly. I assure you this is no big deal as the bend is very slight. Place an elbow on one end of a helix element and slip it on it's upper radial. Line up the lower end with it's lower radial, being sure that when viewed from directly above the radials, it moves from upper to lower in an ANTI CLOCKWISE direction and 90 º to it's starting point. If you get this wrong they will all be wrong. Mark the length to the centre of the elbow minus half the tube diameter to allow for adjustment before soldering. Cut to length and assemble checking if it is the correct element, anti clockwise, 90 º etc. DO NOT solder at this stage.
Repeat for all the elements and when fully assembled lightly tack solder each elbow checking as you progress until it is tacked and looks fair from every angle. When satisfied complete the soldering, being careful not to overheat but apply enough heat to prevent cold joints. Apply sealer to and fit lid. Seal screw heads and apply to any other area as required.
12. Test it by lashing it to a pole, rail or whatever and connect it to the receiver. I taped it to the railing on the front porch, which faces north, with the top of the antenna just level with the guttering and about 10m from the receiver. I received north to south, and south to north horizon to horizon passes with no fading. I didn't believe it at first so left it as it was for three or four days, with no change. It will be necessary to put an amplifier in the line for long COAX runs. I put mine about a metre below the antenna and it works fine.
13. Should you have trouble finding suitable elbows, as I did, then you may build them out of thin walled brass hobby tubing which is usually found in hardware shops. The brass is ok to be used with copper as it consists of copper and zinc. You will get 8 from a length of tube, which is about 20cm or so long. First cut into 8 equal lengths, about 25 to 30mm long then mark the centre of each piece. Cut or file a 90 º vee on the centre but not quite cutting all the way through. Fold each to make a 90 º elbow, and solder the join. The more accurate this is the better the antenna will be.
I believe that if the dimensions are all fairly accurate it will work ok, it is easier to build than it appears. You will be pleased with just how easy it is to build and amazed at the performance.
Top radials & JBox Antenna complete J Box details.
Detail: coax to radials connection, Balun, elbow construction and Radial dimensions.