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2 metre Delta Beam

Three-Element Delta Loop Beam For Two Metres.

By Steve Studdart GW7AAV

 This small and extremely light antenna has surprising gain for its size and may be suitable for those without the real estate or pocket for a huge Yagi. It could be mounted in most loft spaces and still rotated easily, but was designed for mounting on a lightweight mast such as the popular SOTA pole. The SOTA pole is really a fishing pole and is also known in the UK as a ‘Roach pole’ or in other parts of the world as a ‘Squid pole’.  My design as it stands needs some refining for my purposes as I will describe later but it works very well.

The boom is 70cm long and made of 25mm white UVC pipe or conduit but a wooden or even metal boom works just as well.

The spreaders are 45cm fibreglass rods available from kite suppliers but could by made from bamboo cane, the thin top sections people don’t use from their SOTA poles, doweling, or fibreglass tent poles from an old igloo tent.

Each of the spreaders is slotted through two holes drilled in the pipe and glued in place with epoxy resin or hot glue. For a truly portable version some better way of fixing the spreaders to the boom needs to be devised, so they can be easily removed. Possible a flange formed with a wrap of fibreglass and resin around the spreader could be used.

 The driven element is mounted 255mm from the reflector while the director is mounted 566mm from the reflector.

 A 21.5mm pipe clip pop riveted to the boom to allow fitting to a fishing pole mast and a second one at 90 degrees to the first allows a change of polarization between vertical and horizontal. It is a good idea to mount the pipe clips last when the centre of gravity has been established. A little extra nose weight can always be added so the antenna sit right in the form of washers tapped to the high end.

The elements themselves are .5mm PVC insulated hook up wire but could be any type of wire you have handy. For the reflector I used 219cm with the ends soldered together to form a loop and a small piece of heat shrink placed over the join. This was then taped in place on the spreaders so that each side was 73cm long. When I was happy I used hot glue/epoxy to hold the wire more permanently. The same was done with the director using 201cm of wire so each side was 67cm long.

The driven element is 210cm long with each side 70cm long, but instead of soldering the ends they attach to the matching section via a block connector.

The matching section is included because a delta loop antenna has an impedance of about 102 ohms. The matching section consists of ¼ wave on high quality 75-ohm coaxial cable in series with the 50-ohm feeder, which brings down the impedance to nearly the 50 ohms the rig is designed for.

  A quarter wave at 145mHz was calculated to be 492mm but the velocity factor of the coax used was 66 therefore the length of matching section was 325mm. 355mm of the 75-ohm coax was cut to allow for 3cm tails either end which were tinned with solder and screwed into the block connectors. It is perfectly acceptable to splice the 50 & 75 ohm coaxes together, seal the joints with self-amalgamating tape and let the result hang down or form a rough coil, however I prefer to use a piece100mm of 40mm white PVC drainpipe as a former. I wound the matching section and a similar length of the feeder and this then acts as a chock balun. 

 Finally to tidy things up the excess ends of the spreaders were trimmed using a junior hacksaw.


 
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