A Pull-up Bamboo Moxon Antenna for 20m

As much as I love all of the wires up in my trees, I’ve come to realize that a little real gain and directivity would be nice.  However, my trees are really in the way for a a typical tower and a beam antenna, so I am left to consider what resources I have and invent a solution.  What I do have is tall trees…, and a patch of bamboo. The simplest two-element horizontal antenna has to be the Moxon rectangle.  Construction is a single rectangular loop of wire with a couple of breaks made of insulating cord, somehow spaced apart appropriately.

The figure on the right shows the antenna modeled in 4NEC2 for the configuration I ended up with.  You can see the nice broad forward gain lobe and strong front to back ratio that makes this such a nice design.  In my case, there will be no way to rotate it very much; it will be pointing North over the pole into Europe and the Mid East, which have been only marginally served with my wires.

For the construction I need a light-weight structure that I can pull up into the trees.  I make a lot of garden structures out of bamboo, which grows abundantly on one side of my yard.  The strength and stiffness for the weight is unmatched by any other natural material.  I chose a few two-year canes and cut them with a little extra length.  I made a “hub” from a couple of pieces of cedar 2×6, one which I grooved at the right angles so the poles would form the correct crossed structure dimensions.  I placed the bamboo poles in the grooves and clamped everything in place with the second 2×6 and several deck screws.  The wires (#14 THHN) were measured and cut to the desired dimensions and the ends fitted with small soldered loops.  Pieces of parachute cord were used to tie the three wires together into one big loop, two quarter-wave sections for the driven element and a half-wave section for the reflector.

Hub and Pull Point
Feed Point
Corner

I opted to to have the bamboo poles spring downward rather than upward, trusting that gravity should be respected.  I measured the correct spacing from the ends of the wires to the corners where I gave a twist, looped the corner wire over the end of the bamboo, and held it in place with a wire tie.  With the corners in place, the natural spring of the bamboo caused the antenna to more or less assume the correct shape.  

Photography does a poor job with antennas.  It is impossible to see the wires, even the bamboo poles disappear in the branches of the trees.

The Moxon ready to go up.

The best overall view can be seen before we pull the beast up into the trees.  The PVC pipe provides support for the feed line so it will not pull directly on the antenna wires.  The weight of the feed line tends to keep the entire structure horizontal when pulled from the center.

Tennis ball cannon, fishing pole, parachute cord pull line.

The process of establishing the sky hook is an adventure itself.  I used my tennis ball cannon to launch some fishing line over the branches.  Then pull back a line of parachute cord, and finally the main pull rope.  Getting both ends of the final line to fall where you want them is hit and miss and requires ingenuity.  There is always the possibility that something will get snagged way up in the tree and you will be forever living with some dangling rope.  For the Moxon pull I terminated the main support rope with a pulley so I could more easily pull the antenna up and down with a separate rope.  

Sky hook rigging

The pulley end of the sky hook ended up about 45′ high over a small branch, with the main rope another ten feet up over a stout branch.  I was able to get the active antenna wires up to about 35′ with this arrangement.  Another rope from the hub was used to pull the antenna out away from the tree.  The reflector wire just brushes the tree trunk.  A guide line on one of the corners allows the antenna to be aimed over a range of about 45 degrees before it crashes into some form of vegetation.  That allows the antenna to aim just a little west of due north to about 35 or 40 degrees east of north.  This is just about perfect for searching for contacts in the difficult DX zones in the Mid East and Eastern Africa.  

With the antenna up in the air, it’s time to try it out.  I’ll discuss performance in a future installment once I have some experience with it.

Antenna UP!

 

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