Sunday, April 9, 2017

Project - DIY aluminium push up mast

Working portable a lot I was looking for a set up that could help me work the DX that I could mostly copy on my vertical end fed wire antennas but that I had a hard time working - especially in a pile-up situation.

I bought the CB yagi first and changed that into a 4 element 10m yagi. Later I bought the folding hexbeam. Both are very nice antennas to work DX.
Of course I could not put these antennas up with the fiberglass poles I use for the end fed wires.

So I set out to find a portable mast that was sturdy enough to set up without guying (under low wind conditions) and could reach at least 10m high.
This turned out to be a challenge. There is not much out there that is strong enough to set up without extensive guying or it is outrageously expensive.

DIY decision

After a while I decided to build a mast myself from aluminium tubes. I could not find the right material around where I live. The challenge is that you need a nice series of diameters with 1-2mm in between the sections. The other challenge is that you do not want to buy 6 meters of each section (which is the default "industry length"). Luckily it turned out there is a webshop for aluminium stuff in The Netherlands that stocks almost all diameters and sells custom lengths.

What would be the dimensions to choose if you want a sturdy but still portable mast?
After some experimentation I decided to go for the following series (diameter x thickness in mm): 70x2 - 65x2 - 60x2 - 55x2,5 - 48x2 - 42x2 - 35x2,5

The widest tube is 2 meters long - running up to 2,4 meters length for the last tube. I did this to make sure they collapse together nicely with each narrower tube sticking out the previous one.

Okay. So far, so good.

Now how do you clamp the tubes together when you want to extend the mast?
For this I used my angle grinder to grind slits into the top of each section. In the most narrow tube I used 4 slits (the gap between the 42 and 35 is larger), for the other sections I used 2 slits. Note: be sure to carefully file the metal afterwards or you will find that the tubes "stick" when you want to extend the mast.

Tube clamps - right one is the new and more durable version
Over this upper part of each section I slid a heavy duty tube clamp. I started using a version I found in a boat shop but they are made of stainless steel. As this is slightly softer the threads of the bolt used to open and tighten the clamp tend to damage quickly. Now I am using a different kind that seem to be more durable (see the picture above).

The end result: mast of 7 aluminium tubes with 6 cable clamps (5 old types, 1 new type)

Proof of the pudding

How does it work in practice?
Well, the mast is a fully manual operation version :)
Extending the tubes can be hard work, especially under windy conditions. In those conditions the mast tends to bend putting a force sideways on the tube you want to extend. This increases the friction.
The best way to extend the mast is to make sure you position yourself on top of anything practical that brings you on a height where the top of the mast is just above your waist. That way you can extend the mast  

Does it need guying?

In conditions of low winds the mast can be extended completely without guying. It needs to be on a level surface though.
When the wind is somewhat stronger I do not extend the last section and still use the mast without guying. At some point it will need guying, either to compensate for not setting the mast perfectly upright or for windy conditions. For this I added a simple and low cost guy ring out of PVC (as can be seen in the picture). It does what it needs to do: act as an attachment point for the guy wires and stay in place while the mast rotates.

I have been using this mast now for more than a year and I am happy with it. When I set it up, I secure it at the base (I built a mast foot from wood that attaches to the wheel of my car) and at the roof of the car (again using a wood construction) I am still considering changing the base I use. I will write about that one another time.

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