Constructing a simple dipole for 6m (50 MHz)

This was a fun little construction project, and very successful. The objective is a dipole for use on 6m, which I can deploy easily with my Vine Antennas Skypole Max, for temporary use on contests and FT8.

The dipole uses two sections of telescoping aluminium tube for each element. Although a single piece of tube (or rod) could be used, lengths of over 1m are much more expensive to buy, and also attract additional delivery charges. Constructing the antenna using 1m lengths is much more economical, and gives the possibility of adjusting the element length by sliding one tube inside the other... of which more later.

The construction method uses a central plastic box, which provides the following functions:

  • mounting point for the two dipole elements
  • contains a choke balun
  • BNC termination point for the feeder
  • method of attachment to the pole

Parts List

The last four items didn't need purchasing as they were already available in my workshop.

Construction

My intention was to slide the 8mm rod (which turned out to be tube) inside the 10mm tube to make the element adjustable. Then a saw-cut in the end of the 10mm tube and a hose-clip would fix the two parts together to make each quarter-wave element.

When I received the four tubes (2 x 10mm and 2 x 8mm), I slid them together to see if it was a good fit. It was snug, but slid and was adjustable. The next day, I tried to separate the two sections, and they had jammed solid! Nothing I could do, including WD40 and holding one part in a vice, would separate them. So... I went with that, and left them fixed together. No need for the small hose-clips, and no possibility of adjustment. So if adjustment is needed, it might be better to go for a smaller diameter inner tube/rod.

Each of the two quarter-wave elements was cut to 1500 mm. This is deliberately over-length for 50 MHz and will be trimmed down later.

The two parts need to be joined mechanically but not electrically at the centre. A short piece of 8mm plastic rod or tube would be ideal. I searched the kitchen drawer, and found a tube of exactly the right diameter. This was very handy. If you visit a certain high street bank, you might find they are giving away suitable pieces of tubing free of charge! The plastic tube is glued to one tube using Araldite, and the other end is left floating to slot into the other element.



The plastic box has two halves, of equal thickness. I chose to use one half to hold the choke balun and pole mounting, and the other half to mount the elements and the BNC connector.

The choke balun is wound exactly using the instructions I found here: Balun Choke 6m (vk6ysf.com) - except that the diagram is slightly confusing as the antenna side connection comes from below the toroid, not above.

I drilled small holes in two of the hose-clips and the plastic box, and bolted these together to form the mounting point for the pole. A couple of spare solder tags were used to provide a fixing point for some more cable ties, which hold the balun secure. Here is the inside and outside of this half of the case.




On the other half of the case, I drilled holes for four U-bolts to support the elements, and also a hole for the BNC socket. Note that the elements are entirely outside the box, and the electrical connection is via the U-bolts. The two inner U-bolts have solder tags attached on the inside of the case.

The unbalanced side of the balun is soldered to the BNC connector, and the balanced side is soldered to the two solder tags on the U-bolt. So that the box can be opened, I kept the balanced side as short as possible, and allowed a longer piece of coax on the unbalanced side.  Here is the inside and outside of this half of the case, without and with the elements attached.




Adjustment and testing

Next I attached the completed antenna to the pole using the hose-clips:


I connected a feedline, raised the antenna to a moderate height away from surrounding objects, and used my nanoVNA to look for the minimum SWR. The SWR curve looked really good, with a minimum of 1.1 at 47.2 MHz. Next I calculated the adjustment to be made to raise the resonant frequency to my desired 50.2 MHz:

measured freq / desired freq = adjustment factor

47.2 MHz / 50.2 MHz = 0.94

So the elements need to be shortened by a factor of 0.94:

starting length x adjustment factor = desired length

1500 mm x  0.94 = 1410 mm

The final step was to disassemble the elements, cut them down to 1410 mm, and reassemble.

The result is an antenna with low SWR across the 6m band, with a minimum of below 1.1 at 50.2 MHz. Testing on FT8 immediately gave me PSKreporter reports from Cornwall in one direction and Norway in the other... far in excess of anything I had achieved previously. 

Success! Here is the finished item (alongside my HF MFJ mag loop):

 


 

 

 



















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