Ultralight Endfed Triband Half-Wave Antenna

by mfhepp, published

Ultralight Endfed Triband Half-Wave Antenna by mfhepp Dec 7, 2016
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These are all the mechanical parts to build a superlight version of a popular design for a compact tri-band endfed antenna for 40, 20, and 10 m with a loading coil.

The antenna is basically the design described by VK3YY at https://vk3yy.wordpress.com/2014/08/31/end-fed-for-40m20m10m/. The 3D-printed parts allow, however, for a much lighter version with a better handling.

You need to print the following parts:

1 x winder with cover
1 x loading coil tube
2 x loading coil tube caps
2 x clips
1 x insulator (or 2 for inverted vee option).

Here is a brief description of the parts:

  1. The winder and cover is for the T82-43 or T50-43 toroid for the impedance transformer. The 3.2 mm holes are meant for RG174 coax and act as a strain relief, too. Be careful not to bend the coax too sharply or it may be damaged and a high SWR could be the result.

The smaller holes in the winder are meant as a strain relief for the main radiator and a ca. 2m counterpoise, which I found helpful.

  1. The loading coil is to be wound on the 45 mm tube with AWG 26 wire (51 turns) or similar. Glue the ends of the wire to the tube with superglue.
    Then, feed the antenna wire for the 40m part (ca. 2m) and the main part (ca. 11 m) through the holes in the cap part for the coil.

Finally, mount the caps to the tube with superglue. Since this is irreversible and you cannot re-insert the antenna wire afterwards, make sure they are in place and can't slip out.

Solder the antenna wire to the AWG26 and secure the loose cables with superglue. Afterwards, the whole coil and all holes should be protected with shrink tube and ideally liquid electrical tape, as available from sotabeams.

  1. Feed a ca. 50 - 60 mm rubber band through each of the two clips (hooks). The clips serve two purposes: First, they hold the coil attached to the matching round side of the winder during transport. Second, one of them can and should be used to attach the loading coil to the antenna mast (if it is a lightweight, fishing pole type, as from DX wire or sotabeams). This greatly reduces the stress on the top of the mast. The second one can be used to fix the end of the antenna wire to the mast ca. 1.5 m above the ground and guide it to the ground in a sloped style. This is necessary with masts shorter than 10 m because the overall antenna radiator is ca. 12m long.

  2. The insulator is for the top part of the antenna wire when used as a vertical or sloper. If you want to use it as an inverted vee, you may want to add another one shortly after the loading coil (in the direction to the winder/transformer).

When tuning the antenna, start with the 20m section (with the coil and 40m part already attached) and tune it to minimum SWR on ca. 14100 KHz. When done, tune the 2m part of the wire to ca. 7050 KHz.

It is much easier to adjust the antenna wire length at the winder side, even if this means you will have a solder link in the final wire.

Enjoy, and meet you on the bands!

Addition 2016-12-13:

  1. I added a short version of the loading coil, which is just 35 mm long. If you use 0.40mm wire, this will be sufficient to hold the 51 or 51 windings. If you use thicker wire, use the original 45 mm long version. Keep in mind that the length of the coil influences its inductance, so you will need more windings with thicker wire or if you have space between the windings.

  2. I also added a longer coil for the 80m version described by PD7MAA at http://pa-11019.blogspot.de/2012/04/149-transformer-for-endfed-antennas-35.html.

Note that my coil is designed for 0.40 mm wire and thus requires only 138-147 windings to reach an inductance of 110 uH.

There is also a new winder for the 80m coil in the files. This one has room for the coil in the middle of the winder. In this case, you have to put the components for the matching unit into a small extra enclosure of your choice.

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Works best with a heated bed and PLA. The diameters of the holes might be a bit too small with a heated bed; in that case, drill them to size before assembly.

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