It’s fun to take a minimum of equipment out to a high place, lob a half-wave long wire antenna up into a tree, or support it with a fibreglass pole, and off you go – but such a long wire typically has a high (2.5kΩ to 3kΩ) impedance. To make it work, this needs transforming down to match the 50Ω output of a typical transceiver. This is done using a 49:1 autotransformer, built on a small toroid.

This post details my construction of the end-fed half-wave autotransformer and antenna for the 10m, 20m and 40m bands.

The antenna is very effective, can be erected in a variety of ways, exhibits low noise, doesn’t cause any RFI in the shack (use a choke balun in addition), places the feedpoint very near to your transceiver and it’s pretty easy to build.


There are many other articles out there on this subject:

There are also several other pictures of builds I’ve used to assist in this project, particularly from Ian MW0IAN and Michael G0POT. Thanks!


I hope to add some detail and pictures for the terrified constructor in this article, but the above are my source materials. This antenna and its autotransformer are a great project for those new to construction – the toroid winding isn’t hard.

One problem I find in many projects is that they go from diagram to finished construction very quickly with insufficient detail. It’s a bit like “How to Draw an Owl”….

How To Draw An Owl

So I wanted to document what I’d done, in the hope it might be useful to others, especially if you’ve not wound toroids before. The key point is that the count of ‘turns’ relates to the number of times the wire goes through the inside of the toroid.

Parts and materials

I’m building this autotransformer on a FT-140-43 toroid, purchased as a pack of two on eBay from Spectrum Communications, for £9.50 incl P&P.

I first wrapped this in plumber’s PTFE joint sealing tape.

I’m using Maplin 0.9mm 20SWG enamelled copper wire, code YN82D. £8.49 for a small reel (the picture on their page is misleading; it’s not as small as indicated). The source articles used 1.0mm wire; this will allow higher power to be transmitted – I’m mostly QRP, and without calculating it, 0.9mm will be fine for this.

For connectors, I’m using a BNC socket, £2.49 for Maplin’s code HH18U, and a small terminal post, again £2.49 for Maplin’s code FD69A. A small black plastic box, for £3.39 code KC91Y houses the completed autotransformer. This box is a bit ‘tight’, but it does all fit. You’ll also need a 100pF 3kV ceramic capacitor, available for £0.131 each (min order, pack of 10) from RS Components.

Building the Autotransformer

Wire lengths for the toroid winding given here come from reading other articles which describe builds on the larger FT-240-43 toroid. After I’d wound on the smaller toroid as shown here, I had some waste wire that required trimming – you could start with shorter pieces of wire, and I’ve estimated possible lengths for this [in square brackets]. However please note I haven’t tested those lengths.

Measure two lengths of wire, 1.0m, and 22cm. [For the FT-140-43 toroid, and a small box as given above, you could probably use lengths of 80cm and 18cm.] Strip the enamel from the ends for about 2-3mm, clean with sandpaper and tin with solder. Solder the two right-hand ends together.

Solder the right-hand ends together

Holding the soldered end in pliers, twist the two lengths tightly together for 13cm [for the FT-140-43, 9cm may suffice], to form the bifilar winding. ‘Bifilar’ meaning ‘two filaments’, I presume. The remaining length of the shorter piece of wire (at the end of the bifilar winding) being about 6-7cm long. Let’s call this length ‘the tail’.

Now to wind the bifilar part onto the toroid.

Pass the bifilar part through the toroid from the back, so that the ‘tail’ passes under the bottom edge, and the start of the bifilar part is entirely running through the inner of the toroid.

Starting the Windings

Wind the bifilar part back round the top and outer edge of the toroid, and back through the hole. Bend it again over the top. You should have two bifilar passes through the toroid. The end of the bifilar winding (that’s the soldered-together end) will be connected to the outer/shield/chassis connector of the BNC socket. The end of the tail will be connected to the inner/centre connector of the BNC. I’ll get back to the connections later.

The Bifilar Winding

Now, there’s the rest of the toroid to wind.

Wind the long single strand of wire through the toroid so that you have six passes on the inside.

The first part of the rest of the winding

Then pass the wire over the top, and over to the other side, going under the far edge. Loop over the top, and go back through the toroid so that on the far side, you have seven passes, with the remaining end (‘the antenna end’) going under and out.

Ensure that the windings are spaced as evenly as you can.

The final winding

Now tin the connections of the BNC socket and terminal post, and fit them in the box.

Carefully trim the three wires coming from the toroid to fit; scrape off enamel (might need to unwind the end of the bifilar winding a couple of mm to do this properly), clean and tin the ends.

The toroid, trimmed, with box

Solder the bifilar winding to the outer of the BNC; the ‘tail’ at the end of the bifilar winding to the centre pin of the BNC; the ‘antenna end’ to the terminal post.

Solder a 100pF 3kV ceramic capacitor between outer and centre connections of the BNC. Use a hot glue gun to fix the toroid to the inside of the box (not shown).

The assembled autotransformer

Put the top on the box, and there you have it.

The completed autotransformer

Building the antenna

There’s a long antenna wire attached to the autotransformer terminal post, followed by a coil and a further bit of wire. I used fairly rugged plastic-insulated wire; the outer diameter is 2mm, bought years ago on a 100m reel, probably from Maplin.

auto |         Length L1           Coil in uH    Length L2  (fibre-
xfor-|=----------------------------|\\\\\\\\\|------------|  glass
mer  |                                                    |   pole)
-----+                                                    |

For 80m+40m+20m+15m+10m, L1 is 20.35m, the coil impedance is 110μH, and L2 is 2.39m.

For 40m+20m+10m, L1 is 10.1m, the coil is 34μH, and L2 is 1.85m. I built this variant.

These lengths are taken from the diagrams in the PD7MAA article. Also see PA3FRP’s notes.

I had a length of plastic tube with an outer diameter of 15mm; I wound 141 turns of the Maplin 0.9mm 20SWG copper wire on this, and measured 34μH. This was very much a trial and error exercise; I had no idea how many turns it would take, so the initial length was extended by soldering more on, several times.


I attached the far end of the wire to the top of my 7m SOTABeams fibreglass pole, and fed the wire out so that the autotransformer was on the ground – imagine a right-angled triangle with a bowed hypotenuse; I also lifted the autotransformer up onto a small table.

Measurements with an antenna analyser show very favourable SWR readings for the three bands I’m interested in:

  • At 7.305MHz, SWR is 1.21:1
  • At 14.318MHz, SWR is 1.14:1
  • At 27.780MHz, SWR is 1.15:1

With a bit of adjustment, and appropriate positioning – it does seem sensitive to its location above ground (this is explained in the test article by John Huggins) – I think it’d be fine to transmit into this. I’d ideally like to tune it a little more, so I can use it on the exact frequencies I frequent without an ATU.