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Building the EFHW Antenna Kits (EFHW broad band transformer).

Updated: Sep 19

We have written this article as a guide to building a MK1 Radio-Stuff Ultra Light EFHW antenna Kit. The MK2 version and Light Weight SO239 Portable EFHW Antenna are built in a similar way. The only difference to the builds are a) what you get in the kit and b) the length of the bifilar winding. The bifilar winding length varies depending on the core type and size.

If you are unsure how long to make the bifilar winding, use a piece of string, cotton or wire to build dummy winding then measure the length of what you have just used.

Below some articles and videos on core different winding types.

Owen Duffy Small efficient matching transformer for an EFHW.

And from the blog here Two really good YouTube video's on broad band transformers

Just remember efficiency does automatically mean a usable antenna on all frequencies.

The MK1 Ultra End Feed kit comprises of;

Single Fair-Rite T50-43 core
M3 fittings including wing nut and solder tab
100pf capacitor
Approx. 1m enamelled copper wire
BNC connector
Approx. 10.25m green SOTA wire
Kite winder and 30m nylon string

These kits are quite easy to assemble with the exception of the 49:1 transformer which can be a little fiddly due to its small size. Here are a few tips.

Firstly check everything is there. Then cut the wire in half so you have about 50cm to work with. Bend over the end on itself by about 12-13cm to give you a hoop. The size of this loop will vary if you have bought a different kit or are experimenting with more windings.

You will then need to wind the bent section together to give you bifilar windings.

TYo make a 49:1 transformer pass the bifilar windings through the core twice, then five more turns of the single wire giving you seven turns on the toroid. You then pass the wire across through the toroid centre (8th turn), then six more windings to give you a total of 14 windings with 2 bifilar giving a 49:1 transformer, see below.

Cut the excess of the wire giving yourself enough to poke each wire through the solder points. Remove the enamelling at the end of each wire. I use a soldering iron to burn the enamelling off with a little solder, a file or sharp knife will also work (at your own risk, trust me I know).

Push the end of each through the holes. It is important to push the bifilar windings through marked "Bifilar winding".

It's then just a case of placing and soldering all the components in place.

You can see above I have connected the antenna wire by making a knot through one of the strain relief holes and then soldering the wire to the solder tab. The antenna wire is held in place using the nut and bolt and wingnut.

When the transformer is built you can test it with a VNA or Antenna Analyser by putting a 3.2kohm resister between the entenna connection and outer connector of the BNC connector (or GND connection if present). You should see some results like the ones in this article .

When you add your antenna wire you will need to tune the antenna to the bottom end of the lowest band, for example 7mhz if you are looking to have 40m, 20m, 15m, and 10m bands. If you want to use 17m and 12m you can use and ATU, or place traps in series along the wire.

Additional comment for the SO239 board, you will need to solder a small piece of wire from the center of the SO239 to the hole just before the capacitor, see below.

Morten LB0FI has also done a build video for the SO239 EFHW Kit here.

P.S. there is one mistake on the MK1 circuit board. See if you spot it. This was corrected on the MK2 board.

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