Manual 1:4 BalUn 600 Watt for Off center fed antenna (windom antenna)

This Manual for the 1:4 BalUn 600 Watt will help you step by step in building the homebrew kit. If an off center fed antenna (windom antenna) is fed with a coaxial cable from the set it is necessary to use a 1:4 BalUn. BalUn means: Balanced to Unbalanced. As a result, we adapt an unbalanced feed system (coaxial cable) to a balanced antenna system. The characteristic impedance of an off center fed antenna (windom antenna) is about 200 to 300 ohms. The transceiver has an impedance of 50 Ohms, therefore an impedance adjustment must also be made.

This 1:4 BalUn uses a symmetrical 1:4 impedance matching transformer. The 1:4 BalUn 600 Watt is suitable for QRO use. The most important reason to use a good BalUn is to ensure that the coax cable does not become part of the antenna system and therefore radiates as well. This has all sorts of nasty effects, think of: interference, EMI, RFI disrupted radiation pattern of the antenna, restless noise level. This last point is because not only does the coax cable’s shield radiate when you transmit, but the shield also works as a receiving antenna.

Two FT240-43 ferrite toroidal cores are used, one for the BalUn and one for the impedance transformer. The main characteristic of a good BalUn is maximum common mode current reduction and minimal loss of the differential current. There is a lot of information on the Internet and even in literature about making a BalUn, but unfortunately there are also a lot of bad designs that hardly work. This design is one that has proven to work very good!

The enclosure

In this step-by-step guide for the 1:4 BalUn 600 Watt, we begin by marking out and drilling the hole for the coax connector connection. The diameter of this hole should be 16 mm. This is a large hole and therefore it is easiest to create it with a ‘sheet step drill’. (If you have never heard of that, Google is your friend for looking up what you need)

TIP: Place the hole as high as possible (directional lid) As a result, connecting later is easier. (The center of the hole at 1.5 cm from the edge)

After drilling the 16 mm hole, I placed the chassis part into the hole, to determine the location of the fixing holes. I picked a chassis part where I fix the part with 4 screws, but 2 screws could also work. These holes can be drilled with a 3,5 mm drill.

We continue by marking and drilling the holes for the antenna connections and the tensile stools. Especially for longer or permanent antennas it is advisable to use a tensile stool. The hole for the stainless steel eye is drilled with a 6 mm drill and the hole for the antenna connections is drilled with a 5 mm drill. The place where the holes are drilled is to your own liking. After all the holes are drilled all parts can be mounted as shown in the picture below. For the antenna connections above and below the M5 cable lug, use the supplied serrated spring washers (these are both inside the housing and not on the outside), then the bolt will never turn when connecting an antenna wire. Place a flat washer on the outside and then attach a M5 nut.

Because two toroids are used for this 1:4 BalUn, the mounting plate must be raised. Place the M3 spacers as shown in the image below. There are extra nuts around the spacers because otherwise they can damage the housing.

The BalUn

Now it’s time to start the real work, the toroidal cores for the BalUn and the symmetrical impedance transformer. It was chosen to use wire with PTFE shielding for both the BalUn and the impedance matching transformer in this kit. Thus, two cores must be wound. NOTE: The cores look almost the same but are wound differently!

First of all, the 1:1 BalUn (common mode current filter):

We start by securing two wires with the use of a cable tie. This makes it a lot easier. Keep to the bottom about 7 cm wire length, this is more than sufficient. Then wrap the first 12 windings on half of the toroid as shown in the picture below. Make sure the windings are tightly placed together.

Then wrap 12 windings on the second half. Note that the wires are wrapped exactly as in the example. Do not change colors or wrapping directions. The bottom of the photo shows that four wires are on the top of the toroid, the two blue wires against each other and the black wires on the outside. On the top side of the picture this is exactly the other way around, there the wires come from under the toroid and here the black wires are against each other in the middle and the blue wires on the outside. If this is not correct, start again because then something went wrong.

The symmetrical impedance transformer

Now it’s time for the 1:4 impedance transformer:

Start winding twelve bifilar windings on the left side of the core, Keep the colors as shown in the images above. On the left side of the core, the wires start from the top of the core as seen from below and end at the top of the core at the bottom of the core. On the right side this is exactly the other way around, The wires start from the bottom view also below the core and end at the top of the core also on top of the core. I can imagine that this piece of text sounds confusing, it might be better to follow the illustrations carefully. NOTE: So the 1:4 Transformer is wound differently than the 1:1 BalUn.

Wrap the 1:4 transformer as shown in the images above. Now solder together the two inner wires at the top of the image. See image.

Finishing up

HF Kits has developed a solution for fixating the toroidal cores. With the HF Kits mounting plate it is easy to secure the toroids with a number of cable ties. First secure the toroids to the plate before mounting it in the housing.

First, lay down the 1:1 BalUn as shown in the photo below left. ( Blue wires are at the bottom of the image against each other). Next, place the HF Kits mounting plate in the center. On the right is the 1:4 Symmetrical adjustment transformer as shown in the photo below. Check this carefully! Now lay the whole thing on top of each other: On top of the 1:1 BalUn then the mounting plate and on the bottom the 1:4 transformer.

Now connect the two cores with 4 cable ties.

The 1:1 BalUn and the 1:4 Trafo must now be connected together. On the side where 4 blue wires and 4 black wires protrude from the assembly, the connection is made. Solder the four blue wires together and solder the four black wires together. Keep the wires as short as possible. Make sure the two solder joints cannot touch.

Now there are six threads left. Two times blue and black from the upper 1:1 BalUn and one time blue and black from the lower 1:4 transformer. The four wires of the 1:1 BalUn must be soldered to the SO239 connector. The two wires from the 1:4 transformer should be soldered to the antenna terminals.

Place the BalUn in the housing. Accurately make all wires to length.

  • The two blue wires of the upper 1:1 BalUn go to the center pin of the SO239 connector.
  • The two black wires of the upper 1:1 BalUn go to the ground terminal of the SO239 connector.
  • The single blue wire and the black wire from the lower 1:4 transformer, go to the connection points of the antenna. It does not matter which wire is connected where.
1:4 BalUn


A number of antenna variants can be thought of with a 1:4 BalUn. Of course, the antenna should have an impedance of about 200 ohms at the feed point. Some people prefer to feed a Delta-loop or Quad-loop antenna with a 1:4 BalUn, personally I think a 1:2 BalUn is a better choice for that purpose. The 1:4 BalUn is ideally suited for feeding an “Off center fed antenna” also called a “Windom antenna”. The antenna is half a wavelength (or multiple thereof) long and is fed at 1/3 of its total length. For example, a wire with a total length of about 20 meters, fed at 1/3 of the length. So from the feed point a wire of 6.66 mtr and 13.33 mtr. This antenna can be used for the 10, 20 and 40 meter bands. In fact, the antenna is a half wavelength for the 40 meter band, a whole wavelength for the 20 meter band and two whole wavelengths for the 10 meter band. The figure below shows the current flow of this antenna for the previously mentioned bands. The yellow line shows the common feed point of +/- 200 Ohms.

Off center fed current

Fine tuning

Hang the complete “Of center fed antenna” with some extra wire length in the desired location. Adjust the antenna to the correct length using an antenna analyser or SWR meter. Always remember: you can cut, you cannot add easaly. So don’t cut too excited. Always make sure you cut equal amounts from both sides in the correct proportion to keep the antenna symmetrical. That is, cut twice as much from the long thread than from the short thread. So if you want to shorten the antenna 15 cm, cut 5 cm from the shortest piece of wire and 10 cm from the long piece of wire.