Use Safety Wire Pliers to make twisted pair from two strands of #14 THHN.

#16 and #18 bifilar wire can be used for lower power applications, however #14 twisted pair must be used for Legal Limit power.

Twisted pair wire is great for noise canceling and will allow more turns of heavy wire to fit thru the center of a toroid.

(The  needed 15-19 Bifilar windings of #14 will not fit thru a ft-240.)


Glue three FT -240-43 toroids together using cut paper bag as a spacer.

The layer of fiber will facilitate a good bond that can be separated with a razor if needed.

As a general rule, a single FT-240 can handle a transmitter capable of 200w, a pair is good for 500w, and three will handle 1500w with reasonable headroom.

True power handling will be determined by the impedance mismatch per frequency.

Wrap toroid using the twisted pair and terminate as follows…

I was able to get 16 wraps of #14 around the toroid.

15 to 19 turns are needed to make the proper transformation.


You will need one SO-239 connector which can be can be found here.

I like to use copper strap to tie the ground and SO-239 together.

It can be bought here for $4 per 10′.

Solder your connections and tin the wires to be connected to the SO-239.

I like using a strand of wire from inside the THHN to make a “basket” to accept the tinned transformer connections.

It makes it much easier to get a good bond once the transformer is inside the waterproof box.

Mount in to 4x4x2 watertight box.

I fill the center of the toroid with silicone to help hold it in place.

The silicone dries into a “suction cup” that holds better than hot glue or adhesive, but is easily removable if needed.

I use closed cell foam to brace the sides of the toroid to the box.

This is the finished “Legal-Limit” 4:1 UNUN.


I built this 4:1 UNUN to replace my 64:1 UNUN.

The following videos show the 4:1 significantly outperforming the 64:1 on 40m. 

90′ Vertical 4:1 UNUN 100w SDR test Boone

90′ Vertical 4:1 UNUN 100w SDR test Boone

The 64:1 is pictured below.

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