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Glue three FT -240-52 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.

I managed 7 turns (one crossed thru center) using #10 THHN bifilar.

I like to add an additional strand of wire for lashing my solder joints.

This gives a strong “rebar” type mechanical bond in addition to the solder.

#10 Ring terminals are crimped, soldered, and shrink wrapped.

The negative terminal is also heat shrink wrapped.

I use closed cell foam as a pad under the toroids.

I screw down the negative terminal to the SO-239, then I use the long strand to lash the positive terminal to the center of the SO-239.

I flow solder through the lashings to make a solid connection between the two.

Closed cell foam fits snugly over the top of the transformer, and the watertight lid holds it all in place.

 

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.

1. Yaesu 991A with SP-10 Speaker and M-90 Mic
2. ICOM 7610. On top of the 7610 is a Yaesu SP-2000 speaker that works great just not with the ICOM. I am selling it for $125 dollars.
3. Picture of my vertical OCF flag pole antenna @ 16′ shortly being replaced although it tunes great on 80-6M
4. Picture of my Diamond X50A above the shack in the attic.
5. Also pictures is my Yaesu VX-8 mobile transceiver. It is know in the survivalist community as it broadcasts on all VHF and UHF bands.
6. I power the 991A with a Samlex SEC-1235 and the ICOM with an Astron VS-350M
7. You’ll see I am running three monitors to the back of my Macbook Pro. I leverage SDR-Control as seen in the pictures for FT8 (the map), SDR (the specturm) and the far right monitor for my log work.
8. I also took pictures of my RF Filter at the connection point of the 7610 and the common mode choke at the mid-point of my coax wire. I get zero common mode.

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