First, you have to get a T-200-2 toroid.
This means, that it is a toroid of size 200 and material 2. The datasheet says that material 2 gives good results until 30 MHz.
Beginners: do not blindly copy other's designs: understand what you are doing first. In my case, I tried to understand the material first.
Next, pass three wires together around the toroid. I chose enameled 1 mm diameter copper wire. I passed eighteen times the center of the toroid. Other authors use thirteen. My aim was to better serve lower bands, so I put a few more turns.
Also, I verified the electric isolation of enameled wire and it seems that for 20W it is on the safe side.
This is the diagram. The colours names in French mean: Black, Red, and Yellow, from left to right. Refer to this diagram for explaining connections later.
I added another connection in point 2 - (upper red wire)
Choose a connector. I used the leftmost SO-238 one for my unun.
At the beginning, I only connected the upper-black/lower-red wires to the center of the SO-238 connector; the mass of the SO-238 connector is connected to the lower black wire using a bananna connector (upper left, near the "R"), and the antenna is connected to the upper yellow wire (lower left from the ring).
After some trials, in which the 1:9 unun relationship appeared to be a too high transformation ratio, I added another connector to the connection of the upper red and the lower yellow wires. My hope was that it would show a 1:4 unun ratio.
Following the purest Argentinian traditions, I used recovered materials. The case was coming from a ten-years old Synoptics 10 Mb Ethernet adaptor. I noticed that the inner part of the case was Aluminium coated, and that it was conducting. I put some rubber strips recovered from broken bicyicle tyres. The whole fitted nicely inside the case.
The finished unun.
-
From Antenna: The upper bananna connector corresponds to the upper-yellow wire. It is labeled 1:9.
-
From Antenna: The central bananna connector corresponds to the upper-red/lower-yellow connection.
-
To mass: The bottom bananna connector is connected to the SO-238 mass and to the lower black wire. It is expected to give a mass point for radials or a mass point.
-
The SO-238 connector: This one goes to the transceiver. The central SO-238 connector goes to the upper black wire, the mass of the SO-238 goes to the lower black wire / upper red wire.
Some tests being done, before I added the 1:4 connection.
The 7 meter mast is a fiberglass fishing rod. The antenna is made from electrical PVC-coated multistranded wire, according to my calculations it should be OK for 10-20 Watts.
The base is recovered from a Christmas Tree. The 7 m (~21') fishing rod was bought, it cost 15 euros at Decahtlon, a French sports supermarket.
Behind are my other antennas under development.