Mini EFHW transformers

 DG1JAN has shared on GitHub some really nice designs. I went with the MicroUnUn which seems to be a nice alternative of the popular K6ARK.

It uses Amdion FT82-43 (FairRite 5943000601) toroid. The author suggests two other variations:

1. combining 5943000601 with a smaller inner toroid 5943001101  
2. using the double length FairRite 5943000501

I was hesitant about he combined toroid design - the winding was really dense inside the small toroid. So I made few units with 601 and one with 501. I made several configurations and measured the lowest SWR.

Winding	Secondary turns	Secondary / primary	Ratio
10-1-10	21	7	49:1
10-1-10	22	7.33	54:1
11-1-11	23	7.66	59:1
11-1-12	24	8	64:1
12-1-12	25	8.33	70:1
12-1-13	26	8.66	75:1
13-1-13	27	9	81:1

I did my tests in Parc de la Cité with a radiator for 18MHz and SotaBeams Carbon mast. Here are the results of the first round:

Subject	Ratio	Lowest SWR	Frequency
1	49:1	1.43	18.350
2	49:1	1.44	18.300
3	81:1	1.16	18.000
4	81:1	1.12	18.100
5	81:1	1.12	18.200

Second round of tests:

Subject	Ratio	Lowest SWR	Frequency
4	81:1	1.10	18.090
5	81:1	1.10	18.170
6	49:1	1.44	18.190
7	54:1	1.35	18.170

Third round of tests. 

Subject	Ratio	Lowest SWR	Frequency
5	81:1	1.16	18.210
7	54:1	1.38	18.170
8	64:1	1.20	18.150
9	64:1 *D	1.25	18.130

1:64* D uses the double length FairRite 5943000501

These are the indications of my RigExpert. When hooked to KX2, the values are about 0.25 better.