1. First steps with FT140-43
The impedance of an End Fed Half Wave (EFHW) dipole lies in a range of approx. 2k Ohm to 4k Ohm. In order to achieve a reasonable matching at 50 Ohm, a transformer can be used.
As a starting point I simply copied the design of a broadband transfomer, using a ferrite toroid FT 140-43 from Amidon with a turns ratio of 3:24. The transmission ratio, which is the square of the turns ratio (1:8), is 1:64. This means an impedance of 3200 Ohms will be transformed to 50 Ohms. The above mentioned range of 2k to 4k will fall in a range of approx. 30 to 60 Ohms which results in a slight (but acceptable) mismatch.
There are several manuals available, e.g. here or here.
Edit 2021-11-03: There is a discussion on the SOTA-Reflector which indicates, that the cross-over winding technique causes additional loss. Apparently it is more efficient to make all windings as close as possible.
I used 1mm enamelled copper wire, primary and secondary winding are twisted (s. photo). Two 68pF capacitors in parallel are used for compensation on the primary side. VSWR from 7MHz to 14MHz is < 1.5:1 with a load resistor of 3 kOhms.
I use this transformer together with my EFHW-dipole for my activations. The performance seems to be a bit down compared to a linked dipole but still good. At one point I decided to replace the SO-239 with a BNC-connector which is standard for my SOTA equipment.
I never really questioned the design of the transformer but had the feeling, that the Amidon 43 Mix isn’t the best choice. Just recently, a thread about the most suitable toroid for an EFHW transformer started on the SOTAreflector.
2. Experiments with Tx36 – 4C65 from Ferroxcube
In above mentioned thread someone suggested the Amidon 61 Mix for use in broadband transformers. Its loss @ 10 MHz is approx. 4 (four) while 43 Mix shows a loss of about 220 (two hundred twenty!) in the same frequency range (s. graph below).
A big enough toroid with 61 Mix wasn’t at hand, but a Tx36 – 4C65 from Ferroxcube. This material has even lower loss than Amidon 61 Mix (2 @ 10 MHz).
Baseline: Transformer with FT140-43
As a base line I used the design of my first transformer.
- AL = 885nH
- 3 turns primary
- 24 turns secondary
Design with Tx36 – 4C65:
Next, I tried to scale the known design to the properties of a Tx36 – 4C65 toroid. My goal was to keep turns ratio and inductance the same. But it soon turned out that the number of turns (seven primary and 56 secondary) wasn’t feasible.
So I started with five turns primary. The impedance of the primary winding at 7MHz wouldn’t be as high as with the FT140-43 toroid, but reaches almost 200 Ohms which should still be sufficient.
- AL = 170 nH
- 5 turns primary
- 40 turns secondary
With this approach, however, it wasn’t possible to achieve simultaneously good matching on the bands from 7 MHz to 14 MHz with a single fixed capacitor.
Only with a turns ratio of 4:32 all three bands could be matched. A capacitor of 200pF was required for compensation on the primary side. It worked also with a turns ratio of 3:24, but the impedance of the primary winding was now down to 67 Ohms. This semed a bit low and in order to avoid trouble I went back to 4:32.
When I tested the transformer in combiation with the EFHW dipole, the VSWR was >2:1 on all bands. Apparently the impedance of the end fed dipole was lower than the assumed 3000 Ohms, resulting in a resistance of about 30 Ohms on the primary side of the transformer PLUS some reactance, varying from band to band. Surprisingly it wasn’t an issue with the FT140-43 toroid in the very same antenna setup.
The most promising option was to reduce the turns ratio of the transformer to 4:28. With a load resistance of 2.5 kOhms and 2 kOhms the VSWR was now 1.2:1 on all three bands. The measurement with the real EFHW dipole however, resulted in an even worse VSWR and lower resistance. The reason is not yet fully understood but more experiments seem to be necessary.
Finally I gave up the project due to the lack of further improvement possibilities.
3. Variants with smaller FTxx-43 toroids
A very good manual written by Heinz, HB9BCB, can be found here.
I built another UnUn 1:64 with the smaller toroid FT 82-43. The turns ratio is again 3 : 24 but this time I built an auto-transformer.
With a resistive load of approx. 3.3 kOhm, between 7 MHz and 14 MHz no capacitor is required. The reactive component of Z is almost negligible and VSWR is below 1.6 : 1.
I had some concerns about saturation of this tiny toroid but up to 10 watts it seems to be working ok. In the meantime I worked ZL1TM for my all time ODX from a SOTA reference.
The new design saves about 60 grams compared to the version with FT 140-43.