1. First steps with FT140-43
In order to bring the high impedance of an End Fed Half Wave (EFHW) dipole (~ 2000…3000 Ohms) to 50 Ohms, a matching unit is required.
As a starting point I built a broadband transfomer made of a ferrite toroid, FT 140-43, from Amidon with a turns ratio of 3:24. The transformation ratio is 1:64. Since there are several manuals available, e.g. here or here, I simply copied the design.
The wire was 1mm enamelled copper wire, primary and secondary winding were twisted (s. photo). Two 68pF capacitors in parallel were used for compensation on the primary side. VSWR from 7MHz to 14MHz was < 1.5:1 with a load resistor of 3000 Ohms.
I used this transformer together with my EFHW-dipole for a few activations. The performance seemed 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 became the 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 suitable for a shortwave transformer. I tried to build another transformer with a different material mix but the project petered out.
Just recently, a discussion about the most suitable toroid for an EFHW transformer started on the SOTAreflector. I remembered my old project and activated it again.
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 Tx 36 – 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.
Maybe this was the reason why the project petered out 😉
- Data Sheet Ferroxcube 4C65
- Data Sheet Ferroxcube FT36 Toroids
- Data Sheet Amidon 43 Mix
- Description of Amidon FT140-43 Toroid
- Data Sheet Amidon 61 Mix
- Description of Amidon FT140-61 Toroid