EFHW Transformer

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.

balun1_8
Transformer 1:64 made of a FT 140-43 toriod and 1mm enamelled copper wire, turns ratio 3: 24. Primary and secondary winding are twisted. The capacitors are in parallel to the primary winding, have a value of 68pF each and a rated voltage of 1 kV.

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.

IMG_2382
The same transformer now with a BNC-connector.

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).

Amidon-43 Mix properties
Loss (i.e. µ“) of Amidon 43 Mix @ 10 MHz marked with a red circle. This is about 55 times the loss of Amidon 61 Mix.

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.

IMG_2678 - Kopie
Note: In this picture the turns ratio is still 4:32. Later it had to be changed to 4:28 in order to achieve low VSWR in combination with the EFHW dipole.
IMG_2687 - Kopie
The fixed capacitor has a value of approx. 200pF and a rated voltage of at least 1 kV.

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 😉

 


Appendix:

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DM/BW-009 – Kandel – 1241m

Abstract:

October 17th, 2018; 14:00 – 15:00 UTC
My first activation of Kandel.

Wx: sunny,  14°C but limited visibility
Walking time less than five minutes from the restaurant at Kandelsattel.
Ascent/ descent: approx. 40m from/ to the restaurant.

11 CW QSOs on 20m (2 x DX into NA), 3 CW QSOs on 30m, 12 CW QSOs on 40m
Conditions on 20m were good. Activity on 30m was low as almost always, conditions on 40m seemed to be poor.

Rig: MTR-3B / 3W, 1250mAh LiPo (2S), Palm Pico Paddle Single
Ant: EFHW Dipole (40m, 30m, 20m) as inverted L on a 6m GRP Pole

How to get there?

There is a road leading up to almost the summit. I took the bus from Denzlingen to St. Peter and another bus to Kandel.

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Happy anniversary!

50

The activation of DL/AL-003 (Hochrappenkopf) was my 50th Activation of a SOTA- reference. As I started activating on a regular basis in 2015, it has been approximately one summit per month in average since then.

I’m quite happy with this achievement as I have family, job and several commitments, all of them with higher priority than playing radio and climbing mountains 😉

DL/AL-004 – Rappenseekopf – 2468m

Abstract:

September 6th, 2018; 07:30 – 07:50 UTC
My first activation of Rappenseekopf – quite a pleasure after Hochgundspitze.

Wx: sunny with occasional clouds, nice and warm.
Walking time: up 20 minutes, down not known as I continued to Hochrappenkopf.
Ascent: approx. 200m from the col (2272m) between Hochgundspitze and Rappenseekopf.
Descent: approx. 150m down to the col in the West (2320m).

20 CW QSOs on 40m
Conditions on 40m were good. Due to the last- minute change of plans, SOTAwatch reported the reference as DL/AL-005, which was wrong!

Rig: MTR-3B / 3W, 1250mAh LiPo (2S), Palm Pico Paddle
Ant: EFHW Dipole (40m, 30m, 20m) as inverted L on a 6m GRP Pole

How to get there?

From Rappenseehütte follow the track to „Rappenseekopf“. At the col (2272m) turn right and follow the trail marked with red dots. It’s easy.

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DL/AL-005 – Hochgundspitze – 2460m

Abstract:

September 6th, 2018

The activation had to be canceled just outside the activation zone!!!

Wx: sunny and blue sky, 10C at the hut, nice and warm at the secondary.
Walking time: 1:40 h (up and down)
Ascent: approx. 340m from Rappenseehütte (2091m) to the secondary (2430m)
Descent: approx. 160m to the col (2272m).

How to get there?

From Rappenseehütte follow the track to „Rappenseekopf“. At the col (2272m) turn left and follow a faint trail up the S/W ridge. A more detailed description is available upon request via email.

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OE/TI-160 – Hohes Licht– 2651m

Abstract:

September 5th, 2018; 09:40 – 10:30 UTC
My first activation of Hohes Licht – the second highest summit in Allgäu.

Wx: cloudy with occasional sunny spots, 8C at the hut, cool and windy at the top.
Walking time up: approx. 1:00h, down: 1:10h
Ascent: approx. 390m from Grosse Steinscharte (2262m)
Descent: approx. 560m to Rappenseehütte (2091m).

3 CW QSOs on 40m, 3 CW QSOs on 30m, 6 CW QSOs on 20m
Conditions on all bands seemed to be fair, but activity was low. Later I learned that SOTAwatch had been down all morning.

Rig: MTR-3B / 3W, 1250mAh LiPo (2S), Palm Pico Paddle
Ant: EFHW Dipole (40m, 30m, 20m) as inverted L on a 6m GRP Pole

How to get there?

From Rappenseehütte follow the track to „Grosse Steinscharte/ Heilbronner Weg/ Hohes Licht“.
After a while the track divides to Heilbronner Weg (left) and Hohes Licht (right). Follow the track to Hohes Licht. Many people leave their rucksack at the fork and climb the summit without additional weight.

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DL/AL-006 – Rotgundspitze – 2485m

Abstract:

September 5th, 2018; 07:05 – 07:40 UTC
First activation ever of Rotgundspitze!

Wx: sunny and blue sky, 8C at Rappenseehütte, cool at the top.
Walking time up: 1:30h , down: 0:30h.
Ascent: approx. 400m from Rappenseehütte (2091m).
Descent: approx. 220m to Grosse Steinscharte (2262m).

11 CW QSOs on 40m, 3 CW QSOs on 30m, 1 CW QSO on 20m
Conditions on all bands seemed to be fair, but activity was low. Later I learned that SOTAwatch had been down all morning.

Rig: MTR-3B / 3W, 1250mAh LiPo (2S), Palm Pico Paddle
Ant: EFHW Dipole (40m, 30m, 20m) as inverted L on a 6m GRP Pole

How to get there?

From Rappenseehütte (2091m) follow the track to „Grosse Steinscharte/ Heilbronner Weg/ Hohes Licht“. 50m behind the junction (signpost saying „Steeg 3h“) turn left without track. Then head for the notch in the east-ridge. Once you reach the ridge, stay on it until the summit. A more detailed description is available upon request via email.

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Gipfel rund um die Rappenseehütte

Abstract:

September 4th, 2018 until September 6th, 2018
A „Mini- Expedition“ to SOTA- references in the vicinity of Rappenseehütte.

Wx: sunny in the morning, overcast in the afternoon. Summits covered with clouds.
Walking time up: 3:10 h, down: 2:00 h
Ascent/ Descent: approx. 1100m from Alpe Eschbach

How to get there?

From Oberstdorf central station take the Bus „E“ to „Fellhornbahn/ Birgsau“ until the terminal stop „Alpe Eschbach“.
From there, follow the road to Einödsbach and continue on the trail to Rappenseehütte via Petersalpe and Enzianhütte. The trail can be very wet and muddy, even days after the last rain.

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F/AB-336 – Chamechaude – 2082m

Abstract:

August 26, 2018; 08:35 – 09:20 UTC
My first activation of Chamechaude – the highest summit in Chartreuse.

Wx: sunny with occasional clouds, 7C at the car park, windy and cold at the top.
Walking time up: 1:45 h, down: 1:00 h (approx. 750m)

31 CW QSOs on 40m, 5 CW QSOs on 30m, 5 CW QSOs on 20m
Conditions on 40m were fair, 30m was also ok but little activity. 20m was really poor with deep QSB and almost no spots on RBN. Later I learned that a CME hit earth’s magnetic field and caused a k- index of 7 (!)

k-Index_2018-08-25

Rig: KX3 / 10W, 2200mAh LiPo, Palm Pico Paddle
Ant: EFHW Dipole  (40m, 30m, 20m) as inverted L on a 6m GRP Pole

How to get there?

Start from the big car park at Col de Porte (1325m) and just follow the sign posts „Tour de Chamechaude“.

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F/AB-389 – Charmant Som – 1867m

Abstract:

August 25, 2018; 16:00 UTC
My first activation of Charmant Som- the second time I was standing on top.

Wx: Cloudy and fog on the summit, light drizzle.
Walking time up: 30 min. down: 20 min.
Ascent: approx. 200m from the farm (1669m)
Descent: approx. 200m back to the farm.

1 CW QSO on 30m, hence the activation did not qualify 😦
Poor condx on 20m and 30m and a contest on 40m

Rig: KX3 / 10W, 2200mAh LiPo, Palm Pico Paddle
Ant: EFHW Dipole (40m, 30m, 20m) as inverted L on a 6m GRP Pole

How to get there?

Leave the main road at Col de Porte towards Chalets du Charmant Som (1669m). There is a big car park. From there follow the signs to the summit.
Charmant Som  can be very crowded, especially on a weekend as it is easy to climb.

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