Toroid windings for EFHW

Hi,

how long is your wire for the Configuration ? Do you use an Coil for 40m ?

I probably have some deeply burred in some boxes, but Heinz @HB9BCB found even a slightly better one from DX-Wire. It would be interesting to see if the material of these cores have also such big variation as the ones from Fair-Rite, according Heinz.

BTW: when I built my transformers with this core, I didn’t find a company that sold them in Europe, so I ordered them with other bits and pieces from Mouser. The delivery was incredible fast.

73 Stephan

1 Like

No coil, only a 20m long wire. I do not know the exact length … when building the antenna I started with 21 or 22m and cut the wire down in small pieces to best SWR.

3 Likes

Greetings all, it can be noted that “DX Wire” does not have an English version of their web site. Pity, as they have a great range of products.

Geoff vk3sq

DX-Wire is closed until Oct 30, 2022 and does not take orders.

73 Heinz

Unfortunately there is nothing we can do about it if Fair-Rite and Ferroxcube stop producing those two little ferrite cores, if online stores are closed for the holidays, if online stores ship worldwide but only present their website in their native language, if large minimum order quantities are required, if astronomically high shipping fees are charged, etc.

So what can we do then?
Either do without these solutions with ferrite core-based impedance transformers (why not?) or look for a suitable alternative where pressing the buy now button still works, hi.

For QRP applications (max. ca. 15 watts), you could try the next larger ferrite core from Fair-Rite. Compared to the “original” core, this has a core efficiency that is only about 2-1 % lower in the range of 3.5-30 MHz.

BTW, The slightly larger core diameter should not tempt you to unfavorable winding, so the following points mentioned by Owen Duffy should be avoided:

  • thick wire insulation

  • space for flux between the conductor and the core

  • spreading the winding out around the core

  • splitting the winding into parts

  • the Reisert crossover connection

  • wrapping the core with insulation.

3 Likes

First of all, a big thanks to Heinz @HB9BCB for his generous gift in the form of these toroid!

Second, thanks to everyone here (and Owen Duffy in the background) for this discussion and underlying research!

I have been using a variant of Heinz‘s slim coupler for almost all of my activations, and while it performs well, being able to increase the transformer efficiency from 60% or so to 90% means likely a lot. Just thinking how eager we typically are that our transceivers put out a full 5W; yet with most of the popular designs, we are passing only 3-3.5W to the antenna.

It also stroke me how we all are tempted to be satisfied with ‚traditional‘ designs of components (and flawed or suboptimal measurements) in a hobby that is science-based.

So again, thanks for the rigor and discipline in here!

1 Like

Just to make sure: With this you mean that the primary winding should be a tap (true autotransformer-style) instead of using twisted magnet wire for the first two or three turns, don‘t you (as shown on your images)?

73 de Martin, DK3IT

PS: One more question for @HB9BCB and the others here: For a vertical (20-17-15 with traps), feedpoint ca 1-1.5 m above ground (11m mast), would you use 1:49 or 1:64 as an impedance ratio (3:21 or 3:24 turns ratio)?

1 Like

One more question (apologies if this has already been answered in this now substantial thread):

As for measuring efficiency, you basically use two identical transformers connected back to back, right?

And then, do you use
a) an spectrum analyzer plus tracking generator to measure the attenuation, or
b) an RF power signal (5W or more) plus a power measurement at the other end?

My understanding is that only b) would show losses from e.g. secondary effects from heating.

Also, if I remember correctly, there are subtle limitations of the back-to-back measurement setup for EFHW transformers, but I forgot the details.

1 Like

Martin,

Here my answers to your last 3 posts.

Re splitting the winding:
Yes, as an autotransformer with a tap after 3 turns.

Below an example of a (not recommended) splitted winding, meaning the first 3 common turns. Also not recommended is the Reisert winding method, which is intended, for practical reasons, for winding toroidal cores with coaxial cables.
Photo below copied from the beginning of this thread.
toroid_3_21

Re measuring transformer efficiency:
Yes, these points have also been discussed on this reflector, but don’t worry about your question.

Useful and also recommended by Owen Duffy: A transmission measurement (s21) with a single impedance transformer (terminated with the nominal resistive load, or for perfectionists: the nominal load minus 50 ohms to account for the meter input impedance), usually performed with a VNA (photos of my measurement setup below).

Back to back measurements with 2 impedance transformers are often made for practical reasons (e.g. because of the capabilities of the measuring device or the math skills of the tester) and are not good enough for certain purists, hi.

BTW, The transformer efficiency of my very first 1:64 EFHW impedance couplers with a FT-82-43 toroid and 3 primary turns was something like this:

  • 65% at 3.6MHz

  • 70% at 7.1MHz

  • 72% at 10.1MHz

  • 74% on 14.2 and 18.1 MHz

  • 75% on 21.2, 24.9 and 28.5 MHz


The measurement of the transmission properties when the ferrite core heats up due to use and/or extreme ambient temperatures is perhaps a bit too academic for our needs. There are scientific studies to investigate the temperature-dependent behavior of the complex permeability of ferrite toroidal cores (example for NiZn ferrites below), busy readers could read chapters 1 and 4 at least …

So just complicate things as necessary - and use the time to climb mountains and make QSOs, hi.

https://www.researchgate.net/profile/Goran-Stojanovic-2/publication/286162956_Electrical_and_temperature_characterization_of_NiZn_ferrites/links/5e406caa299bf1cdb91bdab7/Electrical-and-temperature-characterization-of-NiZn-ferrites.pdf?origin=publication_detail

3 Likes

Someone asked why the VNA graphs do not also show the loss and efficiency of the measured impedance transformer.

The requested values ​​can be calculated in a simple way.
It should be noted that the load resistance used (3203 ohms) and the VNA input impedance (50 ohms) are also measured during the transmission measurement (s21) of the 1:64 impedance transformer.
In addition, because the impedance of the transformer input is not exactly 50 ohms, there is a (small) matching loss that must also be taken into account.

Calculation example for 7.0 MHz (Marker #2)
Impedance at transformer input 47.56-j0.94 ohms
→ Mismatch loss 0.003 dB (calculator by Owen Duffy see below)

Transformer loss=
-|s21|-load loss(load resistance+VNA input resistance)-mismatchloss=
-18.59-(-10*log50/(50+3203))-0.003=-18.59-18.13-0.003=0.457 dB
Transformer efficiency=10^(-0.457/10)=90.0 %

Calculator by Owen Duffy to
Calculate VSWR and Return Loss from Zload (orYload or S11) and Zo
https://owenduffy.net/calc/ZY2Vswr.htm

For those interested: With the ferrite core and compensation capacitor of the 1:64 impedance transformer (above), when wound as a 1:49 impedance transformer, the transmission values shown below result.
At 7.0MHz, the transformer loss is 0.437dB and the transformer efficiency is 90.43%.

The difference is not exactly dramatic - on a real antenna, the different transformation ratios will be particularly noticeable in the lowest and highest frequency bands - also not dramatic, but possibly relevant when working without an ATU.

4 Likes

I am not capable of doing this so if somebody is.

I am interesting of sumerize all this articles in one short with drawings what to use, both feritte and capacitor, how to wind, what lenght of wire

1 Like

As with most things, that depends on your requirements (bands, efficiency, power handling, …).

I started my experiments with the well written document of Heinz @HB9BCB (in German) and continued my experiments that resulted in the following document:

At that time, I was not aware that Heinz experimented as well with the same toroid core. My main goal was to design a 20m long portable multiband EFHW that is resonant on more bands than the usual 40-,20-,15- and 10m bands and requires a minimum intervention to change the band segments. The solution was to insert one coil with one bypass link or switch.

The majority of transformer designs that one finds on the web and even many commercial products are not very efficient. Just read Heinz’ articles in this reflector and you’ll find out.
They work, but Owen Duffy would say: “Well, in ham radio, everything works. But systems that work better increase the prospects of contacts.”

73 Stephan

1 Like

Well, that is what I would like to avoid. Reading another 20, 30 or 50 pages. I would read them one day, but not today. I want direct recipe what and how to do.

So, I will start with my setup. You may tell what is good and what is bad.


This is my 20m setup. Core is 80-6 and with capacitor and 27 turns secondary resonates on 20m. 3 turns primary. Here is swr curve.

I am using also another transformer 2:14 turns

I wound it on 2x 140-43 core. Choke is wound on one 140-43 core.

With both boxes I use 2x5 meters long counterpoise. It goes into green connector. I am not using capacitor parallel to primary because I use it on 40, 30 and 20m only. I think that capacitor is not needed for this three bands.

I do not know wire lenght. I broke it two days ago. After repair I will measure wire lenght and give swr curve for grey box

1 Like

Hello Martin,

I’m using Wuerth toroids (material 4W620) for a while. Some weeks ago I tested a transformer (UnUn, 2 + 12 windings) with such a toroid. The loss @ 40 m … 20 m was around 0.5 dB ( - 12 %) or below.
My measuring setup was similar to what HB9BCB described.

Now I’ve build a transformer with 3 + 18 windings (autotransformer, z transformation 1:49) using the mentioned wuerth 742 701 0.

The loss is not higher than 0.4 dB (+/- 0.2 dB) @ 80…15 m. The VSWR on the 50 Ohm side is 1.15…1.56 @ 80…20 m and 2.4 @ 15 m. (+/- 0.2 dB due to accuracy of the measuring equipment)

With 120 pF parallel to 50 Ohm the loss is not higher and the VSWR 1.1…1.8 @ 80…15 m. Loss and VSWR @ 10 m are a little bit better than for 15 m.

So in my opinion you may use such a toroid. This toroid (and some more) is available from reichelt.de.

73, Ludwig

2 Likes

As far as I see there are 34 or 35 visible turns on the toroid on your first picture. You mentioned 27 and 3 turns. What is wrong?

73, Ludwig

I do not remember exact number of turns. From my weak memory are 27. Maybe you are right.

It is not very critical. Just wound turns and try. All you need is resonance on the band. Than change the number of turns for best swr.

I connect 3,9 kohm (it may be 4,7 or 3,3) and tune with capacitor for resonance. NanoVna is grat tool and you can watch dip moving as you turn capacitor.

Do not make my mistake and glue toroid before tuning.

Than cut litlle more than half lambda of wire, go out, erect antenna and little by little cut wire until best swr. Again, nanovna is great tool

I am tired of long articles and lot of links. I just want short know how.

Also, if one thinks that something is wrong I do not like: you are wrong and here is why … (link to 50 pages of tehnical explanation).

I would like explanation in two or three sentences and in short what to do. Pictures or drawings also.

It takes more time to drill holes in the box that to wind toroid. Experimenting with different number of turns is very easy.

EFHW antena is simple. 20 euros in toroids, magnet wire and few meters of insulated wire gives you years of experimenting and using antenna in the field

Here are few youtube videos. See the 5 band tuner. One inductor and capacitor from transistor rx

I would like something like this about transformers

1 Like

Ok, very quick:

  1. Most EFHW transformer designs on the WWW are flawed, despite their popularity.

  2. Depending on the band and design, some pass only 25% of your output power to the antenna. That is a loss of 6dB and this can mean that you can only work strong stations.

  3. The new best practice is as described and illustrated by Heinz above:

  1. In short:

a) Use either the recommended toroid, the alternative from Wuerth, or TWO FT-50-43 stacked and glued together with superglue.

b) one winding of 24 turns and a tap at the 3rd turn; the tap is the input.

c) styroflex or other high-quality capacitor 68-100pF in parallel to the input.

d) tight windings (see Heinz‘ perfect example), not too thick wire (0.4 or 0.5 mm); thicker wire will not lay as closely on the toroid and hence cause losses.

Simple enough? :sunglasses:

73 de Martin, DK3IT

PS: Two stacked FT-50-43s perform surprisingly well in case you have difficulties sourcing the other toroids.

3 Likes