EFHW Transformers

[DV9HF]

What’s the difference among 2:14, 3:21 & 3:24 transformer ratios in terms of efficiency, SWR & bandwidth?

[EA2BD]

Hello,

lots of useful info on the reflector if you make a search.
Just to provide a short way, just check here first, and read on in the post:

A SOTA Centric End Fed Half Wave Antenna (Part 1)

Ok Stephan, my project to improve the EFHW couplers used so far (in the slim design) lasted from spring 2018 to autumn 2019, that was a little longer than expected (…). For this purpose, after a prior evaluation using the toroidal core calculator from Owen Duffy, prototype couplers 1:49, 1:64, 1:69, 1:81 were built, measured on the workbench on purely resistive loads and back-to-back and finally tested on real EFHW antennas for 80-10 m. The following ferrite cores were considered: Amidon 2x …

Take your time to read deeply and avoid mistakes like loose coil winding, separated turns, etc

73

[OE5REO]

I asked the same question some weeks ago here:

Toroid windings for EFHW

Hello all, i recently purchased a DIY EFHW kit by HB9ZHK: Having never built a 1:49 transformer by myself before I wonder if there is any difference between winding 2:14 or 3:21 turns on the toroid. I found different winding methods which should all be leading to the same result: [toroid_table] [toroid_2_14] [toroid_3_21] [toroid_k6ark] So, which method should I go for? Can I expect any difference between 2:14 and 3:21? Thanks for your replies. 73 Martin, OE5REO

The suggested video by Colin MM0OPX was very interesting and helpful:

End Fed Half Wave Transformers.......Are they as good as we think? #hamradio #hamr

73 Martin, OE5REO

[DV9HF]

Thank you for the link.

[EA3BV]

2:14 is ratio 1:7, 49:1 transformer, about 2450 Ohms from 50 (49x50).

3:21 is about ratio 64:1, 3200 ohms, 50x64.

Every wire has his own impedance at determinate frecuency in each location.

You must create the nearest ratio transformer to get the best value ratio for your own wire.

I make custom ratio adding or removing secundary turns to get Z=50 ohms.

I thinks better performance will get when less ratio transformer, better 2 turns than 3 in the primary, for example.

But It is more dificult test it at field. The toroids allow loosses of course

[VK3AFW]

Two turns may be insufficient to provide low magnetising currents. Three turns is usually recommended. High magnetising current wastes power in the core. Of course the core cross section is equally important.

[HB9BCB]

I’m sure Owen Duffy wouldn’t be happy with the statements made above.
He says the really essentials in his 2 posts below.

Fast readers please note:

• ln(x) is not log(x)

• Math.log(x) means ln(x)

https://owenduffy.net/blog/?p=11786

https://owenduffy.net/blog/?p=14376

EDIT
Sorry, Owen Duffy’s post discussing the leakage inductance is of course on the same level as the two posts above:
https://owenduffy.net/blog/?p=23594

[EA3BV]

You probably got the right, indeed I just made another 49:1 with a FT82-43 with 3 turns in the primary.

Thanks for support!

Left: old, right: new modelo (still without enclosure or PCB and without C.

Finished:

[M5RJC]

Hi

I experimented lots and ended up with 3:22 turns for ~ 1:54 ratio

Cheers

Rick

[VE6VID]

Rick,
I have found that the 3:22 works well as well.

Malen
VE6VID

[SP2RM]

There is interesting videos from DL2MAN (one of developers of uSDX).

Antenna building #1 - EFHW Antenna - theory and practice of the broadband transformer

Antenna building #2 - EFHW Antenna - figuring out the right core size

For me, I the selected 3 primary and 24 total and 150pF from Owen Duffy’s posts for FT-82-43. The same winding is used by QRPGuys EFHW kits (which important for me, because this is commercial product).

I was able to tune to 1:1.1 SWR with such winding. With 3 primary and 21 total it was about 1.5.
But also, it is important a length of the counterpoise (feeding coax line before common mode choke).

[M5RJC]

Hi

I prefer to use a separate counterpoise and isolate the coax outer (either a separate choke or separated un-un winding). Then I can use several options of coax depending on the situation without impacting the performance.
I have a weight and a crocodile clip on the end of the counterpoise wire so I can throw it, and if there is a convenient metal fence I can attach it (assuming it’s not an electric fence !)

Rick