LF: Re: [Lowfer] [TECH] Help needed on toroid trasf. construction

[Frank Gentges KØBRA <fgentges@mindspring.com>]

Alberto,

To wind such a transformer you can use the Amidon FT50-J core with an 
initial permeability of 5000.  for a 10 kHz bottom end you will need about 
17 turns for a 50 ohm source/load.  For the other 4:1 side you will need 4 
times that.   With separate windings it will start dropping out at about 10 
MHz.  To extend it higher you will need to twist wires together for a 
5-filar winding.  The 4x winding is made up of 4 of those all connected in 
series.  I use Radio Shack wire wrap wire here which is readily 
available.  However, to get 17 turns of 5-filar wire wrap wire on that 
little core may be too hard.  You may either go to smaller wire or a larger 
core such as an Amidon FT87-J core.
In any event I will be glad to wind up such a beast and send it to 
you.  Before I start, let me know which side is 50 ohms and is the other 
side 4x50 = 200 ohms or is the turns ratio 4:1?  Or is the 4x side 50 ohms 
and the 1x side 12.5 ohms or 1/4 the number of turns?  How much space is 
available?
I wound a similar 1:1 replacement for the TenTec RX320 which made a real 
difference.
Frank K0BRA

At 01:34 PM 6/11/01 +0200, Alberto di Bene wrote:
> Hi all,
>   I need some words of wisdom and a bit of help about the
> construction of a small toroid transformer.
>
> First the antefact : I swapped recently my TS-850 for a TS-950SDX,
> a very fine radio, but much less sensitive on the LF band than the TS-850.
> Looking at the schematics, and doing some filter simulation with
> Electronic Workbench, I think to have found the culprit, i.e. L3, a small
> toroidal transfomer, with ratio 1:4, which is inserted in the signal path,
> after the input attenuator, and before the filters bank. With all probability,
> its  response at 136kHz leaves much to be desired. The TS-850 doesn't
> have any such transformer, the signal goes straight to the filters.
>
> So I have decided to replace it. Looking in the MiniCircuits catalog, I have
> found what seems to be the ideal replacement, the T4-6T-KK81 model, which
> has a -3dB response from 10kHz to 100MHz. Problem is that this transformer
> is not carried in stock by the Italian distributor, and both they and the
> MiniCircuits sales office have a minimum quantity amount for an order.
>
> At this point the only choice is to build it myself. And this is were your 
> help could
> come in my rescue. I need to know what your experiences are in building
> such a wideband kind of transformers. What is the best type of material,
> caveats to not forget, optimal number of turns, you name it.
>
> TNX for any help,
>
> 73  Alberto  I2PHD
>
>
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Frank Gentges KØBRA
fgentges@mindspring.com