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

To: "Alberto di Bene" <[email protected]>, [email protected], [email protected]
Subject: LF: Re: [Lowfer] [TECH] Help needed on toroid trasf. construction
From: Frank Gentges KØBRA <[email protected]>
Date: Mon, 11 Jun 2001 10:06:03 -0400
In-reply-to: <[email protected]>
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>

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
I wound a similar 1:1 replacement for the TenTec RX320 which made a real
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
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