If you have access to Amidon cores I would try a ferrite toroidal core with
the #72 or #77 material. For wideband applications these materials are
rated for 0.5 -30 MHz. From my experience the frequencies given by Amidon
are 0.5dB or at worst 1dB points, so for -3dB the range might be something
like 0.2 - 70MHz
Below 200kHz loss will increase, but at these frequencies the noise level
is so high that you won't notice the few dB extra.
Besides the material it is the number of windings that determine how
'wideband' a transformer is, more windings will improve the LF behavior (at
cost of extra loss at the HF end) and vice versa. Experiment with the
number of windings.
A final tip : use a rather large toriod. It will allow you to space the
windings and thus reducing the inter-winding capacitance (that often limits
the upper frequency).
73, Rik ON7YD
At 13:34 11/06/01 +0200, you wrote:
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
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
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