Topband: broadband transformer help please

[Greg - ZL3IX]

Felipe Ceglia - PY1NB wrote:
> Hi,
>
> I am about to make a pair of 3:1 (900:300ohm) transformers using a 
> binocular core for a waller flag receiving antenna.
>
> The fact is that I only have one pair of similar binocular cores 
> available and I don know their permeability values.
>
> I measured: 1.22uH for one turn, and 4.48uH for 2 turns on it.
>
> So, I ask... What is the equation for finding out permeability numbers 
> for it? I would like to have this xfmer working from 1.8 to 7.5Mhz, 
> would it be possible with this core?
>
> thank you for your attention,
>
> Felipe - PY1NB
>
>
> ps: I found this document, but it was OCR'd and the formula got screwed. 
> It shows on the last page.
> http://www.oselectronics.com/downloads/Broadband%20Transformers.pdf
>
>   

> Hi Felipe,
>   
The material permeability can be determined, but you will need to know 
the effective length of the magnetic path, and the effective cross 
section area.  My guess is that you should be more interested in the Al 
value, 1.22 uH in your case.  You could try these cores, but on 160m you 
will need at least 10 turns to keep the magnetising inductance up.  The 
coupling capacitance would probably be a bit high.  I would suggest that 
you try and obtain some FairRite cores type 2873000202, as recommended 
on W8JI's web site.  These are available from Amidon by mail order.  I 
replenished my stocks only a couple of weeks ago, and paid USD 0.80ea + 
postage.  The transaction was by Internet.

73, Greg, ZL3IX