[TowerTalk] ferrites for Beverage transformers

[Jim Brown]

GM David,

I don't consider myself an expert on this topic, but I just wound four 
transformers for VE3DO loops using #43 cores that I had in my parts 
stash. Winding guidelines were for binocular cores and a ferrite 
material with greater mu, so I increased the number of turns by about 
50%. They all work just fine. If all that I had were #31 cores I would 
have used those instead. The key to this is knowing u' and u'' (u' is 
mu, u'' accounts for the loss component) for the material. Fair-Rite 
publishes this data for all of their materials as a graph of the values 
as a function of frequency in a dozen or so pages near the front of 
their excellent catalog.

One VERY important point that everyone seems to have missed in winding 
these transformers is that stray capacitance between windings couples 
common mode noise from the coax shield to the antenna, then back down 
the coax to the RX as a differential signal.

A ferrite toroid has such a high mu that it contains virtually all of 
the magnetic flux, so there is no need for one winding to lay on top of 
the other, and doing so greatly increases the interwinding capacitance. 
The transformers I wound are on opposite sides of the toroid core. I 
used small gauge wire (#28?) from a hunk of multi-pair in my junk box.

73, Jim K9YC

On Mon,4/10/2017 9:08 AM, David Robbins wrote:
> I want to get some cores to be able to repair two wire Beverage transformers
> and reflection transformers.  I have been reading this:
> http://www.qsl.net/n1eu/topband/BeverageXfmrs.htm and some other articles
> and trying to find the referenced parts, in particular the 412-5250
> binocular core sounds good for the impedance transformers, but google is
> failing me when trying to find a source. who stocks them??  I haven't seem
> much about reflection transformers either, would that same core be good for
> reflection transformers or what else is recommended??
>
>   
>
> David Robbins K1TTT
> e-mail:  <mailto:k1ttt at arrl.net> mailto:k1ttt at arrl.net
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