[TowerTalk] Binocular Cores vs Toroidal Cores

[Tom Rauch]

> Working on a pushpull receiving amp that uses binocular cores
> of 43 material in its design.  Is there any reason toroids,  of say, a
> 43 mix or MN8CX core could not be used as well with the same number of
> turns?

Binocular cores are better in broadband applications because more 
copper is INSIDE the core window where it adds to "useful" 
impedance. The length of copper hanging outside the core, which 
mostly contributes to flux leakage and stray reactances, is 
minimized with a binocular core. 

43 material is the wrong material for use in a non-resonant 
transformer of almost any application at HF. The initial permeability 
is far too low for most low flux density HF applications, including 
baluns and even normal KW level broadband baluns and matching 
transformers.

With high power / high flux density you might want 61 or 65 
material, but for receiving you want a Q=1 about 2Mhz, if it is a HF 
preamp. 43 material is good at VHF, if you are building a TV set 
amplifier or balun.   

I'm not sure what a MN8CX core is (it is a non-standard "house" 
mix number). Some books suggest a ridiculously high ui, using 
cores with an impedance peak at LF or VLF, which results in a 
lack of impedance at HF and a very high loss tangent. Misek and 
other books suggest 10,000 or higher ui cores, which is also not a 
wise choice. The impedance generally rolls off far below the lowest 
planned operating frequency, and that means you need more turns 
for a given impedance. They also saturate easier, and are more 
prone to IMD. That's why the cores in the Beverage Handbook and 
other places require so many turns, something like 15 or 20 if I 
remember right. 

What is needed is a core with the impedance PEAK at the lowest 
frequency of planned use. That would normally be a 73 material 
core for low power (or low flux density) operation on 160 meters. Q 
is 1 at about 2MHz, and for a given core size it results in the 
lowest turns in a system, widest bandwidth, and best performance.

The Beverage and preamplifier transformers I use only require one 
or two turns for the 50-ohm winding, because they are the correct 
core material.

Save yourself some trouble and get a 73 material binocular core 
that is about 1/2 inch deep, and you can use a one or two turn 50 
ohm winding. The 450 ohm winding in my beverage transformers 
only has 5 turns! Easy to wind, and it actually works better and 
costs less.

I use them so often I have hundreds of them in my parts pile, if you 
can't find any. Ameritron would sell you some also, they are used 
in the HF solid state amplifiers.
73, Tom W8JI
W8JI@contesting.com 

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