[TowerTalk] New Larger Fair-Rite #31 Toroid

[Jim Brown]

Jim,

  On Tue,3/28/2017 9:00 AM, Jim Thomson wrote:
>   Danny,  E73M, and myself looked at the new  4 inch OD torroid, and discarded it soon after. The ID is simply too big, being a whopping 3 inch ID,

The primary virtue of the larger core is that it makes it possible to 
wind more turns of larger cable.

> so even though its twice the length of the smaller  2.4 inch OD x 1.4 inch ID, it buys you nothing when you measure both of the torroids, with 1-7 turns of  393 or 213 .

Study the Fair-Rite data a bit more carefully. Yes, the two plots look 
quite similar, but it's a log-log plot. Five turns on the larger core 
resonates at 20 MHz, but at 25 MHz on the smaller one, and the resonant 
peak is greater in amplitude in the larger core. This is the result of 
the core being longer. And also remember that resonance changes as the 
square root of inductance and capacitance.

>     The smaller torrid is only half the length, and works 99% as good.
> If you say had a requirement for  6 x 2.4 inch cores... = 3 inches long..... you would require 5 x 4.0 inch cores to do the same thing.....  =  5 inches long.

That depends on the nature and the diameter of the cable that you're 
winding through it. Larger coax will have greater capacitance between 
turns, which lowers the resonant frequency. Power cables will likely 
have less capacitance between turns (the capacitance in big coax is 
greater because of the diameter of the shield), thus a higher resonant 
frequency. And nothing changes the physics that for the same material 
and the same wire/cable passing through the core, inductance is directly 
proportional to the length of the conductor within the core, so the 
inductance with three 1-in long cores should be very close to the 
inductance with six /12-in long cores for the same number of turns with 
the same cable.

73, Jim K9YC