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[TowerTalk] W2DU Style Balun

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Subject: [TowerTalk] W2DU Style Balun
From: (Tom Rauch)
Date: Thu, 7 Feb 2002 07:14:38 -0500
> One.
> >How many beads for the same thing operating from 80M-10M?
> One.
> Use an FT-240-77. Wind the coax on the core. For a 50 ohm feedpoint,
> about 6 turns should be adequate for 40m-10m. 8-9 turns for 80m-10m.
> This isn't the W2DU design, but the W1JR design. Same principle,
> different execution.

I'm sure one 77-material core will not handle 1500 watts (or even 
less) in all cases, so we have to be careful using that material. 

77 material is very low Q, with Q reaching unity below 160 meters. 
Being such, it behaves mostly like a resistor at HF. 

We have to keep voltage across the winding down, or the core will 
get hot. One nice thing about 77 material is the curie temperature 
is high. It will handle twice the temperature of 73 material before 
losing its magnetic properties.

For high electrical stress applications, we should design around 
cores that have higher Q or better (and cheaper) yet just use air 
core chokes when we have room.

73, Tom W8JI 

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