[TowerTalk] 80M loading coil - open coil or toroid?

[Jim Lux]

On 1/1/13 12:43 PM, N2TK, Tony wrote:
> For my 80M 4-sq with a Comtek box I use a 3.6 uh coil to resonate the
> shortened (41') radials (minimum power in a dummy load at 3.76MHZ. In
> addition I add a 4.1uh coil to resonate the radials at 3.53. Presently the
> coils are 2" OD open coils made with tinned #14 wire.
>
> Is there a reason I should use something like a toroid coils instead of
> open coils, other than the weather making the coil forms brittle after
> several years?
>
>
>
> I found some cores in my junk box but not sure if they would work. They are
> T-200, light blue. If I put a layer of glass tape and wind 6 turns of #12 on
> the core utilizing the entire core I get 3.6uh. If I squeeze the turns
> together so they only occupy half the core I get 4.1uh.  The problem is I
> have no idea if these would be appropriate for full power on 80M. I don't
> know if the color and/or inductance gives a clue as to what these cores are.
>
>
>
> Any suggestions about using these cores or perhaps a better core for this
> application or just buying or winding some open coils for this application?
>

Interesting question..

At first, I'd say that you'll have less losses with the air coil (no 
loss in the core).  However, it occurred to me that to get a given 
inductance in an air core coil will take more wire than on a magnetic 
core, so it might be that the increased resistive losses from the longer 
wire would be bigger than the core losses.


as to figuring out the material

Typically, you back out the AL value (uH/100 turns).. inductance goes as 
the square of turns, so if you had 100 turns instead of 6, you'd have 
about 1100 uH  (multiplying your approximate 4uH * (100/6)^2)

You then hunt through the tables looking for a T200 that has an AL 
around there..

I see a powdered iron #26 material (AL =920) as the only plausible 
candidate at http://www.catzco.com/toroids.htm