Topband: Wideband x-former...

[Rick Karlquist]

Jim Brown wrote:
> On Mon, 30 Mar 2009 19:38:05 -0700, Richard (Rick) Karlquist
> wrote:
>
>>Even easier. You can simply get a big 43 matl toroid and wind an
>>autotransformer on it.
>
> Hang on a minute. The original post said "can handle some power,"
> but didn't say what frequency. #43 "will handle some power" on
> 160M and 80M, but it will begin making more and more heat as
> frequency is increased above 40M.

> Fair-Rite #61 and #67 are suitable for HF transformers that can
> "handle some power." Read the descriptions on the first link, then

Yes, these low permeability materials are necessary to get maximum
power handling and (similutaneously) wide bandwidth.  However, in
order to make such optimum transformers, it is also necessary to
stack up multiple cores, use controlled impedance windings, and
use both low frequency and high frequency peaking capacitors.
This would clearly be beyond the abilities of the original poster.
Since the post was made on the topband reflector, it should be
assumed that the transformer needs to cover down to 160 meters.
#43 is the lowest practical permeability that will go down this
low in a simple design.

I have some homebrew transformers that I use with my open wire line
that use stacked 61 cores, controlled impedance windings, and peaking
capacitors.  They have a SWR of <1.2:1 from 1.8 to 54 MHz and handle
the legal limit.  However, the design and construction is quite non
trivial.

Rick N6RK