Topband: 73 mix cores

[Tom W8JI]

> Fair-rite also states that multi-aperture ferrite cores (like this one)
> have greater bandwidth  vs. a single-hole core made of the same material. 
> I
> cannot explain that.
>

Increased bandwidth comes from minimizing the wire outside of the core 
window.

When I was experimenting with multiple phased loops and reversible Beverages 
in the early 1970's, prior to Fair-rite tooling that core, I made my own 
binocular cores from stacks of shield beads.  I initially used a string of 
beads in a single line, but over half of the wire length in the windings was 
outside the core material. Even back then, I realized over half the 
conductor length was outside the core where the wire did nothing good and 
everything bad.

 I thought it would be better to use side-by-side rows, so only a small 
fraction of wire length was outside the core material.

I also used a circle formed from large shield beads (and still do) in high 
current chokes, where the high current demands large conductors.  It's easy 
to build a 100 ampere or larger choke using what looks like cores parked 
like a circle of wagons, and obtain wide bandwidths with very high current 
chokes. This also works for baluns and matching transformers when conductor 
size is large.


> I can verify that the losses in any single-core transformer that I've ever
> made from that core, regardless of the Z ratio, was less than 1 dB. (0.9 
> dB
> was an extreme case.) And I've measured losses as low as 0.2 dB. ON4UN in
> his latest book wound transformers on that core with even less loss, IIRC.

I've been using systems like that for at least 40 years, and I always 
measure things.

> I used a stack of three once when I experimented with transmitting on a
> Beverage. Amazingly, the core never approached the Curie temperature even
> feeding it with 800 watts of CW and heavily processed SSB. Photos of that
> core test are at
> www.w0btu.com/files/misc/Binocular%20core%20power%20vs%20temp%20test/ .
> I cannot find the measured loss on that core at the moment, but it was
> certainly low enough (~0.4 dB, IIRC) that it never exceed 225 degrees F
> even with 850w key-down, with no air movement and a layer of heat shrink
> which reduced heat radiation. And I'm using that same transformer on the
> input of my swamped-grid amplifier. It barely gets warm even with 100w CW.

If power levels or impedances are high enough, you'll sometimes have to move 
to a core with higher Q over the operating range. This narrows bandwidth a 
bit, but greatly reduces heat.

This is a major issue with baluns, which are always a compromise of several 
things. If you use a system with extreme choking impedances, even 10,000 
ohms, cores can become hot enough to reach curie temperature. If voltage 
across a winding is 1000 volts, easily obtained with a kilowatt and common 
antennas, a 10k ohm choke would have 100 watts of core heat. At high power, 
we can't "impedance" ourselves out of a poorly planned feedline and antenna 
combination.

High power baluns that run the coolest are often baluns that are not even 
acting like baluns!

If we need an extreme impedance, we probably have a poorly planned system. A 
poorly planned system with an extreme choke or balun will not work as well 
as a properly planned system with a modest impedance balun or choke, so it 
is most valuable to learn how this stuff really works as a *system*.

Some systems, most notably line isolators, give advice contrary to what is 
best for the system.

73 Tom