开云体育

With a current balun or CM choke, it is the reactance (inductance) that is
mostly responsible for the balun action. In the case of the choke balun,
beads installed along the coax at the feed with 31 or 43 material, they
form a reflective 'filter'. There is some absorption, but most of the
action is due to reflection from the inductive reactance they form
installed on a conductor. As such, they form a high-Z isolation point
between the feeder and the antenna center, assuming they are installed at
the feedpoint of the doublet. In the case of the CM choke, the common mode
currents are reflected by the inductive reactance of the windings as with
the current balun and the balance of current between the two conductors is
forced through induced opposing magnetic currents within the cone. This is
the reason I prefer the CM choke for the purpose. In either case, the
common mode current is reflected to a large extent by the inductive
reactance back where it originated. Installation of a balun at the
feedpoint of a doublet does not make the CM currents go away, it just
establishes a 'fence' for those currents between non-antenna associated
currents (on the outside of the feedline) and the radiating structure.

I can assure you the core Amidon sells with their balun kit is a red core,
Typie 2 material:
[image: image.png]

Ferrites designed to introduce loss or absorption are good EMC/RFI
suppression devices. In the case of these ferrites, current is induced
into the ferrite structure where resistive losses to the induced magnetic
currents turn that energy to heat. These ferrite structures become rather
hot when used in the balun application for transmit energy.

If the balun action of the various cores were absorptive, they why are
'good' baluns specified as having , for example, 1000 ohms series
'resistance' at the lowest frequency of interest (or at least 5X the system
impedance). This should be specified as +j1000 to be correct. It is the
reflection of the CM currents that this high-Z addresses, not the
absorption of CM energy. If absorption were the requirement, then why the
higher +jX for better balun performance?

Dave - W?LEV


On Sat, Nov 7, 2020 at 10:36 AM Martin via groups.io <martin_ehrenfried=
[email protected]> wrote:

On Fri, Nov 6, 2020 at 04:40 PM, David Eckhardt wrote:

Yes, many use 31 material for this purpose. I've wound many like CM

chokes

using red cores, 31, and 43 material. Problem is with the higher ?r
materials, I end up with a virtual tesla coil at the transition of my

open

wire feeders and the output of the matching network. Too much inductance.
The red cores I use once wound as the CM chokes measure in the vicinity

of

+j1000 or greater, even at 1.8 MHz. The red cores are the correct cores

to

use for this purpose. These are also the same core material Amidon offers
in their kits.

Hi Dave,

Maybe the reason you get a high voltage when using higher u materials is
that they are working effectively and creating a high impedance to common
mode current.

I'm concerned that your choking impedance is mainly reactive as this is
not likely to be as effective as a choke with a mainly resistive impedance.

Iron powder is not the correct material to use for this particular
purpose, irrespective of who is selling it, and it is not clear from
Amidon's website which cores are intended for specific designs which adds
to the confusion.

Here are some good references regarding choke Baluns and why highly
reactive common mode impedances should be avoided.








Regards,

Martin - G8JNJ

--
*Dave - W?LEV*
*Just Let Darwin Work*