On Sun,4/26/2015 12:08 PM, Kelly Taylor wrote:
If the choke bead is getting too hot, it doesn't mean you shouldn't use a
choke. It means the choking impedance isn't high enough. Either the beads
aren't the proper ferrite material or there aren't enough of them. With many
commercial bead chokes, it's sometimes a combination of both errors.
The ONLY ferrite material I know of that works for a string of beads
choke on the HF bands is Fair-Rite #73. It is the material that W2DU
chose for his common mode chokes that he called "current baluns." The
#73 material is resistive in the HF spectrum, which satisfies one of the
requirements of an effective choke. The other requirement is that the
resistive impedance must be high enough to suppress current enough.
Overheating means that there is not enough resistance.
If anything, the heat is proof a choke is required, because if that energy
wasn't making the bead choke hot, it would be on the outside of the
feedline.
Exactly right.
Remember -- any inductance in a choke resonates with the capacitance of
a feedline that is shorter than a quarter wave, or some multiple that is
capacitive. To be effective in all situations, the choke must be RESISTIVE.
73, Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|