... or just simplify things and use COAX feeders!
Frankly I have never got along well with open wire feeders.
Bry
On 12 May 2015 at 11:28, Jim Brown wrote:
> On Tue,5/12/2015 11:01 AM, Hans Hammarquist via TowerTalk wrote:
> > Gentlemen,
> >
> >
> > I have seen many articles how to build and/or buy common mode chokes for
> > coax cables but I don't remember any articles how to build (or buy) and
> > choke for a 400 ohm ladder line.
>
> That's because no one has thought of one that works. An important
> component of the problem is that the Zo of practical two-wire chokes
> wound on ferrite toroids is much lower than 400 Ohms. For example,
> close-spaced enameled wire yields about 50 ohms, close-spaced THHN
> (house wire) yields about 90 ohms.
>
> Another important component of the problem is that, because the
> feedpoint is off-center, the common mode voltage gets VERY high if
> you're running much power, so the dissipation in the choke is likely to
> fry it.
>
> A third reason is the concentrated differential mode dissipation in the
> turns of line when the antenna is poorly matched to the line.
>
> > The common mode current on a ladder line is causing the same problem as a
> > common mode current on a coax.
>
> Exactly right.
>
> > i have a choke between my tuner and radio which reduced noise quite good
> > but wonder now how much RF power I radiate and signal I receive from the CM
> > current in the feeder to my OCF dipole.
>
> The only other practical way to kill common mode on high Z line would be
> with a conventional 2-winding transformer capable of handling the power.
> Fair-Rite #61 might be good enough for moderate power levels below about
> 12 MHz, but its losses increase above that frequency. Fair-Rite #67
> would be a better candidate -- its losses remain quite low up to about
> 40 MHz, and are not bad at 50 MHz. On the graphs below, u's is the mu
> that defines inductance, and u''s is the loss component in a series
> equivalent circuit.
>
> http://www.fair-rite.com/newfair/materials61.htm
>
> http://www.fair-rite.com/newfair/materials67.htm
>
> http://www.fair-rite.com/cgibin/catalog.pgm?THEAPPL=Inductive+Components&THEWHERE=Closed+Magnetic+Circuit&THEPART=Toroids#select:freq1
>
> 73, Jim K9YC
>
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