The bifilar-wound chokes on a 2.4-in o.d. Fair-Rite #31 toroid are the
best choice for this application. To be more conservative with respect
to power handling, build two of them and connect them in series near the
feedpoint (that is, up in the air). Twelve bifilar turns is "right" for
your frequency range.
To wind them, measure out two lengths of #12 THHN, allowing 2.25 inches
per turn plus another six inches for termination, use small ty-wraps to
hold them together to form a transmission line, then start winding from
the center in both directions until you have 12 turns of that line.
Then use a suitable means of strain relief and connectorization to
insert them in the line, cutting off the excess length. If it's a fairly
long line and you have a long wire or vertical antenna for 80 or 160
nearby, you might choose to use one of the chokes at the feedpoint and
the second one further down the line as an "egg insulator" as suggested
in the tutorial to prevent interaction with other antennas.
73, Jim K9YC
On 4/25/2012 8:27 AM, Jim Lee wrote:
> *What is your suggestion for an appropriate choke to use at the feed
> point of a _LADDER-LINE_ fed doublet ? This would be 80 m through 15
> m operations a full legal limit.
> On 4/25/2012 10:19 AM, Jim Brown wrote:
>> On 4/25/2012 6:41 AM, David Robbins wrote:
>>> "Assuming" that there is an adequate choke so that the currents are
>>> balanced it shouldn't matter which conductor from the coax goes to which
>>> side of the antenna
>> I generally agree, and want to emphasize the importance of a serious
>> ferrite choke at the feedpoint of ANY antenna, including this one.
>> Remember that the primary reasons for using a choke are minimizing RX
>> noise and RFI (from us to others), and that is equally true whether the
>> feedline is coax or parallel conductors.
>> 73, Jim Brown K9YC
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