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Topband: K2AV 160m Folded Counterpoise (FCP), parts and winding for isol

To: Bob Garrett <>, J A Ritter <>, TopBand List <>,
Subject: Topband: K2AV 160m Folded Counterpoise (FCP), parts and winding for isolation transformer.
From: Guy Olinger K2AV <>
Date: Fri, 30 Dec 2011 20:50:48 -0500
List-post: <">>
As requested.

Parts:    #635  Double polyimide insulated
#14 AWG, 15 feet.   #12 AWG standard wall teflon
tubing, 15 feet.   T300A-2  #2 material powdered iron

You can also use Micrometals T300-2D, or a PAIR of Micrometals T300-2
stacked and taped together with fiberglass tape, which are the same as
Amidon T300A-2.  You can often find the Micrometals cores on eBay.

Note: The core material, bifilar winding turn count, and core dimensions
are chosen to facilitate a "simple" 160 installation that has approximately
1/4 wave wire, a toroid wound balun-sized isolation transformer feeding the
coax, and a folded counterpoise attached, that delivers an impedance that
is close enough to 50 ohms resistive to reasonably use regular coax as a
feed.  If you change the turn count, or the core material, or core
dimensions, this balance for the "simple solution" is defeated, and the
conditions which were tested in our two year research period no longer
apply.  You might substitute configurations which we put up ourselves,
tested, and specifically rejected for cause.  Like burned it up, was lossy,
wouldn't stay tuned, was worse than the original antenna, etc, etc

You can't use a stack of smaller diameter cores with the same total of A
sub L numbers because you can't get the required 20 bifilar turns on the
inside diameter of the smaller cores. 20 turns fills up the inside diameter
of the T300x-x form factor.  If you don't use 20 turns, then you will get
less inductive reactance to cancel the FCP's capacitive reactance for the
simple installation and need more wire in the radiator to compensate.
Dropping only one turn on the core will add 12 or 13 feet to the "pruned"
length for resonance you would have had otherwise.

The #2 powdered iron core material has been very carefully chosen for 1.8
MHz QRP and QRO, drawing on advice and published work by W2FMI.  Do not
substitute ferrite or other powdered iron materials. #2 powdered iron cores
are always painted red and easily identified.  A core without paint or a
different color will not work.


First, have a look on W0UCE's site for a good picture of one of these
correctly done . (Pix worth 1000 words and all that...)
Scroll down for the pictures.  Note the appearance of the bifilar pair when
done right.  Think of the bifilar pair as an exotic "zip cord".  You will
be winding the PAIR as if you were winding with zip cord.

The professionally wound version from Balun Designs with enclosure and
hardware can be seen at:

Cut wire and tubing in half and slip the wire inside the tubing to create a
pair of parallel 7.5 feet teflon-sleeved wires. Some find it easier to
handle the wires in winding if you tape them together.

Tightly wind twenty bifilar turns around the core.  This will use all the
space in the inner diameter.  Keep the bifilar pair turns separate and
uniformly spaced on the outside.  ALL the wires should be laying flat on
the toroid, with NO twist flips where the wires do an "over-under".

IMPORTANT: When properly connected there is NO connection between the
antenna/FCP side and the coax side.  CAUTION: If you get that WRONG when
you hook it up, you will STILL be able to hear on it, but the system won't
work right and you will loose valuable dB's.

!!! VERIFY THE SEPARATION !!! with an ohmmeter BEFORE you start pruning the
antenna wire or hurling electronic curses at dog, family, neighbors, or me.

To wind one of these for a 16+16 version FCP for 80 meters, and you are
doing the "simple" version with 67 feet or so radiator above the FCP,
evenly space *fifteen* bifilar turns around the toroid.

73, and I'm looking forward to a lot more 160 QSO's in the contests,

Guy K2AV

On Sat, Dec 10, 2011 at 1:38 PM, Bob Garrett <> wrote:

> Hello Guy,
> When the dust settles, will you post a list of parts and sources so we can
> duplicate this antenna and the unique matching network?  73, Bob K3UL
UR RST IS ... ... ..9 QSB QSB - hw? BK

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