[TowerTalk] Fair rite materials for choke baluns

[jimlux]

On 7/5/16 9:17 AM, Jim Brown wrote:
> On Tue,7/5/2016 7:22 AM, Jim Thomson wrote:
>>> >http://k9yc.com/VerticalDipole.pdf
>> ##  Look at the PDF.  Think of a dipole.. but turned vertical.
>> With coax  going to center, and center conductor bonded to upper wire
>> half,
>> BUT, the lower wire leg, is replaced by the braid of the coax itself.
>> A  CMC is inserted where the lower insulator would normally be placed
>> on a wire dipole.
>> Below the  coax CMC... is just a continuation of the same coax... all
>> the way back to the xcvr.
>>
>> ##  it’s a unique way to build a vertically  polarized dipole...using
>> a coaxial CMC  as the lower
>> insulator. It also places the CMC at an extreme high V / high Z
>> point...and Im surprised the
>> CMC  actually survives.
>
> Yes. This is EXACTLY why I built my first vertical dipole in 2007 -- a
> worst case dissipation test for a ferrite choke wound with coax.  It was
> cut for 40M, and suspended in a tall redwood next to my house, and fed
> with RG59 (a Belden RG59 with real copper center and copper braid). The
> choke was one I had measured, 7-8 turns through a big clamp-on. It
> lasted about 15 minutes in a CW QSO at 1.5 kW. Failure mode was melting
> of the coax. I replaced the coax with a transmitting RG6 (two copper
> braid shields and copper center with Pasternak's name on it bought at a
> surplus store in Si Valley) and wound two of the same chokes on it. That
> combination held the TX power just fine. Double the resistance, so half
> the current, dissipation divided between the two chokes.
>
> BTW -- this feed method could also be used to rig a dipole horizontally
> from a high window or roof to a tree.
>

It's also been used in one of ARRL Antenna Compendiums for an 
inductively loaded 4 square, except there, they ran the coax up inside 
the lower half which was tubing.   Same idea, though..

So you do need a fairly high Z on the outside of the shield for this.