Topband: Shunt fed towers and common mode chokes

DAVID CUTHBERT telegrapher9 at gmail.com
Thu Dec 6 16:56:58 EST 2012


Google G3TXQ COMMON-MODE

Dave WX7 G
On Dec 6, 2012 1:40 PM, "Steve London" <n2icarrl at gmail.com> wrote:

> My 160 meter shunt fed tower project is essentially done. However, I have
> an issue with the 80 meter antennas hung off that tower. In a nutshell, the
> current baluns (ferrite beads) feeding these antennas don't have enough
> common mode impedance on 160 meters. They heat up, and the SWR of the shunt
> fed tower changes as they heat up.
>
> After some reading, I think what I need are some RG8X toroid baluns, wound
> on #31 ferrite material. If they are going to replace the existing bead
> baluns, then they will need to be placed at the feedline/antenna junction.
> However, if I want to completely isolate the 160 shunt fed tower from the
> 80 meter feedlines, shouldn't I place the new baluns as close to the
> tower-mounted antenna switch as possible, and leave the bead baluns in
> place at the end of the feedline to choke off the 80 meter common mode
> energy ?
>
> To add more complications for adding the RG8X toroid balun, my 80 meter
> antennas are switchable 2 element wire beams, with each element fed with
> 18' of RG-8. That dimension is critical, as it provides the proper amount
> of capacitive reactance at the feedpoint to make the element a director.
>
> Comments ?
>
> 73,
> Steve, N2IC
> ______________________________**_________________
> Topband reflector - topband at contesting.com
>


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