The goal is to have 4 square for 80m with vertical dipoles a la K8UR,
without compromising the performance of 160m shunt feed on the same tower.
My tower is 100ft. If vertical dipoles have baluns at the feedpoint, the
coax to vertical dipoles is at about 45 ft and loads the tower at low
height, reducing performance on 160m. Having a balun by teh 4 sq box, in
the middle of the tower, cuts the coax from the tower but the cable to the
dipole will radiate.
For those who want to have a dipole at the end of tower and at the same
time shunt feed the tower, normal baluns do not work, and tuned baluns are
used as in ON4UN book. The alternative is a transformer. I tried a
transformer made of 8 bifilar turns on FT140-43. Heating on 80m at a KW
level was minimal.
Ignacy, NO9E
On Wed, Apr 8, 2020 at 2:43 PM Jim Brown <jim@audiosystemsgroup.com> wrote:
> On 4/8/2020 11:04 AM, Ignacy Misztal wrote:
> > I am trying to decide where a balun (or transformer) is needed.
>
> What are you trying to accomplish? Kill common mode current on
> feedlines? This minimizes RX noise AND prevents their interaction with
> other antennas (like your 4-square).
>
> For transmitting, serious ferrite chokes (NOT "baluns") are the best
> solution. See http://k9yc.com/2018Cookbook.pdf for specific
> recommendations, including a few for 75 ohm coax to feed 4-squares.
>
> Transformers are less effective -- for the high efficiency needed for
> transmitting, windings must generally be bifilar, which adds capacitive
> coupling from one winding to the other, providing a path for common mode
> current, decreasing the effective isolation. There are measurements
> demonstrating this effect for RX transformers, which can generally
> tolerate the 1-2 dB of loss added by placing windings on opposite sides
> of an efficient core like Fair-Rite #61.
>
> http://k9yc.com/RXChokesTransformers.pdf
>
> 73, Jim K9YC
>
>
>
>
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