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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