I found that the measured common mode impedance ona supposedly identical
arrangement to the W2AU balun gave rather less common mode suppression.
However, the losses about 5%, or about 0.22dB. That still represents 50
watts at 1kW however, and the beads at the balanced end did get warmer
than the ones at the unbalanced end. Overall, the temperature rise wasn't
that excessive - after 5 minutes, you could still comfortably hold the
beads. Of course with 75 of them, that's less than a watt each on average.
Now that is in a matched system. In an arrangement with a high SWR, you
can get a very large 'common mode' voltage across the balun, with
resulting currents to be suppressed that can cause problems. For this
reasons, I'm very leery about baluns with high SWR. Far better to have a
proper balanced tuner.
The balanced L network that Rich favours can, for some antennas, give
wider bandwidth without retuning than the classic parallel or series tuned
circuit. Feeding my 80m dipole on 40 (the feeder is 64 feet of open wire
line), the balun feeding a balanced L network has lower working Q than a
parallel tuned circuit, while on 80, the tuned circuit is better.
Not,Rich, that you'd approve - the balun feeding the L network is a
ferrite bead balun, and the L network inductors are wound on powdered iron
toroids 3 inch o.d., 2 inch i.d. and 1 inch thick! I did do the sums
though to check that the flux density would be low enough to avoid
problems.
73
Peter temporarily SM/G3RZP
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