Chris writes:
> I have been infected with the idea that the
> ferrite bead baluns have high loss and relatively low impedance,
> and that a better design uses coax wound through one or two binocular cores.
Hope you didn't catch it from me :)
Other things being equal, sleeve (W2DU) baluns/ununs aren't lossier than
multiturn baluns, given that a bead sleeve is just the N=1 case
of an N-turn balun. As long as the choice of coax and ferrite
compound is reasonable, sleeve baluns don't have much loss.
The advantage of a multi-turn balun at QRP is that the common-mode
impedance increases as the square of the increase in the number of
turns. Doubling the number of turns raises the common mode impedance
by a factor of four; tripling by a factor of nine, and so on.
With 5 turns of coax on a one-inch stack of toroids, you've got about the
same common-mode impedance as 25 inches of bead sleeve, other things
being equal. Thus the multi-turn balun can be smaller and lighter
by comparison with a W2DU balun having equivalent common-mode
impedance.
Being smaller, a multi-turn balun or unun has less surface area than an
impedance-equivalent W2DU balun, so it doesn't radiate heat as well.
That doesn't matter at QRP, but it's easy to cook a physically small
multiturn balun or unun at QRO if it's running against a high resistive
mismatch or imbalance.
The multiturn balun trades away power handling capacity for a decrease
in size, which is usually a safe and advantageous thing to do at QRP.
Incidentally the W3EDP isn't expected to have a low SWR on multple
bands. You just tune it up with the ATU and go. What's nice about
the W3EDP is that it's not a half-wavelength on any of the amateur
bands, so it doesn't present thousands of ohms of impedance on
those bands. As such, it can be tuned by a simple, low-excursion
L network without resorting to the sort of tank circuit that you
usually need to tune end-fed halfwaves.
73,
Ken
K3VV
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