>My question: Does anyone have any data and/or comments about the Sevick 2:1
>step-down balun? Anyone ever built or used one? Any data comparing
>efficiencies with the narrow-band (Q-section and hairpin) approaches?
Hi Charlie:
I have been looking at a similar problem for my rotatable 40M dipole.
I want to have a resonant 30M dipole (about 48ft), then switch it to center
loaded 40M (via relay). I have looked at the step down xfmr, and also the
'hairpin' (center loaded) type of matching.
The problem really becomes the impedances you are dealing with in the 40M
case. The reactance of the 30M dipole at 40M is quite high (about -j400 ohms,
or 50pfd) together with the 25 ohm resistance component. In any case, before
you can use the 2:1 transformer, you have to get rid of the reactance. In your
case, you have acheived that by the linear loading. Therefore, it should work
fine. I don't think it would be much different than your Q-section however.
The losses in the Q-section can be made low also, and the loaded dipole will
be inherently narrow band. I haven't studied it, but there may be a way to cut
the Q-section such that it can do a bit of tuning to make the thing appear to
have a wider bandwidth. Same thing for the hairpin match. The hairpin match is
actually a form of the 'loading coil' design discussed below. It depends on
the resonator being tuned near resonance so that the amount of capacitive
reactance can be adjusted to an exact value.
I have had great success with a home made 40M dipole with center loaded half
elements (the loading coils are near the middle of each side). I even tuned
the loading coils such that they were traps at 15M, making it a 15m dipole
also! The design yielded a direct 50 ohm match on both bands, however it was
only about 100khz wide on 40M. The coils had a Q > 250, so the losses were not
significant.
BTW: In trying to use center loading the problem becomes voltage handling at
the high impedance center point, which is about 5000 ohms with a loading coil
to tune out the 50 pfd capacitance. (R parallel = Q squared times R series)
At 1000 watts, the voltage is about 2200 VRMS!.
I have looked at a Mosley 40M dipole center loading coil, and it is
impressive! Should easily with stand the legal limits. It uses link coupling
to make the impedance transformation from 5000 to 50 ohms. I will probably
try something similar.
Hope this helps.
73,
Bob, W5LT
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