> Moving the balun from the output to the input of a floating
> L or T changes things, but it does NOT make things better
> for the balun. Say I have a string-of-beads balun on the
> output of a T network and with a given load on low
> frequencies measure core heating and balance. If I go
> through the hassle of floating that network and moving the
> balun to the input....nothing significantly changes. On the
Moving the balun to the input does not affect core loss or
balance. It does however change the operating conditions for
*differential* mode current. If the balun is made of 50 ohm
coax + ferrite, then it will operate at 1:1 VSWR when located
at the input. When located at the output, it will operate at
some high VSWR. However, since it is a short (<< 1/4 wl)
transmission line, the usual handbook graph of additional loss
due to VSWR doesn't apply. In fact, if the load impedance at
the tuner output is higher than 50 ohms, the coax loss will actually
be *less* than if it were matched, because the current is lower.
This is because the loss is nearly all conductor ohmic loss
rather than dielectric loss. If the load impedance is less
than 50 ohms, then the coax loss will be higher at the output
than the input. However, in that case, the ferrite loss is
likely to be very small due to the low voltages involved, thus
it is OK to have more ohmic heating. You could imagine a corner
case of 5 ohm load at 28 MHz with one foot of RG58 at 1500W
The loss would be something like 60W. Even that might be OK
if you switch to teflon coax. The tuner itself might not be
able to handle this corner case.
Conclusion: don't bother moving the balun to the input.
Rick N6RK
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