In article ,
Jeff Liebermann wrote:

Thanks. 50 ohm coax does not make it a matching section to a 200 ohm
folded dipole. However, the ferrite beads are a good way to simply
block the reflected power from the folded dipole so that it looks like
it's matched. In any case, that reflected power is lost (converted to
heat) in the ferrite beads.

Ummm... I'd disagree (or at least quibble) on at least two grounds.

Ground #1: the ferrite beads would only block non-balanced current
flow back down the outside of the coax. They will have no effect at
all on power which is reflected back down the inside of the coax (the
center conductor and the inside of the shield) from any impedance
mismatch where the coax meets the folded dipole.

In effect, the presence of the beads (if they're choking the coax
properly) actually ensures that the transmitter *does* see the true
effect of any impedance mismatch. The transmitter is just as likely
to see a higher SWR than a lower one, when the beads are added.

Ground #2: the beads do not necessarily result in a significant loss
of power. If their RF impedance is high enough at the frequency of
use, then RF current flow through them will be negligible, and there
won't be a loss of power. Power loss in choke-ferrites tends to be
worst when the RF impedance is both resistive, and too low for the
application (i.e. still allows substantial current flow, which then
results in dissipation of power inside the choke).

--
Dave Platt AE6EO
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