[TowerTalk] Ferrite Cores in Trapped Dipoles
jimlux at earthlink.net
Wed Feb 7 11:33:09 EST 2007
At 08:23 PM 2/6/2007, Michael Tope wrote:
>----- Original Message -----
>From: "Dan Zimmerman N3OX" <n3ox at n3ox.net>
> > Yet many of us are using autotuners (which all have powdered iron
> > inductors) on pathological matching cases, probably with high
> > circulating currents.
>You can get reasonably good Q's with iron powder. They are often
>used in linear amplifier tank circuits for instance (most of the Alpha
>amps use an iron powder core for the low frequency portion of the
>tank circuit). This will give you a smaller inductor (what counts in a
>desktop amp), but not necessarily a lighter inductor (what counts
>in a trap). Also, I think iron powder is pretty hard to saturate
>compared to ferrites due to the distribute air gap effect (iron
>particles suspended in a low mu binder).
>It seems like an interesting trade space to explore.
It does... I would imagine the domimant loss mechanism in any trap is
IR losses in the coil, and the fact that a air core inductor is
physically larger might mean that even if you have some losses in the
core and need to use smaller wire for the windings, you might come up
with overall lower loss.
It's also kind of interesting from another standpoint, in that at
some frequencies, the current will be high and the voltage low (where
the trap isn't trapping, but passing the power to be radiated) and at
others, the voltage will be high and the current low (where the trap
looks like an open). In the former, I would think IR losses are
dominant, in the latter, voltage standoff.
You can build small ferrite cored inductors with good voltage
standoff (e.g. TV flyback holds off 20 kV and is a couple cubic
inches), although it might require techniques that aren't very common
in the typical ham shop, like vacuum encapsulation/potting, but
certainly not all that exotic.
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