[TowerTalk] Ferrite Cores in Trapped Dipoles

Jim Lux 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.

Jim, W6RMK 

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