Dear Roger, LF Group,
I used T-130-2 toroid cores (with 63 turns) in the output filter of my 200W
PA, which proved to be quite conservative for this power level. T-200-2
cores have proved adequate at 1kW output. The main limitation to using
smaller cores for low powers is that the required number of turns
increases - this means either finer wire or multi-layer winding. Fine wire
will reduce Q , multi-layer windings increase stray C, which reduces
supression of higher-order harmonics. How important these are depends on the
type of PA design you intend to use. But I would guess the T-94 or T-106
size toroids should be fine at the 20W level at 136k, and are not
particularly bulky compared to other components likely to be in the design.
The #2 material seems to be the best widely available choice for this job.
Pot core inductors, or other kinds of ferrite core, are quite feasible, but
need to be designed with an air gap to get a reasonably high Q and stable
inductance, which complicates things a bit.
Air-cored inductors are quite feasible too, but require quite a large
diameter, say around 50mm, and spacing from other components, so would
probably be quite bulky for a 20W TX.
For receiver input filters I have mostly used fixed inductors of the type
manufactured as filter chokes. For example the Panasonic "ELC" series sold
by RS components that are small ferrite bobbins with an open winding usually
have a reasonable Q of about 50 - 100 at 136k. It isn't too hard to change
the inductance by removing some turns, or to add a link winding if required.
The fixed inductors that look like resistors are rather low Q at LF for
tuned circuits, however. Bear in mind these inductors are not screened.
Cheers, Jim Moritz
73 de M0BMU
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