[Amps] Tuned Input ?

Steve Katz stevek@jmr.com
Wed, 20 Mar 2002 07:46:59 -0800

> w3nr@vol.com asks
> >	Is it really necessary to use silver mica's for a tuned input ? ,
> >or will
> >disc ceramics suffice ? If silver mica's are needed, does anyone have a
> >surplus supplier or is a trip to Mouser in order. Thanks 73...
> Depends.  SM are usually more forgiving of RF and power and more stable in
> value than ceramics.  Having said that -- NP0 ceramics change value less
> from heating than SM.
	::NPO ceramics are stable because of the mix used.  The key is that
"silver mica" all use the same dielectric material, and "disc ceramic" do
not -- there are many variants of ceramic capacitors and some behave badly
with RF.  I've found the cheapie but common Z5U, Y5V, etc (rather unstable)
types are usually not very good RF performers, while X7R types can be, and
NPOs generally are.  NPOs should be available in the smaller values such as
would be used for tuned input on 10m, but may not be available in the larger
values such as would be used for 160m.  If you find this is the case, you
might try high-quality X7R capacitors which have a similar barium titanate


> So your answer is  "it depends," based on a lot of information not in the
> original question.  If you're not going to destroy thousands of dollars of
> stuff if you blow up a cap, I'd start with whatever was in the junk box or
> was readily available.
> Of course, Mouser ships fast, is relatively inexpensive, and won't kill
> you
> on small orders.
> Jim N6OTQ
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