Dave,
For the low bands, the #2 red is pretty widely used. The 3M tape for that
(forget the number) is pretty expensive but it's strong and won't breakdown
in moderate heating. For the higher bands, air core types are small enough
to fit even into some tiny spaces so there's typically no good reason to put
up with the compromises (lower Q and heat loss) that a toroid necessary
involves.
The big mistakes made on these are trying to use a single toroid for 2
bands - as Carl says below, one toroid - one band. And not providing any
consideration for heating if you are into high duty cycle modes like RTTY.
You can find a lot of comments on both of these issues in the last couple of
year message history.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Jim Brown
Sent: Saturday, March 24, 2012 6:08 PM
To: amps@contesting.com
Subject: Re: [Amps] Guidelines.....toroids for tank ckts
On 3/24/2012 3:24 PM, Carl wrote:
> I would not recommend toroids above 40M and even there its iffy at high
> power. And of course use one per band.
Yes. I don't know anything about powdered iron materials, but all
ferrites get increasingly lossy with increasing frequency. A few
ferrite mixes are designed to handle high power, and are pretty low loss
at low to medium frequencies, but each of them has a high frequency
limit, beyond which their loss has increased to the point that they are
not very useful. For example, Fair-Rite #61 starts getting lossy above
about 10 MHz, while their #67 starts above 20-30 MHz. In general,
losses will couple from the core to the wires, and will show up in the
equivalent circuit as resistance.
Another issue is voltage breakdown -- ferrites are semi-conductors, and
each mix is different there too. Some are pretty good insulators, others
are fairly conductive. It's worth studying the Fair-Rite catalog, which
is really excellent. Fair-Rite data sheets include data for resistivity,
permeability and permittivity. If you have a solid EE background, it's
also worth calling Fair-Rite's technical support people. But study their
catalog and applications notes first so that you know what questions to
ask and can understand the answers.
73, Jim K9YC
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|