On Tue, 26 Feb 2008 09:51:05 -0500, jeremy-ca wrote:
>Manfred, my reply was assuming use at high power since this is not a QRP or
>receiver forum. In that case iron powder is the only accepted material as
>has been proven time and time. Even Sevik was forced to back down from his
>ferrite obsession when he was challenged by many and it became obvious that
>he never tested his designs at power.
Power amps have input and interstage circuitry that may handle relatively low
power. I've studied ferrites a LOT with respect to RFI suppression and common
mode chokes, but I'm not an expert on their use in transformers. That said,
there are some ferrite materials that offer VERY low losses at HF, most
notably Fair-Rite #61 and #67. W8JI has observed that if the transformer is
NOT handling power (for example, a Beverage transformer), a lossy material
may be more suitable, depending on its mu over the frequency range of
interest.
Another VERY important point with respect to mu (permeability). When single
numbers are quoted, they are for the mu at low frequencies. But the mu of ALL
of these materials varies over a wide range as a function of frequency. Low
loss materials like Fair-Rite #61, #67, and #68 have relatively low values of
mu, but #61 maintains its value to about 20MHz, and the other two hold theirs
to nearly 100 MHz. By contrast, high loss materials designed for suppression,
like #43 and #31 start out with low frequency mu values of 800 and 1200
respectively, but have dropped to 200 at 10 MHz and 100 at 100 MHz. More
important, #43 is more lossy than inductive above 10 MHz; for #31, that
transition is around 4 MHz.
ALL of this information is in the Fair-Rite catalog. My RFI tutorial includes
a long discussion of ferrites and what the data sheets mean.
http://audiosystemsgroup.com/RFI-Ham.pdf
73,
Jim Brown K9YC
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|