----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <amps@contesting.com>
Sent: Tuesday, February 26, 2008 2:51 PM
Subject: Re: [Amps] how to wind an HF broadband 10:1 transformer
> 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.
Very true and that observation goes back to way before JI and is in the
original Micek Beverage book. OTOH it does not fit all installations.
At a prior QTH I had 1000-1500' feedlines and using 61 material allowed me
to not have to use a preamp most of the time; 75 or 77 material was too
lossy. Ive also used my Beverages all the way to 10M when either atmospheric
or snow static made reception impossible on yagis. I considered remote
preamps but induced voltage from lightning storms even miles away put an end
to that idea.
At this QTH I run several 200-400' RG-11 feedlines to a remote switch and
then 700' of 1/2" hardline to the house. Im still using the same
transformers using FT114-61 cores. The trade off between flat VSWR response
(at the feedpoint) to high frequency efficiency appears to be worthwhile in
a rather non critical receiving application.
In a broadband low level receiver or transmitter path core losses can easily
be recovered with more gain. However the smaller you get with the ferrite
the more prone they become to non linearity events, primarily in receivers.
Carl
KM1H
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