More precisely, you need to keep the LOADED 'Q' relatively low - preferably
not much over 10. This in turn requires keeping total input capacitance of
the
pi network low, which may not be possible when using an conventional
"bread-slicer" type of air variable big enough for 80-160M. The variable's
Cmin may be great enough to make the lowest achievable loaded Q of the pi
as high as 25 or even greater. That's why we use split caps in the 87A,
etc.: the small section used for higher bands has much lower Cmin than a
big cap. It also provides a slower tune & load rate on the high bands
without requiring gear or friction reduction drives.
Provided you can keep Cmin low enough to achieve a loaded Q of about 10,
you can get all the harmonic suppression needed more efficiently by adding
a simple "L" section (i.e., using a pi-L tank) of Q ~2-3. The L typically
will add another 15 dB or so of harmonic suppression, whereas raising the
pi net Q high enough to do that requires substantially higher pi "Q,"
increased loss and tank heating due to circulating current, and more
critical tuning.
73, Dick W0ID
-----Original Message-----
From: Mark S Graalman [SMTP:wb8jkr@juno.com]
Sent: Thursday, October 22, 1998 7:28 PM
To: zak@goes.com
Cc: amps@contesting.com
Subject: Re: [AMPS] Minimizing circulating currents on 10M.
Minimize the "Q"
Mark WB8JKR
On Thu, 22 Oct 1998 19:41:08 -0400 "W/Z Kornienko" <zak@goes.com> writes:
>
>How does one minimize circulating currents in the pi-net
>of an amplifier, especially on 10M?
>
>
> 73 de Walt Kornienko - K2WK
>
>
>
>
>--
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>
>
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Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
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