On Wed, 20 Aug 1997 17:44:05 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> From: km1h@juno.com (km1h @ juno.com)
To: <amps@contesting.com>
>> Date: Wed, 20 Aug 97 16:42:51 +0000
>
>> Inductance is easy to determine with B&W coil stock. All you need is
>the
>> diameter and TPI and look it up in a B&W sheet. Since the inductance
>is
>> linear all you need to do is look up the total L for a full 10"
>length
>> and then measure your lengths.
>
>Not a good method in the real world.
Maybe in your version of reality but not in the world the rest of us
inhabit.
The standard B&W coil tables have been a part of amp design for over 40
years and a "few" successful amps have resulted....including many QST,
Handbook and even Eimac articles.
As hams we are not striving for some Nirvana level of perfection nor
interested in a constant barrage of bull.
Follow the classic articles and you wont go wrong. Follow the crap and
you will wind up with a Mississippi Nightmare....or worse.
Tom makes a few very good points but since the average ham has no access
to the lab test equipment it is mostly moot. A reasonable error in the L
will make very little difference in reality. As Tom has mentioned
recently, a Q of 8 or 12, etc is no big deal. If your variable C cant
handle that much of a fudge factor then YOU have a problem that needs
correcting.
A select few have strived to put amp design into an arena reserved for
the self annointed gods. T'aint so. Get your hands dirty and find
out...start small and make a few mistakes...it is part of the learning
curve. Just remember to disharge the B+ while learning!
And learn to seperate the chaff from the chiff...or is it the other way
around?
73...Carl KM1H
>Inductance changes by the square of the turns if the inductor has
>100% flux linkage from end to end. If it has no flux linking the
>turns, it goes up linearly. Stray C and flux coupling to other
>objects also affects the results.
>
>The poorest way to do a tank is to set the inductor by calculation.
>The best way is to set the capacitors and "prune the tank coil in" to
>thoise values under simulated operating conditions.
>
I agree with this concept but again it is somewhat out of the "amateur"
realm. For the average 500-1500W type amps from reputable publications,
just follow the article. Having a GDO, L/C bridge ( operating at 10MHz of
course...not 1 KHz) and other lab toys is fine but not a show stopper.
'nuff said today...Im off to lobsters with the family. (a benefit of the
laptop while Im up here in York Beach, Maine for the day )
73....Carl KM1H
>If the inductor is short compared to the diameter, inductance
>decreases by nearly the square root of the turns ratio decrease. B+W
>coils of L/D ratios of around 1or less normally behave this way.
>Half the turns can approach being 1/4 the inductance.
>
>If the inductor is very long compared to the diameter, the change is
>less and is nearly linear with length. Inductance also varies with
>the mounting location and turn to turn capacitance. In that case half
>the turns is half the inductance.
>
>In fact, the ratio can actually vary far beyond these examples.
>
>The best idea is the MEASURE the inductor at the operating
>frequency. Since most equipment can't do that accurately, the person
>has to use a known fixed C and measure the resonant frequency to
>calculate the actual L.
>
>Values from a turns chart can be far off, unless the coil is long
>(compared to diameter) and mounted in the clear. Fortunately loaded Q
>isn't very critical to performance.
>
>73 Tom
>
>--
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>
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