> 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.
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.
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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