[AMPS] Q Swing

Wed, 20 Aug 1997 17:44:05 +0000

> From:          km1h@juno.com (km1h @ juno.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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