# Re: Directions for tesla coil research

```Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

> How does a bigger top load increase Q.    if Q = Xl / R      and     Xl  =
> 2 * pi * f * l.
>
> from these equations, one would assume that if frequency goes down, due to a
> shift in resonant frequency, then Q decrease because Xl decrease.  I know
> I'm missing something important here, right?
>
> Shaun Epp

I think so.  The discussion was about reducing the frequency by
increasing the top loading, but you have the wrong expression for Q in
this case.  The Q is the parallel resistance divided by the reactance
so, for a given resistance (same power in the streamers) the Q will
increase.  Your expression is for an inductor with a resistance in
series with it.  If you look at the equivalent circuit you will see that
increasing the parallel resistance does lower the series resistance,
with the same effect of increasing the Q a bit. For coils with a Q much
over 10, as are all the ones under discussion here, the product of
series resistance and parallel resistance equals the square of the
reactance.

Rp Rs = X^2

if I didn't make another typo.

Ed

Ed

```