# Measuring secondary resonant frequency

```Original poster: "Kelly & Phillipa Williams by way of Terry Fritz <twftesla-at-uswest-dot-net>" <kellyw-at-ihug.co.nz>

Hello All,

I have a question relating to the use of a frequency generator and an
ocilliscope for measuring the resonant frequency of the secondary coil and
topload. This method is described by Malcolm Watts in an article on Tesla
Coils in "Wireless World".

For this test, you connect the "signal" lead from the frequency generator to
the bottom lead of the secondary coil, and leave then "ground" lead
disconnected.
You then suspend the "signal" probe of the ocilliscope about a yard/meter
away from the toroid, leaving the "ground" lead sitting on top of the
ocilliscope casing.

Initially, you should see an almost straight line on the ocilliscope. As you
tune the signal generator closer to the resonant frequency of the secondary
you see the straight line on the ocilliscope screen become a sine wave. As
you reach the resonant frequency, the sine wave grows very suddenly to a
huge amplitude, then dies away to a straight line again as you tune the
generator past the resonant frequency. The higher the "Q" of the secondary
coil, the more suddenly the peak amplitude on the ocilliscope appears.

The precise frequency of the peak amplitude on the ocilliscope is the
resonant frequency.

The resonant frequency of my 9" by 45" secondary coil and 25" by 9" toroid
is about 55 kHz according to this method. However, if I leave the signal
generator set on the resonant frequency, and walk close to the toroid (1.5
meters / yards), the high-amplitude wave on the scope dies down to a
smallish sine wave. As I walk away again, the wave on the scope goes back up
to a large amplitude.

Question - does this mean that when my coil is operating, and I walk closer
to it and further away from it, (while remaining safe from arcs) I am
actually changing the resonant frequency slightly?

Also, if you touch the toroid during the test, the wave on the scope cuts
completely to a straight line. I suspect that this is similar to what
happens when the secondary coil is arcing to a grounded rod, when the Q of
the coil and the output voltage drops.

BTW, this method of testing the resonant frequency can also be used for the
primary circuit.

Thank you very much,

Alan Williams

```