From G3PLX:
Gary:
You say you don't have a Q meter, but it's very easy to improvise one, if
you have a means of generating RF at 136kHz and a means of measuring RF
voltage.
Set up your LF transmitter to deliver an output of 1 volt across a suitable
low-value resistor, for example a 1 ohm resistor which is in series with the
earthy end of a 50 ohm dummy load. Check with your RF voltmeter that it is
1 volt.
Next connect one end of the inductor you want to measure to this 1 volt
source, then a suitable capacitor down to ground, so you have an L-network.
Move the voltmeter so it's across the capacitor, and tune to resonance at
136kHz by adjusting the capacitor. In other words, tune for maximum RF
voltage across the capacitor.
If your source voltage was indeed 1 volt, then the voltage across the
capacitor at resonance is numerically equal to the Q factor of the circuit.
If the source voltage wasn't exactly 1 volt, or it's dropped a bit due to
the load of the test circuit, then calculate the Q from the ratio of the two
voltages.
Note I said "Q of the circuit". Clearly you need to be sure you are using a
good quality capacitor if you want this technique to actually measure the Q
of the coil. And of course the voltmeter must not load the circuit, or it
will reduce the Q. You could even improvise the voltmeter, since you are
just measuring a ratio. The actual voltage itself doesn't come into the
equation.
What I have described here is actually what happens inside a traditional Q
meter.
Note that if your transmitter doesn't output a sinewave, then there will be
some error in this measurement. You could minimise this by replacing the 1
ohm resistor by a capacitor. A capacitor of 1 ohm reactance would be 1uF at
136kHz, but don;t just use any old capacitor. Don't use an electrolytic, use
a polyester one if you have one that big, or if not, use the biggest one you
can find.
73
Peter
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