The Grid Dip Meter

The Grid Dip Meter

The grid dip meter also known as a grid dip oscillator is variable oscillator circuit that uses a variable capacitor for tuning and external coil for the L part of the circuit. Most also have a potentiometer to control the level of oscillation of the circuit. The value of the coil determines the frequency range of the meter and most meters come with a set of coils that are calibrated to the frequency dial on the meter. The actual values of the coils is not critical if you have a receiver with a calibrated readout on it to compare to. The indicator meter on the unit measures the grid current in the circuit and is what is used to make measurements. You can find used meters at hamfest and swap meets in the price ranging from $5 to $50 depending if they have the coils and the quality and condition of the meter

Well the grid dip meter is used to check the resonant frequency of a tuned circuit whether that circuit be a pi-network in an amplifier or a filter or the most common an antenna. How this is accomplished is that rf energy is coupled into the circuit under test and you vary the frequency of the oscillator in the meter until resonance is found. At the point of resonance the circuit will absorb the most rf from the meter much like an antenna will take rf from a transmitter that is matched. Once this happens the grid current of the tube in the oscillator will drop thus the "dip".

If you want to know the value of that inductor or coil you just wound then you can use the meter and a little math to do that. What you would do is take a high quality capacitor of known value and put it in parallel with the unknown inductor and now you have a tuned circuit. Then you just take your meter and find the resonate frequency of that circuit. Now you have a lot of info now what? Well here's the formula to find the inductance of that coil in uH.
.............1
L=--------------------
..........4π2 f2 C

π=3.1416
f is in MHz
C is in uF
L= unknown in uH

The same thing can be done with a known inductor for capacitance and the formula is the same but reverse value C and L in the formula.

You can find the Q of an inductor and here's how to do it:
Find resonance of the industor then tune lower and then higher until the dip is reduced by 30%. When you go higher this is F1 and then lower is F2 and the formula is as follows....

.........F
Q=------------
......F1 - F2

and that Q!!!!!!