One of my projects needs a 70Mhz. Low Pass Filter. I plan on using air wound coils for its construction. Only problem is that my RLC meter does not measure down to the inductance values I need. I remember reading somewhere that they used a RLB with a pickup coil to get similar results to what you would have with a Grid Dip Meter.
I made up a small pick up coil using an old plastic 35mm film canister, 4 turns of wire and a SMA connector. I connecting it to the RLB and SNA jr. I quickly wound about a few turns of wire around a old pen, and soldered a 330pf cap across the coil. The film canister is large enough that I could insert the coil and capacitor inside for maximum coupling.
Doing a couple of wide band sweeps in the normalized mode I saw a small dip in the signal around 10 MHz. Narrowing the sweep I found a nice dip and measured 9.259MHz. Since I knew the value of the parallel capacitor, I plugged those values into an online LC resonance calculator. Came up with a value of 1.01uh for the coil. Squeezing and spreading out the coil I came up with values that looked appropriate. With this method I should be able to wind the coils I need for the LPF. And have another tool to play with.
The pick-up coil worked well for air wound coils, but I wondered if I could modify it for doing toroid coils? Looking at the manual for a Grid Dip Oscillator, I saw that for toroid coils they used a coupling loop around the GDO coil connected through a couple of turns through the toroid. I wound a few turns of wire around the end of the film canister next to the pickup coil ,and soldered the ends to a couple of pins from a good quality IC socket. I knew these pins would be a good fit for the connectors on the solderless bread-board jumpers I have. The connectors on these jumpers are also small enough to fit through the toroid with windings in place.
I wound up a toroid coil and soldered a 47pf capacitor across it.
Running the bread-board jumper through the toroid a couple times I
connected to the pins on the modified pickup coil. Did a wide band
sweep and saw a little dip around 20 Mhz. Narrowing the sweep I was
able to find a dip at19.819 Mhz This dip is not as deep as with the air wound coil, but is deep enough to use.
Using the LC Resonant frequency calculator, I got a value of 1.37 uH. Going to a toroid coil calculator for the type of core I was using I entered the value of capacitor and number of turns in the coil. This gave me design values of 20.18 Mhz. and 1.26 uH. Looks like this simple pickup will work for both air-wound and toroidal coils.