I am trying to develop an AM transmitter but I am having problems with getting a variable capacitor and inductor (for the rf circuit) in my location. I was able to find a choke coil with an inductance of 150uH but most of the circuits I was able to find in the net have 200uH inductors. Is it possible to use this inductor?


Here is the link to where I am going to base my circuit on: http://www.zen22142.zen.co.uk/Circuits/rf/amtx.htm

and here is the link to the inductor I am planning to use: http://www.e-gizmo.net/oc/index.php?route=product/product&product_id=45861&search=150uH

for the capacitor of the rf amplifier, only 75pF trimmer capacitors are available in our nearby shop so can I use constant capacitors in parallel so I could still control the capacitance? enter image description hereenter image description here

  • 2
    \$\begingroup\$ You give us no details about how you're going to use this inductor, there is no schematic. We have no idea what this inductor is (include a link to a datasheet). Because of this your question is unanswerable. \$\endgroup\$ Mar 18, 2019 at 12:45
  • \$\begingroup\$ I'm sorry for the lack of information. I plan to use it as a replacement for the 200uF for the rf amplifier. Here is the link to where I could buy this coil: e-gizmo.net/oc/index.php?route=product/… Again. I am really sorry since it's my first time trying to ask here and have very low experience in designing RF circuits. \$\endgroup\$ Mar 18, 2019 at 13:57
  • \$\begingroup\$ Much better now with the information, though it would be even better if you could put the schematic into the question because external websites tend to disappear after a while. \$\endgroup\$
    – pipe
    Mar 18, 2019 at 14:21
  • \$\begingroup\$ Thank you for your recommendations and feedbacks to make my question more understandable \$\endgroup\$ Mar 18, 2019 at 14:32
  • \$\begingroup\$ At 1MHz, you can be sloppy, as long as the bottom of C9 ties directly to the bottom of R1, which also ties directly to the coax shield (1" wire at most). Make a 1" diameter form (could be wood), that is 1" long. Wrap 20 turns of insulated wire, shoulder to shoulder, on that form. This produces about 6uH. Using finer wire, lay down 40 turns along that 1" long form. This produces about 25uH. There should be little of no wood visible between the turns of wire. \$\endgroup\$ Mar 18, 2019 at 14:43

1 Answer 1


Yes, you can use 150 uH inductors if you adjust the values of the parallel capacitors to keep the resonant frequency where you want it.

Yes, you can add fixed capacitors in parallel with your variable capacitors to "center up" the adjustment range around your desired output frequency. Of course, the adjustment range will be much smaller (more narrow). You can use the calculator on the website you linked to determine how much external capacitance to add. The calculator uses total capacitance, so you should subtract 1/2 the max value of the variable capacitor to see how much external capacitance to add. This will optimize the adjustment range.


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