I have a question on Inductors for VHF Amplifiers and Transmitters. I often see circuits on the web, where several aircore enameled copper wire inductors are used. The specification looks something like this:
L1 = 5 Turn 26 SWG Enamelled Copper 10mm diameter aircore
I understand all the attributes of this specification. But the problem arises when a specific guage of wire used in the circuit, is not available with me. In such scenario I thought of using the calculators available online (like this one) to calculate the inductance of the Coil. For that inductance, with simple reverse calculation I will try to determine the number of turns and coil diameter for a different guage wire which is available with me. The inductance may not match exactly, but it reaches very close to the desired inductance. My question is, is this approach correct? Can I really replace an inductor in RF circuit which is expected to be x turns of y guage wire of z diameter by an inductor which is x1 turns of y1 guage of z1 diameter. The question might be silly for subject matter experts, but I am not originally from Electronics Background. So, just wondering if apart from the inductance of the coil, anything else matters to those RF Circuits.
I also see that some circuit uses 'RFC' or 'FM RFC'; and often no other specification is supplied. I understand it means Radio Frequency Choke. But is there any standard inductance for an RFC? If I don't have an RFC for say 100 MHz, how can I make one myself?
Best Answer
That calculator is no good for what you want because it assumes all the "turns" are stacked on top of each other and that doesn't work for VHF and above because of inter-turn capacitance causing unwanted resonances that will fundamentally ruin performance expectations.
At VHF you need to use spaced turns and this largely solves the problem of unwanted resonances but significantly alters the relationship between inductance and number of turns. With a helix type inductor, all the turns don't couple to each other so instead of inductance being proportional to turns squared it's somewhere between "turns" and "turns squared". You need a proper "air-cored" (or helical) inductance calculator that not only has number of turns as a parameter but the overall spacing dimension for those turns such as this one: -
Within reason you can use a similar wire gauge but why bother when you have the above calculator - mimic the inductance by tweaking length or number of turns to give you what you want.
Here is another calculator that looks good: -