Hello, all,

I'm just beginning to learn about designing practical filter circuits. Using tools like Elsie, I think I can choose the components sizes for filters. However, I don't know how to size the components for the expected amperage (to choose the wire size for hand-wound inductors) or voltage (for voltage limits of capacitors). I'd like to learn the formulas for calculating these specifications.

As an example, attached are the Elsie output for a 7.3MHz LPF (just above the 40M Amateur band). The standardized values for the inductor is 1.5µH and the capacitor is 620pF. If I want to design this filter for a typical radio amateur transceiver that outputs 100W into a 50Ω impedance, what amperage should I plan for in the inductor, and what voltage could the capacitor experience? Assume a dummy load of 50Ω and a perfect SWR of 1:1. What formulas would I use to calculate these values? I also think that these values change if the SWR is not 1:1. How would the formula change if the SWR was 5:1? Is there a worst-case formula?

Thanks so much for your advice and guidance. If you know of a website or resource that explains this, don't bother repeating it, just point it out to me.

-Kevin

 

https://web.archive.org/web/2011120.../HF Filter Design and Computer Simulation.pdf

 

https://web.archive.org/web/20111203120818id_/http://www.ee.iitb.ac.in:80/uma/~bhagwan/antennas and microwave/HF Filter Design and Computer Simulation.pdf

Ah, thanks, @nsaspook , for this 447 page book, which I'm certain contains the answer to my question somewhere in there. Any hints on where? Looking through the table of contents, nothing jumped out at me.

But, thanks for the book, which I'll add to my collection. I didn't know about it.

-kevin

 

Unfortunately, it is not as simple a matter as you seem to think it is. Doing this with "cookbook" methods is not likely to lead you to the results you expect. I would at least take a crack at chapter 1 to see if you can make it through the rest of the book. One of the complications you will encounter is the "skin effect". High frequency currents only travel to a certain depth in a wire rendering most of the cross-sectional area useless. The same is not true for AC currents at power line frequencies of 50-60 Hz.

ETA: See pp. 54-59 of the above referenced document for the introductory material on the skin effect.

ETA2: If you are interested in learning about filters and simulating them, I can recommend at least two free simulation packages that you might consider.

By all means approach the problem any way you think is appropriate, but remember to refrain from complaining about the results, since those will be entirely your responsibility.

 

Playing with high-power RF Circuits can be a really mooshy bucket of worms.
There are no Formulas that "just work every time",
there are only "guidelines", and what tricks others have worked-out by experience.

You will be doing a whole lot of "Cut & Try" experiments
before You get all the really strange Bugs worked-out.

The following Chart may get You to thinking about why Copper-Tubing or Pipe is not uncommon.
.
.
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