Re: Small loop antenna -- Question for Kevin S.

Bill M <radioexray@...>

FWIW, I keep a LC calculator on my website. It comes in handy when I can't find my old timey cardboard Allied L/C Slide Rule :)


gkinsman1 wrote:

Hi Kevin,
Yes, this is very helpful. The equation and example you provided are exactly what I was looking for.
I really appreciate you taking the time to answer all of my questions today, as well as providing sources for the loop element and tuning capacitor. I think I'll build one of these mini loops.
--- In ultralightdx@..., "Kevin S" <satya@...> wrote:
Hey Gary:

Here's a site:
About halfway down, under "Resonance Effect", is the formula:
f = 1/((2*pi)*(sqrt(LC)))

I have found the following to be much easier to use, which is a simple rearranging of the above equation:
L*C*f*f = 25,330
f is in Mhz, L is uH (usually around 250) and C is in pf (usually 10-365 or some such). So, pick two of the values, and the third one pops out. For example, if L is 250 an you want to tune down to 530 khz, the require C value is 25,330/(250 * 0.53 * 0.53) = 361 pf.

For an average project, you can figure 20 or so pf for "distributed
capacitance" (i.e., the wires of the loop interacting with each other, acting as a capacitor). So, for a loop of say 250 uH with 20 pf distributed capacitance, in theory the cap would be 35 to 361, but with the 20 pf added capacitance of the loop, the required cap value would be 15 to 341. This is a typical range I've seen quoted.
I just checked my little loop in the green box, and I did link the two sections of the cap together to make 532 pf max capacitance. With all that capacitance, it tunes down to 440 khz, and easily reaches 1700 khz on the high end. The beauty of the Litz coil is that it has very little distributed capacitance owing to the winding pattern, so your high end (>1700 khz) is not endangered.

Hope this helps - Kevin
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