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

Kevin S <satya@...>

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

Thanks for the info, Kevin.

Pardon my ignorance, but where does the 90,175 pf*uH come from? Do you
have any web sites that discuss this?

--- In ultralightdx@..., "Kevin S" <satya@...> wrote:
I think that's correct - I must have tied the two sections together to
make a 532 pf lower end. Multiplying inductance times cpaacitance, you
need about 90,175 pf*uH. Since the inductance of the coil is around
266 uH, that leaves 339 pF needed to get down to 530 khz.
Since there is so little distributed capacitance in the Litz coil, tying
the two cap sections together means that you should still be able to
tune the top end.


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