Note: Observe 555 minimum and maximum R/C values (

Test example: Assume the inductor has negligible resistance and that upon carrying out the procedure described in the above-referenced page, the frequency that resulted in a half-amplitude reading was 1,175,000 Hz (enter values in Hz, not KHz or MHz). Entering this value in the frequency box below without commas (leave the resistance box empty) results in an inductance measure of 3.9 μH, which happens to be the value that the inductor in question should have had.

If the inductor has a significant internal resistance (can be measured with an ohmmeter) then enter this value in the resistance box before clicking ‘compute.’

Note: The experimental method is fully described at the page linked in the introductory paragraph above.

Select units for inductance.

The formula reproduced above includes an exponential term, so the inverse must necessarily contain a logarithm—I thought this as good as reason as any to refresh my fading memory of logarithms. A detailed derivation entails several steps, but can be satisfactorily summarized in two. The first is the direct or immediate inverse of the formula for diameter, while the second substitutes the natural logarithm for log base 92, and combines constants in order to simplify the resulting expression.

Formula (3), giving AWG as a function of diameter in millimeters, has excess precision—more than the original formula. However, the calculator (below) rounds the answer to the nearest whole number (i.e., wire gauge number).

Here is another backwards calculation. This one is more a mental exercise—It doesn’t really merit a calculator, but just for fun. Many sources (for example, Wikipedia) give the following formula for VSWR in terms of reflected power (or, more precisely, the ratio of reflected to forward power):

Assume that the ratio of reflected to forward power is a positive fraction less than 1. The question is: “For a given VSWR, what proportion of the power is reflected?” As I said, this is normally a mental calculation, unless VSWR is extremely high, in which case it is probably too late to have asked the question! First, simplify the formula with a substitution:

Use ordinary algebra to solve for Γ and then square both sides.

An example will demonstrate how easy it is to calculate reflected power from VSWR. Suppose VSWR is 3:1. 3 minus 1 is 2. 3 plus 1 is 4. 2/4 = 1/2 and (1/2)

You are given a principal loan amount (P), an
interest rate (i), and a number of equal payments (N). Interest is
expressed as a fraction or proportion per payment. For
example, if the loan is to be paid monthly and the interest is 6% per
year, fractional interest would be 6÷1200 or .005. The divisor
1200 reflects conversion from percent to fraction (÷100) and from
annual to monthly (÷12). The number of payments (N) refers to the total
number over the course of the loan. For example, if payments
are monthly and the loan is for 6 years, N = 6×12 = 72. Let A stand for
the equal payment amount:

Many years ago (i.e., before Internet and before personal computers) a calculator salesman asked me to program this problem for his machine, and promised a case of scotch whiskey as payment. I never saw a drop of scotch, but working out a program for the problem was reward enough. Back then it wasn’t JavaScript!

Example: Suppose you wish to borrow $25,000.00 at an annual periodic rate of 6%, and make monthly payments for 6 years. Enter 25000 as a number into the first box (omit the dollar sign and commas), .005 in the second box (see first paragraph above), and 72 in the third box. Then click the ‘Compute Amount’ button. For these input values the monthly payment should be $414.32. This amount consists purely of principal and interest. It does not include any add-on charges that may be included in a real loan, such as taxes, insurance, and so forth.

The author makes no claim as to the accuracy or completeness of information presented on this page. In no event will the author be liable for any damages, lost effort, inability to reproduce a claimed result, or anything else relating to a decision to use the calculators or supplemental information on this page.