Use the calculator below to calculate the padder, trimmer and inductor values to bandspread a tuned circuit to a specific frequency range.

This diagram shows the arrangement of the tuned circuit. **C**_{V} is
the variable capacitor. **C**_{P} is
the (series) padder. **C**_{T} is
the (parallel) trimmer. **C**_{S} is
the circuit stray capacitance which includes the self-capacitance of
the inductor.

Refer to the following pages for the underlying theory of these
calculations:

Bandspreading Part 1

Bandspreading Part 2

Enter the desired frequency range in the top two fields.

Enter the tuning capacitor minimum and maximum values in the next two fields. (

*Note that a frequent error is due to entry of the min and max values in the wrong order. If you get a calculation error, check this first.*)Optionally, enter the circuit stray capacitance in the fifth field. If you leave it blank, a value of zero is used.

At this point the calculator will display in the middle section, the allowable range of inductance, and series and parallel capacitances. If red asterisks are displayed after the above fields have been entered, it means that the variable capacitor range (combined with the stray capacitance) is too narrow to give the desired frequency range. If this happens, then please change your frequency range or variable capacitor paramaters and try again.

Any time that you change a value in one of the input parameter fields, the allowable component ranges will immediately recalculate and update.

Decide which component you want to be a fixed value, and select it from the popup menu in the Exact Value Calculation section.

Enter the value of the chosen component in the next text field, making sure you stay within the allowable range. Then click the Calculate button.

The exact values of the components are displayed at the bottom of the Exact Value Calculation section.

**Note**,
when you enter a Padder (series) capacitance value of zero, it means
that there is no padder present. That is, **C**_{S}
is shorted out. Similarly, if the calculation produces extremely high
values for the padder (more than 100 times the *C*_{V}*
*maximum
value), it will be displayed as zero, indicating that a padder will
have no significant effect.

A graphic depiction of the radio dial scale is displayed,
showing the linearity of the tuning. This is generated when the
* calculate*
button is clicked. The linearity will change depending on the type of
variable capacitor. There are four types to choose from:

- Midline (also known as centerline and centraline);
- Straight Line Capacitance;
- Straight Line Wavelength;
- Straight Line Frequency.

A popup menu allows you to select which kind of variable capacitor you
are using. The * Midline*
is the most common kind used in broadcast band radios from the early
1930’s until present day. It should be noted that there is no
single official midline curve. There were variations on this
characteristic, and a number of different RMA (Radio Manufacturers
Association) standards for the midline curve. The variations were
fairly minor, and the curve used here is fairly typical. The

Suppose we have a tuning capacitor with a range of 15-365 pF, and we want to
tune the 49 meter band from 5900 to 6200 kHz using an inductance of
15 *µ*H
and assuming a stray circuit capacitance of 25 pF.

Enter these values: 5900, 6200, 15, 365, 25 in the order shown, into the five fields of the Input Parameters section. Once the last value is entered, the middle section of the calculator will show the allowable ranges for the Trimmer, Padder and Inductor. The values are:

Trimmer: 0...3316.34 pF (If present, must be 3316.34 pF or lower)

Padder: >= 9.07 pF (If present, must be 9.07 pF or higher; 0=No Padder)

Inductor:
0.20...21.50 *µ*H
(Must be in the range of 0.20 to 21.50 *µ*H)

We see that the 15 *µ*H inductor that we want to use is within the allowable range. Choose
“Inductor” from the popup menu, if it’s not already selected.
Enter “15” into the field immediately below the popup menu. Click
the Calculate button. The exact values for the Trimmer and Padder are
calculated, and all values are displayed in the bottom section as:

Trimmer: 64.22 pF

Padder: 24.87 pF

Inductor:
15.00 *µ*H

The
resulting radio dial scale is graphed in the area below these
numbers. The default dial scale assumes a * Midline*
variable capacitor characteristic. By choosing a different
characteristic from the popup menu, the graph will update to show its
effect.

Back to:

Calculators

Bandspreading Part 1

Bandspreading Part 2

Radio Theory

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This page last updated: January 31, 2023

Copyright 2010, 2023 Robert Weaver