Bandspread Calculator


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. CV is the variable capacitor. CP is the (series) padder. CT is the (parallel) trimmer. CS 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


How to use:

  1. 1.Enter the desired frequency range in the top two fields.

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

  3. 3.Optionally, enter the circuit stray capacitance in the fifth field. If you leave it blank, a value of zero is used.

  4. 4.At this point the calculator will display in the middle section, the allowable range of inductance, and series and parallel capacitances.

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

  6. 6.Decide which component you want fixed and select it from the popup menu in the Exact Value Calculation section.

  7. 7.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.

  8. 8.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, CS is shorted out. Similarly, if the calculation produces extremely high values for the padder (more than 100 times the CV maximum value), it will be displayed as zero, indicating that a padder will have no significant effect.

Example 1:

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 µH 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 µH (Must be in the range of 0.20 to 21.50 µH)

We see that the 15 µH 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 µH

Example 2:

Suppose we have a tuning capacitor with a range of 10-100 pF and we want to tune from 9500 to 9700 kHz. Assume we have a stray capacitance of 15 pF. Enter these values as in Example 1. Immediately we see that the allowable range of component values is:

Trimmer: 0...2090.23 pF

Padder: >= 3.39 pF (if present)

Inductor: 0.13...15.36 µH

In this case let’s assume we want to fix the value of the Padder at 270 pF. Therefore, select “Series Capacitor (Padder)” from the popup menu, enter 270 in the field below, and click the Calculate button. The exact values are calculated and displayed as:

Trimmer: 558.51 pF

Padder:  270.00 pF

Inductor: 1.36 µH

Back to:

Bandspreading Part 1

Bandspreading Part 2

Radio Theory


This page last updated: February 25, 2021

Copyright 2009, 2012, Robert Weaver