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

This calculator makes use of HTML5 graphics features, to draw a radio dial scale which shows the tuning linearity (or non linearity) resulting from the various component values. Most current versions of popular web browsers support HTML5. However, this is relatively new territory for many of them, and there may be a few glitches along the way. This should be considered a beta version for the time being. If you encounter problems, please let me know. If you prefer you can continue to use the Original Bandspread Calculator.


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. 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.

  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.

Dial Scale Graphic

A recently added feature is a graphic depiction of the radio dial scale 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:

  1. Midline (also known as centerline and centraline);

  2. Straight Line Capacitance;

  3. Straight Line Wavelength;

  4. 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 Straight Line Frequency and Straight Line Wavelength types were used in the 1920’s and give linear adjustment of those parameters. Equal increments of shaft rotation provide equal increments of frequency or wavelength respectively. The Straight Line Capacitance characteristic is common in certain test equipment and other non radio tuning applications, such as in antenna matching networks, though it is sometimes used in tuners for very narrow band tuning. By choosing the different characteristics from the popup menu, you can see how these affect the overall tuning linearity. In some web browsers you can save the dial scale image as a graphics file by right clicking on the image and then using the Save Image As... menu item. You can also see the effect of changing values of the various circuit components. By entering different values for the trimmer, padder and inductor, and then clicking the Update button, the dial scale chart will be updated to show the new tuning range and linearity. If you change the variable capacitor type, the calculator will automatically redraw the graph based on the calculated values. So, you will have to click the Update button to redraw with the manually entered values after changing the variable capacitor type.


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

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:

Bandspreading Part 1

Bandspreading Part 2

Radio Theory


This page last updated: July 10, 2016

Copyright 2013, 2016 Robert Weaver