radio8z

The body of this report is posted as a PDF attachment in order to preserve the formatting.

Mar 7 This post has been edited to contain new information.

The body of this report is posted as a PDF attachment in order to preserve the formatting.

Mar 7 This post has been edited to contain new information.
Mar 17 An error was found on the schematic for the modified output stage. The values for C3, C4, and C5 were given in nanofarads. The correct units are picofarads. The schematic has been corrected and is marked as Rev 1.

Abstract:

Ramsey AM-25 Efficiency Enhancement

Increasing the field strength of a radiated signal will, in general, increase the usable range of the signal with the range increasing approximately as the square root of the output power. Since the DC input to a Part 15 AM transmitter is limited to 100 mW under the Part 15 rules (15.219) increasing the power out is not simply a matter of increasing the power in. Another approach is to improve the efficiency of the final stage so the output power is increased without exceeding the 100 mW input power limit. Changes to the Ramsey AM-25 are described here which resulted in more than double the original circuit efficiency and which thereby doubles the power output while maintaining compliance with Part 15.219.

It was determined by measurement that the original efficiency was 29% driving a 50 ohm load and 23% driving a 29 ohm load. After modification the efficiencies were 73% and 53% respectively indicating more than double the efficiency with the modification.

I hope you will find this useful and let me know if it works for you.

Neil