Saturday, December 21, 2013

Amplifier for the Kanga/M0XPD DS shield

It was time to look at an amplifier for the Kanga DDS shield . I found their "Sudden TX Shield" on the web site. This would have worked fine but would have taken a couple of weeks to get to Texas and would have cost 24.95GBP plus 20GBP shipping. A homebrew amplifier was needed. 

According to the M0XPD blog the Sudden shield is based on the G-QRP-Club Sudden Transmitter. The manual is available at http://www.gqrp.com/Sudden_TX_Kit_manual_40m.pdf . The junkbox had some 2N3904, 2N3906 and 2N5109 and I also had some plugin bandpass modules from my G0XAR Ulimate 2 Beacon transmitter. A visit to Radioshack produced some double sided PCB (Note it is not glass fibre PCB,  if you try to cut it with tinsnips it will crack in random directions) to build it deadbug style. The trackes were made with a scalpel and steel ruler.

Initially the amp was tried on 30m with a 2N3904 and a 2N5109. 1mW input produced over a watt of output from the LPF but driving it from the DDS shield gave just 100mW. Looking in the modamp box I found some MSA0385 which would produce 10mw output with 20dB gain. I added this to the PCB and found it had too much gain when driven by the DDS shield so an 7dB attenuator was added to the input. The "gain" preset on the 2N3904 allowed the output to be set from -10dBm to +32dBm which peaks at 0.5A at 13V. The 2N5109 got very warm but W5LUA provided a top hat heatsink from his collection on a recent visit

With the amplifier running 200mW the MEPT has been seen all over the USA when transmitting on the Alexloop



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