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For the PACC contest 2005, we set ourselves to further improve our 160m station. For the CQWW CW contest in 2004, we already
build a vertical and 2 K9AY RX loops for this band, all we needed now was a little more power. We didn't have any serious amplifier for this band, and the 200W barefoot power just didn't make it trough the high QRM levels often
experienced on the other end of the line. So we decided to build a 160m extension to one of the available amplifiers. We choose the good old L4B by Drake. This amplifier gives a nice 1 kW output from 80m up to 10m. The best
solution for us was to sacrifice the 80m position, and modify it for 160m. The build was done by Jelmer PA5R.
The Build The idea was simple: just add enough capacitance in parallel to the existing anode and
antenna loading variable capacitors, and increase the inductance in between. As a starting point, all values should be roughly doubled, and then fine tuned for best performance. Of coarse, we are dealing with a high power
amplifier, so only suitable materials and components for these power levels can be used. The RF currents flowing in an amplifier output tank circuit can be massive!
After a lot of experimentation, the end result was an
additional 160pF at the anode capacitor, an extra inductor made of 24 wdg heavy gauge Teflon coated wire on a big Amidon T-300 core, and an additional 1200pF in parallel with the antenna loading capacitor.
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