2010-December-01 - More Power with the EB104 design

I have been using the Gateway 40 with some success, but 50 watts is not enough in many situations. I can make some contacts when conditions are good, but in marginal conditions it is a struggle to just get a signal report. It's time to build an amplifier.

The 50W FET amplifier went together easily and worked without too much trouble. I did blow a few of the FETs, but that was when I fed them into a mismatched antenna at the old QTH. At the new QTH, with a good dipole, I have not had any issues. Like many people, I have reviewed the classic FET designs by Helge Granberg, in particular the EB104 design which uses four MRF150 MOSFETs and produces 600 Watts output. That should be enough. There is also a kit available from Communication Concepts for the RF deck. I'm not a fan of kits. I would rather build from scratch. But making an accurate double sided PCB is tricky and I dont know how well it will work if I build it ugly style.

Before I build the amplifier, I need to consider if it is sensible to amplify the output of a phasing transceiver further. Is the opposite sideband suppressed enough to warrant amplification? If it is properly adjusted, then it should be 40-45dB down.

631 Watts is 28dBW
After 40dB of suppression the opposite sideband would be at -12dBW or 63mW.

The questions - is that acceptable? 45dB suppression is better, at 20mW. Or 50dB at 6mW. The FCC does not regulate opposite sideband suppression. Double sideband is permitted after all. It seems sensible to follow the guidelines for harmonic suppression though. Harmonics must be suppressed 40dB and below 50mW. That means we need 41dB of sideband suppression or more.

The key challenge will be building the power supply, which needs to deliver 50Volts and 25Amps. I don't want to just buy an old telephone PSU off ebay. There is no fun in that. Instead I'm going to build a linear PSU. Tom, W6KAN, has a great example of the amplifier that was featured in QST a while ago. Like him, I have ordered the transformer from Ameritron, part number 406-1246.

I'm looking for a suitable case for the Power Supply, which I will build first.

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