Today, I finished a project I started over two years ago. It’s not that it took two years to build, it’s just that its taken me two years to get the parts together and build in my spare time! The upcoming 2nd 70MHz contest and PW 70MHz contest prompted me to get this finished.

Its a 300w 70MHz amplifier, using a BLF 278 MOSFET. Power supply is 50v @ 10A. The amplifier has 9dB attenuation at the PA input to bring the gain down to something usable, as without the attenuation, 2.5w input gives 300w output (21dB gain)! The input & output power, with 9dB ATT (total 12dB gain) is:

• 5w = 100w
• 7.5w = 150w
• 10w = 200w
• 15w = 300w

The amplifier uses ~7.5a @ 48v for 200w O/P, so it’s around 80% efficient. Bias is set to 750mA. The maximum power here in the UK on 70MHz is 160w, for which this amplifier requires 8w of input. Below is a picture of the inside. I will tidy up the cables at some point!

300w PA & LPF

I need a larger Bird element to measure the 300w output on the Bird 43!