I recently built a quarter wave ground plane antenna, so that I could get back on the 4m (70MHz) band. I have made a page showing some images and construction details of this antenna which can be seen here.
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!
Hmm, what to do on a bank holiday Monday? Well, we decided to purchase the aluminium to build a 15m and 10m Yagi about 6 months ago, so it was time to make something of it. Dave M0TAZ is building a 4 element 15m Yagi, and I am building a 3 element 10m Yagi. Dave started constructing his 15m beam this week. It is a DK7ZB 28Ω feed-point 4 element Yagi, with a forward gain of ~6.4dBd. The elements are tapered with 3 different sizes of tubing and the boom is 40mm square and 5m long. All that’s needed now is the feed-point arrangement and it’s ready for testing! Still, it looks great and is very rigid. Cant wait to get it on the mast! Here are a few photos for now…
Today, I finally built a ½ λ dipole for 4m. I have previously been using a folded ½ λ dipole made out of 450Ω balanced feeder, suspended in the loft, which is not ideal. I still had the coax outside on the roof and the pole which were both used previously for my 2m/70cm co-linear, which has been relocated to the chimney, so once it was built, it was simple enough to put up. (more…)