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.
RSGB 2nd 70MHz Contest
We had some great September weather today for the 2nd 70MHz contest. We setup at the Kelvedon Hatch ‘Secret Nuclear Bunker’ as seen above, using my new 70MHz amplifier at 160w and my 4 element DK7ZB 12.5Ω Yagi.
Conditions seemed quite normal, we worked no Scotland or Ireland but our best DX (as many others) was PA4VHF at 449km. Other highlights were GJ3YHU in Jersey and M1CJN/P in the North York Moors. Total contacts was 46.
A very enjoyable day of operating and testing the new amplifier. Next week, it’s the Practical Wireless 70MHz contest, so lets hope for the same weather and some band openings perhaps? Maybe I’m asking too much 🙂
The Claimed Scores are available. Thanks to Dave M0TAZ, George M1GEO, Dave M0YOL and Chris G8OCV for help & company, and of course everyone we worked (or tried to work!).
Two years in the making – 70MHz 300w Amplifier
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!
Update to Slim Jim calculator
I have added the ability to include a known velocity factor into the Slim Jim / J Pole calculator, to help with building them with 450Ω or 300Ω feeder. Also, here is a tested and reproducible Slim Jim for 70MHz (4m) made with 450Ω balanced feeder:
Working Happisburgh Lighthouse on 70MHz
Selim M0XTA went to Happisburgh Lighthouse for the Lighthouse On The Air weekend 2015. He set up a 70MHz station on top of the lighthouse, using a ⅝ λ vertical. I had a listen from home and could just make out a signal in the local noise I get on 4m at home, so I decided to go out portable and give it a try.
I set up a slim jim made from 450Ω feeder on a 10 metre fibreglass pole on a small local hill (away from the QRM) and used the Anytone AT588 with 50w on FM. There was some slow QSB, but we made the contact. He was 105 miles from me (see map). Not bad for no beams and FM!
I took a tiny bit of video on my phone, which is below.
70MHz Cumulatives – 9th August 2015
Some amateur friends (that sounds odd, but you know what I mean!) were playing radio this weekend from the Kelvedon Hatch Secret Nuclear Bunker in Essex. I decided on Saturday to build a 70MHz Yagi and have a go in the 70MHz Cumulatives contest. I had some spare aluminium and wanted an antenna with good performance that wasn’t too bulky and therefore easy for portable operation. I decided to build a DK7ZB design as I have had great success with them in the past.
I decided on Version 1 on this page. Its a 12.5Ω design and has around 8.24dBd gain. The gain vs size is excellent, matching Yagi’s almost twice the length, except this one I can fit in the car. The compromise is the narrow bandwidth, and therefore tolerances were very small and build was rather critical. I adjusted the antenna slightly in EZNEC to suit my ½” tube. First tests with the antenna showed that it was a perfect 1.0:1 SWR at 69.5MHz and 1.7:1 at 70.2MHz. I trimmed 10mm off each element and that got it spot on 70.2MHz. We did well in the contest and a QSO map is available. We worked 40 stations in 2 hours, which I think is good for 70MHz. Our best DX was PA4VHF at 449km.
Thanks to George M1GEO for allowing me to use the site and Dave M0TAZ for the use of his mast!
I finally put up a dipole for 70MHz (4m)
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…)