As featured in Practical Wireless December 2017.
A handy guide showing the aluminium sizes required (in both metric and imperial) and the total order lengths required is available as a guide. You will need to verify these sizes for yourself, information provided as a guide.
This year’s beam project was 18 MHz, a non-contest band and one that often provides very good propagation into the USA and South America. The design was once again from DK7ZB website detailed here, Its a 28 Ohm version on a 5m boom with 5.7 dBd of gain so 400w in should give around 1.5kW ERP.
I selected this design for a number of reasons, the website contains great detail and the antenna fits onto a standard 5m boom making construction a little easier. It also means you can just have one boom joiner section, allowing easy transportation in the car.
The aluminium was ordered from Aluminium Warehouse, they have a good range of sizes and include lengths up to 5m. Delivery or collection is both possible, and they will cut to size if required. The aluminium required needs to concertina starting with 30mm, then 25mm, 20mm and finally 16mm.
Some careful planning is required to ensure the inside diameter is sufficient to allow this construction. This can be especially tricky as some of the sizes need to be in imperial and others in metric depending on what sizes are available. The initial stage involved checking the internal diameter, do each of the sections fit inside each other, thankfully they did although some sections were quite a loose fit. Chris G8OCV cut all of the sections using his chop saw, making the process much quicker and easier than the hack saw method used on my 21 MHz version. The band saw was then used to cut some slots in the elements, and with the use of a jubilee clip, this should clamp the elements into each other.
Looking at this picture you could be mistaken for thinking the antenna isn’t that large. The boom is 38mm box, and the reflector is around 8.5m long.
Thanks to Chris G8OCV, George M1GEO, Peter G0IAP and John M0UKD for their assistance with the project. Without their time, effort, encouragement and technical input this project would not have been successful.
The next phase will involve bolting the sections together, completing the matching section, and hopefully its first on air tests.
Part 2 final construction and on air testing is available here.