XXT Mega Loop - VHF gain for the HF contest bands
I have two things to write about tonight; the concept of the XXT Mega Loop and the engineering hilarity of getting very high lines attached to 80 feet (plus) trees.
Allow me to start with the XXT Mega Loop. James and I came across this design whilst hammering MMANA one night. Many of you will know about my absurd fascination over loops and I was sure that there must be some properties hidden away in a high loop that would allow a horizontal loop to exhibity gain somewhere, assuming we had height to our advantage.
After a few false starts, James thought back to his theory and gently enquired if a rhombic style antenna may work, but instead of a diamond shape, how about half a diamond? An equilateral triangle? All of a sudden, some strange properties started to develop inside MMANA. Very real (high) gain in some directions, but only if multiple wavelengths and height were present. And a multi-band antenna too? Weird.
Later that night, after James had gone home, I hammered the numbers and worked out that 28.5 meters per leg at 20m in height will develop an extremely efficient antenna from 40m up through the traditional contest bands at or near 200 ohms impedance. A 4:1 balun would cope with this. Facing this triangle in the right direction could be a killer! FInding somewhere that had three trees in the right place would be the problem though.
Here’s the design anyhow:
Even 80m will load comfortably into less than 2:1, of course at this height, a regular bubble of RF will develop like any decent NVIS antenna, however, can you imagine 3dBi gain at 10 degrees on 40m? This is stuff I can only dream of.
In my other article, I discuss how we erect this aerial, this article only discusses the theory.
Here’s the screenshots, make you own mind up:
20m Plot:
15m Plot:
10m Plot:
Stop press, we built it yesterday and made 2 QSOs (20m and 15m) to US, then took it down and replaced it with one twice the size - it only JUST fits in between the tops of the trees and grows our theoretical gain by quite a margin. 40m now goes to 5.5dBi to USA with a take off angle at 10 degrees. We look forward to some interesting runs to the US in CQWW.
73 from M0XXT JOTA Station
Callum.
