# 40m 3 Element Vertical Yagi Antenna – Part II

We’ve built some large antennas before but never this big; a 3 element 40m vertical array with raised radials. We made it a raised radial system for a) a quick match to 50 ohms and b) it needs to be a “field” system. We can’t permanently leave our antennas in a public park.

First, you need to get into the scaling to believe it: Take a 6 meter scaffold pole of 48mm diameter (21 feet x 2 inch). Stick it upright on the ground and sleeve inside it, an inch and a half (30mm?) 4 meter (13 feet) pole. On top of this, sleeve a 10 meter (30 foot) fishing pole blank.

You will now have a structure that is effectively 20 meters tall (65 feet). Now then, the fishing pole blank will become the vertical part of an antenna which happens to be a quarter wave for 40 meter band. Being a raised antenna, we need radials and since we’re closer to the ground than a wavelength, we need a more than the traditional two radials to counteract the ground losses. We decided that 8 x radials will be about as good as 60 or so regular ground mounted radials. Do we have the maths right? We think so.

The radials have been modelled at 10 meters length each since they are essentially part of the circuit and will have currently on them, hence the quarter wave dimension. They slope to the ground at approximately 45 degrees. We need to attach some paracord to the ends of the radials and extend them down a further 7 meters before we finally hit the ground. Imagine how far away you are now from the original scaffold pole? I can tell you, it’s 13 meters (42 feet). The diameter of just one of these then spans 26 meters (84 feet) and we have three laced together at a spacing of 10 meters each (three element array).

Today, we did all the hard engineering and measured out all the bits and pieces, ready for a trial the week before CQWW. James and I laughed at the thought of how big this monster really is – and then wondered if it actually fit inside the park so as not to distrupt the walklers? Thank goodness we checked. For those of you unlucky enough to have been to our Scout Hut, you will know that when leaving our front doors, you will notice an oak tree in the distance that houses one corner of our mega-loop. James and I measured from the grass outside the doors to the last radial and we were only about 5 meters from the oak tree. Bloody hell!

We’ve had to re-engineer where we had planned to fit this monster into the park since it has a total wing-span of 52 meters (170 feet).
A picture tells a thousand words, so feel free to check the pics.