We finally built the 40m array on Saturday and conducted a full test on Saturday night, in preparation for CQWW this weekend coming (24th/25th Oct 2009). The good news is that signals bearly audiable on 40m on our standard Mega-Loop came up a few db on the array, to around 5 and 7.
The front to back ratio could be higher. Germans were crawling all over us, working Worked All Germany contest which was a good test, but it did hamper our out-and-out gain tests to US. Many of them probably switching off and saving their energy for next weekend. The modelling we went for gave us maximum gain at 10 degrees take-off. We could have gone for slightly less low-angle gain and instead aimed for a very high front-to-back ratio dialing in up to -15dB off the back. As it is, this current antenna only gives us around -6 db gain off the back. There are some benefits though, like working VK “off the back of the beam”, which I’ve never said before on 40m. Great fun.
Essentially, we have the gain of a non-steerable 4-square array so we should have fun into the top end of South America as well as all of North America, right up to Vancouver and Alaska.The proof of the pudding though, is this weekend and needing 40m to work all through the night, from 7 at night through to 7 in the morning – possibly more. This antenna needs to give us 12 hours out of each 24 hour slot, a must for a Multi-Two entry. Station #2 will have 160m, 80m and 20m to play with all night.
Remember to dial your logging program to update GetScores.org. A live scoreboard is hilarious fun for teams and will keep you on your toes all weekend.
Good luck and have a blast.
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.
We’re putting together a Multi-Two entry for CQWW this year and it’s pretty clear from our experience with CQWPX that we not only need gain to the US on 40m but we need excellent front-to-back ratio. A new antenna was required.
A two element yagi was considered but we don’t have a tower for such a beast. We did though have various 10m fishing rod blanks and a load of aluminium scaffolding tubes. With some analysis, we feel we can build a high gain array utilising Yagi’s principles of a driven element in the middle and a reflector and a director element front and back.
Original modelling was conducted with MMANA however, the team has recently started to convert to modellilng with NEC2. James’s NEC model confirmed my 5db gain using MMANA at 10 degree take-off angle.
The feedpoint for each element will be at 9 meters above ground using 6 meter scaffold pole sleeved with a 4 meter inch and a half steel pole. The 10m Sky Blue Leisure flag poles sleeve to the inch and a half steel poles. I’ll take some pictures tomorrow of the build.
Make no mistake, these will be monsters with 8 raised radials per element. Today, I made the insulated guying blocks for the radials. This is going to wipe the floor!
Watch this space.