Project Y. So there is another top secret Project X in the pipeline…
After losing the WARC dipole and looking at various options on dxbeam.com and optibeam.de I considered taking the homebrew route. These antennas are worth their price but it is still a lot of money for a simple dipole, either monoband or dual band 30/40. The main issue would be the coils. I have a friend who owns both 30m and 40m rotary dipoles so I gave him a call. I asked him if he wanted to take some pictures and dimensions of the coils. Then he told me he only uses the 30m dipole and changes the end tubing to the 40m version when he wants to play in a contest. The part beteween feedpoint up to the coils included is the same for both antennas. So as it turns out, he had an almost complete 40m antenna in spare. He offered me to pick it up and try it.
Now, that is a huge step forward in ‘homebrewing’ the antenna. OK, I admit: no homebrew, just put the antenna on the tower. But I didn’t buy it (yet). A few practical problems needed to be overcome. Since this antenna is rather long, I cannot put it parallel to the yagi’s boom anymore. I would not be able to tilt over the tower anymore. So the dipole needs to go parallel to the yagi’s elements, perpendicular to the boom. This raises the chance that the dipole will couple with the yagi. So I need to get the most spacing I can get. But the longer dipole needs guying. So if I want maximum spacing ànd guy the dipole, I needed to extend the mast on top of the tower.
My first plan was to turn a piece of plastic (commercial brand name: ‘Ertalon’) down on a lathe and extend the mast. But the mast is actually 2 tubes welded into eachother to get 10mm wall thickness. The welding makes for a not perfectly round inner diameter. Turning down a rod on a lathe would not fit. A collegue of mine who teaches machining and knows all about lathes and machining parts, looked into his junk box and I went to buy some parts and the final result was a ‘top tube extender’.
It’s an aluminum coupler that is screwed into the mast with 3 M8 bolts. Inside is an M20 threaded rod fixed with a lock nut on the lower side, and the upper side there is a piece of M30 ertalon with inner M20 inner thread to screw over the threaded rod. This is used to fix a scrap piece of 38x30mm aluminum tubing. To make sure this tube does not rotate over the rod, I had it MIG spot welded on the coupler. A little PCV cap on top to keep H2O out to finish it all. The tube holds an aluminum plate with stainless steel hardware to receive the 5mm nylon rope used to guy the dipole and minimize the sag.
The plan was to build and install it ASAP but a lot of things got in the way delaying the project. I am still waiting for parts in back order. I managed to gather enough parts from my ‘leftover stock’ but it took some time assembling it all and fitting all pieces of the puzzle. So I could only install it on Wednesday BEFORE ARRL DX CW. I really wanted to use the 40m rotary dipole in this contest because I’ve been told more than once by more than one voice I respect that such an antenna at half a wavelength above the ground would beat everyting I have used before. And I have used quite some 40m antennas in the past: shorty vertical dipole loaded with coils and capacity hats, vertical with ground radials, vertical with 2 elevated radials, inverted V dipole, a delta loop and a couple of vertical wire dipoles sloping down from the tower. The latter has brought me a lot of pleasure in the contests. But this rotary dipole should beat ‘m all. So the stakes are high and I hope it’ll work out.
ON4BHQ offered a helping hand which I gladly accepted. Not only is it much safer to have some help, it makes the work go smoother and it is much more fun. The WX worked with us: no wind, not cold and a shining sun. I could use a friend’s scaffold which made the work much easier than doing it all from a ladder. Still the work took quite some time. From unscrewing the tower’s safety bolts, over tilting it and installing the dipole to putting the tower back straight up again and measuring SWR, it took us about 3.5 hours. The drilling for the tube extender needed to be done carefully. Any offset between the bolts spaced 120° would have to be compensated by drilling with a bigger diameter which would not provide a tight fit to the M8 bolts anymore. So after drilling each hole, we put the extender back up to mark the second hole, then take it off and drill. And a third time for bolt #3. This takes time but the result was an perfectly tight fit. This won’t come off soon.
The time had come to hook up the antenna anayzer. Fingers crossed! I was VERY glad to see a perfectly flat SWR plot between 7.000 and 7.200. And I was even more glad to notice no difference in SWR-plot on the 20/15/10m yagi. With the tower straight up, a critical eye notices a minor shift between the dipole and the 20m driver on the yagi. It’s only a minor offset but I always try to make the antennas look perfectly. I will evaluate the antenna now. How it plays in a contest. How it behaves in strong winds. If this thing truly does was people tell me, it’ll be a keeper and I will adjust it.
All in all, thanks to playing the scenario in my mind a couple of times, the installation was nothing spectacular. It is not as easy as it may seem and in my setup it is not smart to make it a one man show. We took our time to do things right and I believe we succeeded. Thanks again to ON4BHQ for taking an afternoon out to support ON5ZO and providing mechanical insights and a pair of helping hands.