Okay, okay. The postings about computer checkers haven’t exactly been all that popular with you guys, so I thought I’d write up something that I am currently working on. Moonbounce Day is coming up April 16, 17, and 18, and I thought it would be a good opportunity to try to hear the station operating from Arecibo, Puerto Rico. I don’t have any equipment that does 70cm SSB/CW except for my trusty FT-817, so my idea is to just listen and record, and to use the minimal antenna that might conceivably work.
I of course continue to have my trust Arrow antenna, but I thought something a bit longer could be better. So, I have designed a DL6WU yagi consisting of a six foot insulated beam with 9 directors tuned for 432 Mhz and matched with a folded dipole/hairpin match. A trip to a local welding supply house netted me enough welding rod to make all the elements for $5, and another $3 netted me a nice six foot length of 3/4″ square hardwood to serve as the base. A few quick experiments last night showed that a miter box and a hacksaw could result in reasonably accurate cuts, but that maybe I should cut them 1mm long, and then trim them using my benchtop belt sander.
I’ll try to nab some pictures as it progresses. With any luck, by the end of the weekend I’ll have this mounted atop an old camera tripod, and I’ll be able to test it out a bit by maybe aiming it at some LEO sats and seeing how well it works. I’ll also have the complete dimensions later.
For now, coffee and breakfast.
Update:
Okay, here is the design that I came up with using VK5DJ’s Yagi Calculator. I specified an insulated boom 0.75″ in diameter, round elements 3.2mm in diameter, and a design frequency of 432 Mhz. One slightly annoying thing is that the design software only produces metric measurements, so I took them, wrote a little program to convert them into properly rounded values expressed in 1/32nds of an inch, and then dumped them all out. Here we go.
338mm | 13 5/16" | Reflector 327mm | 12 7/8" | Radiator (single dipole) 305mm | 12" | Director #1 302mm | 11 7/8" | Director #2 299mm | 11 25/32" | Director #3 296mm | 11 21/32" | Director #4 294mm | 11 9/16" | Director #5 291mm | 11 15/32" | Director #6 289mm | 11 3/8" | Director #7 287mm | 11 5/16" | Director #8 285mm | 11 7/32" | Director #9 686mm | 27" | Total folded dipole length 343mm | 13 1/2" | Center of the dipole 154mm | 6 1/16" | Center -> bend of the dipole 149mm | 5 7/8" | End -> bend of the dipole 25mm | 0 31/32" | Bend Diameter 30mm | 1 3/16" | Boom position of Reflector 169mm | 6 21/32" | Boom position of Radiator 221mm | 8 11/16" | Boom position of Director #1 346mm | 13 5/8" | Boom position of Director #2 495mm | 19 1/2" | Boom position of Director #3 668mm | 26 5/16" | Boom position of Director #4 863mm | 33 31/32" | Boom position of Director #5 1071mm | 42 5/32" | Boom position of Director #6 1290mm | 50 25/32" | Boom position of Director #7 1519mm | 59 13/16" | Boom position of Director #8 1758mm | 69 7/32" | Boom position of Director #9
I cut the elements by marking the lengths, then putting them into a little plastic miter box and cutting them with a hacksaw. A few are very slightly long, so I’ll go back and fine tune those by a little judicious use of my benchtop belt/disk sander. I then turned to the boom. I must admit that on second glance, I’m not very happy with the hardwood boom: it’s got about 1/4″ of wave/twist in it as you sight down the length. But oh well, this is an experiment anyway. I marked all the positions, and drilled them out with a 1/8″ drill bit. This proved to be too tight of a fit for my elements, so I dug up a 9/64″ bit, which is very slightly too large, and allows some wobble. When I made a cheap yagi before, I secured these with hot glue, which is what I suspect I will do later.
That’s where the antenna stands now. I have to work on the driven element a bit more. Bending this by hand is a bit imprecise.
I’ll have pictures up when there is something to see.
Mark,
I can’t help myself sorry; but you Americans and your continued use of non-SI units! Apart from some high-profile incidents caused by this, wouldn’t it be so much easier for trade and general communication if you just adopted SI like most of the rest of the planet has? Burma and Liberia are the only other hold-outs AFAIK?
That said, I have imperial drills in my toolbox along with metric ones, and often catch myself talking in inches and feet, even on my blog. I blame that on culture, my Father was a builder and worked through the metric conversion here in AU (“completed” around 1977) so I’ve been exposed to both systems since birth. It isn’t too hard to remember 25.4 and 2.2 conversion factors for the most common units, but I do find the duodecimal-centric imperial system needlessly complex.
On the flip-side, I feel your pain: US-sourced work frequently uses imperial units, to utilise them I must convert them to metric. While most scientific work is already in metric, hobby-related materials, like ARRL books are especially annoying in this regard. The ones that have some nasty (and typically unexplained) decimal constant in them to get the right answer out in whatever units the author has chosen are particularly frustrating, requiring substituting in all the conversion factors to SI base units then aggressive algebraic simplification to refactor the formula to SI.
I find such utilitarian formulae also tend to hide to physical basis which generally is far easier to see when SI units are used. (While not a unit problem, collapsing multiples of constants such as Pi, e and c to decimal approximations are especially evil as well.) From a purely pedagogical point of view this is a major failing of much amateur work. Maybe this is cultural too, with a cookie-cutter replication mentality dominating much of the modern amateur experience.
Regards,
Alan
The rare chance for fairly easy EME is this weekend, I hope Milhouse doesn’t interfere with your antenna project.
73, Elwood, WB0OEW
Hello Mark. I stumbled onto your blog as I too was designing a yagi from the same calc software. I ended up w/ a 10 director yagi made from a pine boom and 1/8″ aluminum wire. Initial tests determine that I can follow an Oscar from horizon to horizon full quieting on my 817.
So even if I hear nothing from Oscar-1 this weekend, at least I have a very good AMSAT antenna.
Best
Jim – AB9LM